Anticorrosive Coatings E-Book

Jörg Sander

• Herausgeber: Vincentz Network
• Kategorie: Wissenschaft und neue Technologien
• Serie: European Coatings TECH FILES
• Sprache: Englisch

Seitenzahl: 447

Veröffentlichungsjahr: 2014

Beschreibung

This eBook brings together all the disciplines involved in the creation and use of corrosion protection coatings for metals. It presents the latest insights into the quality and chemistry of surfaces, the proper way to prepare them by conversion treatment, the function of resins and anticorrosion pigments in paints, and novel concepts in corrosion protection. As an added bonus, all the standards and directives mentioned in the text are grouped together in their own chapter for a better overview.

Leseprobe

Vincentz Network GmbH & Co KG

Jörg Sander | Lars Kirmaier | Mircea Manea | Dmitry Shchukin | Ekaterina Skorb

Anticorrosive Coatings

Fundamentals and New Concepts

Cover: Simon Cataudo – sxc and Sergey Kolesnikov – Foltolia.com

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Sander, Jörg; Kirmaier, Lars; Manea, Mircea; Shchukin, Dmitry; Skorb, Ekaterina

Anticorrosive Coatings

Hanover: Vincentz Network, 2010

EUROPEAN COATINGS TECH FILES

ISBN 3-86630-805-1

ISBN 978-3-86630-805-3

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EUROPEAN COATINGS TECH FILES

Jörg Sander | Lars Kirmaier | Mircea Manea | Dmitry Shchukin | Ekaterina Skorb

Anticorrosive Coatings

Fundamentals and New Concepts

Foreword

Metals, in particular steel and aluminium, are among the most important construction materials to be met in everyday life. However, these versatile materials are prone to corrosion, resulting in safety impairments, aesthetic failures, and, on the bottom line, economic damage. A major part of surface engineering of metals therefore is focused on corrosion protection.

Organic coatings have been used on any substrate for design purposes as well as to preserve the outward appearance. When applied to metals, corrosion protection becomes their most important technical feature. Though, in this respect, the primary effect of a coating is to form a physical barrier, there is no simple rule of “the thicker, the better”. Effective corrosion protection provided by an organic coating requires proper preparation of the substrate surface, expert formulation of treatment and coating chemicals and appropriate application processes, as well as adaptation to different uses and service environments.

A lot of literature has been published on single aspects of corrosion protective coatings. However, corrosion protection by organic coatings is a truly cross-functional issue. Whereas, to the lead author’s opinion, a unified approach to this task is lacking that highlights the role of all disciplines involved in the creation and use of corrosion protection coatings for metals.

The intention of this book is to provide this missing synopsis. It features an up-to-date picture of the quality and chemistry of a substrate surface, its proper preparation by conversion treatment, the function of resins and anticorrosive pigments in paints, and novel concepts for corrosion protection. It is addressed to all parties involved in metal surface and coatings engineering, both the supplier and the user, both the expert as well as the student in any of the single disciplines. It is intended to contribute to a better understanding of the mutual roles and responsibilities in corrosion protective organic coatings, and pave the way to a more durable and sustainable preservation of our valuables and resources.

Velbert, Germany, April 2010

Jörg Sander

Contents

1Introduction

1.1Why corrosion-protective coatings

1.2Literature

2Corrosion protection coatings

2.1Principles of function

2.1.1Electrochemistry of corrosion inhibition

2.1.2Metal oxide formation

2.1.3Cathodic protection

2.1.4Passivation and conversion coating

2.2Design of organic coating systems

2.2.1Diffusion barrier features – humidity uptake and electrolyte permeation

2.2.2Active pigments

2.3Function of individual coating layers

2.4Literature

3Surface preparation

3.1Industrial cleaning

3.1.1Importance of cleaning process

3.1.2Contaminants

3.1.3Surface energy and tension

3.2Mechanical cleaning

3.3Chemical cleaning

3.3.1Plasma and corona processes

3.3.2Solvent cleaning

3.3.3Chemistry of aqueous cleaners

3.3.3.1Mechanism: alkalinity, saponification and metal dissolution

3.3.3.2Ingredients of aqueous cleaners

3.3.3.2.1General considerations

3.3.3.2.2Surfactants

3.3.4Physics of aqueous cleaning, bath life and rinsing

3.4Literature

4Organic coating materials

4.1Ingredients of organic coating materials

4.2Resins

4.2.1Alkyd resins

4.2.1.1Alkyd resins manufacturing process

4.2.1.2Decay of alkyd resins

4.2.1.3Composition of alkyd resins

4.2.1.4Curing of alkyd resins

4.2.2Chlorinated rubber

4.2.3Polyvinyl chloride

4.2.4Epoxy resins

4.2.4.1Raw materials for epoxy resins

4.2.4.2Manufacturing process for epoxy resins

4.2.4.3Cross-linking of the epoxy resins

4.2.5Epoxy esters

4.2.6Acrylic resins

4.2.6.1Manufacturing of acrylic resins

4.2.6.2Thermoplastic and thermosetting acrylic resins

4.2.7Polyurethanes

4.2.7.1Reactivity of isocyanate group

4.2.7.2Waterborne polyurethanes

4.2.7.3Isocyanate free polyurethanes

4.2.8Polyaspartics

4.2.8.1Polyurea systems

4.2.8.2Polyaspartic systems

4.2.9.1Nomenclature of the silicon chemistry

4.2.9.2Manufacturing of alkyl silicates

4.2.9.3Reactions of alkyl silicates

4.2.10Polysiloxanes

4.2.10.1Reactions of siloxanes

4.2.10.2Manufacturing of siloxanes

4.3Pigments – Introduction

4.3.1Corrosion protection pigments

4.3.1.1Lead and chromate pigments

4.3.1.1.1Lead pigments

4.3.1.1.2Chromate pigments

4.3.1.2Phosphate based pigments

4.3.1.2.1Zinc phosphate

4.3.1.2.2Modified orthophosphates

4.3.1.2.3Modified polyphosphates

4.3.1.3Inorganic/organic synergies

4.3.1.4Wide spectrum anticorrosives

4.3.1.5Phosphites and phosphides

4.3.1.5.1Zinc hydroxyphosphite

4.3.1.5.2Iron phosphide

4.3.1.6Borates

4.3.1.6.1Barium metaborate

4.3.1.6.2Zinc borate

4.3.1.6.3Calcium borosilicate

4.3.1.7Molybdates

4.3.1.8Ion-exchange pigments

4.3.1.9Zinc cyanamide

4.3.1.10Hybrid anticorrosive inhibitors

4.3.2Barrier pigments

4.3.2.1Micaceous iron oxide

4.3.2.2Aluminium flakes

4.3.2.3Zinc flakes

4.3.3Sacrificial pigments

4.3.3.1Zinc dust

4.3.3.2Magnesium

4.3.4Colouring agent

4.3.4.1White pigment, titanium dioxide

4.3.4.2Red pigments

4.3.4.3Yellow pigments

4.3.4.4Green pigments

4.3.4.5Blue pigments

4.3.4.6Black pigments

4.4Extender pigments

4.4.1Carbonates

4.4.2Sulphates

4.4.3Silicas

4.4.4Silicates

4.4.4.1Talc

4.4.4.2Kaolin

4.4.4.3Wollastonite

4.4.4.4Mica

4.5Other additives

4.6Solvents

4.7Raw materials for powder coatings

4.8Literature

5Film formation

5.1Physical drying

5.2Chemical curing

5.2.1Thermal cross-linking: chemistry, mechanism, imparted properties

5.2.2Radiation curing

5.2.2.1Chemical principles and intrinsic properties

5.2.2.2Applications

5.2.2.3Equipment

5.3Literature

6Mechanism of protection and properties of organic coatings

6.1Measurement of physical properties and influence

6.2Dry film thickness

6.3Adhesiveness

6.3.1Role of adhesion and factors of influence

6.3.2Measurement of adhesion and elasticity

6.3.2.1Industrial methods

6.3.2.2Laboratory methods

6.4Permeation through organic coatings

6.5Corrosion protective performance

6.5.1Titanium and zirconium fluoro complex based pretreatments

6.5.2Weldable corrosion protection primer for automotive sheet

6.5.3Thermally curing 2-in-1 primer-pretreatment

6.5.4Chromous based pretreatments – chromiting

6.5.5Active pigments, ion exchangers and scavengers

6.5.6UV-curable primer-pretreatment

6.6Degradation and ageing

6.6.1Weathering

6.6.2Electrochemical degradation

6.6.2.1Cathodic disbonding: oxygen reduction

6.6.2.2Anodic disbonding: filiform corrosion

6.7Literature

7Testing of organic coatings

7.1Performance testing

7.2Accelerated corrosion tests

7.2.1Overview

7.2.2Constant climate tests

7.2.2.1Salt spray

7.2.2.2Constant climate, humidity

7.2.2.3Filiform corrosion

7.2.2.4Condensation

7.2.2.5Boiling, water soak

7.2.3Cyclic climate tests

7.2.3.1Cyclic humidity

7.2.3.2Prohesion

7.2.3.3VDA test

7.2.3.4UV test, weathering

7.3Electrochemical testing

7.3.1.General remarks

7.3.2.Electrochemical potential

7.3.2.1Standard potential

7.3.2.2Cyclovoltammetry

7.3.3Electrochemical impedance spectroscopy

7.3.4.Electrochemical techniques with high spatial resolution

7.3.4.1Scanning vibrating electrode

7.3.4.2Height-regulated Scanning Kelvin Probe

7.3.4.2.1General technique

7.3.4.2.2Blister test

7.4Outdoor exposure tests

7.5Literature

8Chemical conversion treatment

8.1Substrates

8.2Chemicals for pretreatment

8.2.1General remarks

8.2.2Alkaline passivation

8.2.3Phosphating

8.2.3.1Iron phosphating

8.2.3.2Zinc phosphating

8.2.4Chromating

8.2.5Anodising of aluminium

8.2.6Chromiting

8.2.7Chromium-free pretreatment

8.2.7.1Titanium and zirconium fluoro-complex technology

8.2.7.2Other chromium-free pretreatments

8.2.8Hybrid pretreatment coatings

8.2.8.1Silane/siloxane coatings

8.2.8.2Combined thermal processes for primer-pretreatment

8.2.9Surface preparation of other substrates – copper alloys, white metal, magnesium, stainless steel

8.2.10Environmental considerations

8.3Application of pretreatments

8.3.1Immersion and Spray Treatment

8.3.2Pretreatment application for coil: spray/squeeze, spray-cell, roll-coating

8.4Literature

9Organic coatings for maintenance

9.1Surface tolerant coatings

9.1.1General considerations

9.1.2Surface tolerant coating materials

9.2Organic coatings on residual rust and old coatings

9.3Literature

10New corrosion protection concepts

10.1Thin films

10.1.1Self-assembling monolayers

10.1.2Conducting polymers

10.1.3Biopolymers

10.2Nanomaterials

10.2.1Nanocomposites

10.2.2Sol-gel derived ceramic and hybrid coatings

10.3Self-healing coatings

10.3.1Self-repairing polymer films

10.3.2Inhibitor release

10.4Conclusions

10.5Literature

11Standards and guidelines

11.1General information

11.2General norms

11.2.1Norms on mechanical testing of organically coated metallic work pieces

11.2.2Norms on corrosion testing of organically coated metallic work pieces

11.3Selected European legislation on environmental protection

11.4Literature

Authors

Index

1Introduction

Jörg Sander

1.1Why corrosion-protective coatings

Man has used metal since about 6,500 B.C. Dating back to this time long past, copper items were found in the neolithic settlement of Çatal Höyük (Catalhoyuk) in Anatolia, where their appearance coincided with the obvious decline in the use of stone tools[1, 2]. While copper and its alloys served for decoration, everyday objects, cutlery, tools and weapons of their era, widespread use of metal only started with the establishing of iron making in the second half of the second millennium B.C.[3].

Iron has dominated history, culture, engineering sciences and industrial manufacture grace to its properties, combining availability and convenient refining, versatile workability by casting, forging, rolling and machining, and, last but not least, structural strength and ductility. From Damascus steel scimitar blades to Stephenson’s Rocket, from the common passenger car to the Sears Tower Building, from a washing machine to luxury cruise ships – the applications of iron and steel are innumerable.

However, since the discovery of iron and the invention of its relative, steel, man has also been confronted with the degeneration and decay of this so very useful material by corrosion. The corrosion of iron and steel is a very fast process that destroys valuable goods every day and causes a lot of economic damage. The prevention of corrosion therefore is of paramount importance.

Organic coatings offer a very attractive road to the corrosion protection of metals. Their purposes and applications span from the temporary protection of surfaces during storage, transport and onward processing, over the role as protective primers that may provide a number of additional features like surface structuring, biostatic finish, conductor properties, (dry) lubrication etc., cosmetic coatings for colour, haptic and gloss design of surfaces, and finally surface finishes that provide anti-graffiti, anti-fingerprint, or wear resistant properties. Organic coatings provide a unique combination of aesthetic appearance and protection against corrosive decay for man’s most important construction material – metal.

A lot of literature has been issued on corrosion prevention, and a lot of scientific and engineering effort as well as public and institutional funding are spent to understand corrosion and deterioration phenomena and to find ways for their prevention. The protective coating of metals primarily intended to retard the start of corrosion damage has often been discussed. In particular, the organic coating of metals has been described in basic reference works by representatives of the paint industry, in papers by the scientific community, in guides to surface engineering, and in monographs dealing with single aspects of use. All authors had their specialist perspectives on metal and metal finishing, for example in steel making, on passivation and surface treatment, on the vast number of uses and their respective anticorrosive measures, and the expertise to formulate and apply coatings. However, it has proven difficult to obtain the big picture that would demonstrate the interdependence and the interplay of all parties.

The intention and the purpose of the present book is to provide a synopsis on all aspects of corrosion protection coatings for metal, from the substrate quality and its chemical properties, the importance of proper cleaning and surface preparation by conversion treatment, the role of paint ingredients, in particular novel resins and anticorrosive pigments, surface engineering techniques, and the ongoing research for novel trends that will lead the way into the future of corrosion-protective coatings.

Five authors, each of them with a good record in the industrial and scientific community, have joined in this work.

1.2Literature

[1]Zohar, M., Catal Hüyük, Residential Structures, Mortuary Customs, Material Culture, Artistic Expression, Catal Hoyuk, a Neolithic Town in Anatolia, Net Industries, Kingston ON, Canada 2010 (www.jrank.org/history/pages/5978/Catal-H%C3%BCy%C3%BCk.html) (28-03-2010; 15:35 h)

[2]Weipert, H., Palästina in vorhellenistischer Zeit (Palestine in Pre-hellenistic Times) in: Hausmann, U. (ed.), Handbuch der Archäologie (Handbook of Archeology – Middle East) 2, 1st Vol., Beck, Munich 1988, p. 120

[3]Coghlan, H. H., Prehistoric Iron Prior to the Dispersion of the Hittite Empire, Man 41, Royal Anthropological Institute of Great Britain and Ireland, 1941, pp. 74 ff

2Corrosion protection coatings

Jörg Sander

2.1Principles of function

2.1.1Electrochemistry of corrosion inhibition

Corrosion is an electrochemical process that destroys the surface of metals by dissolution reactions and formation of corrosion products. On a coated metal surface corrosion takes place at the interface between substrate and coating. Consequently, a coating must provide sufficient protection to retard an onset and propagation of corrosion reactions. Strategies for corrosion prevention therefore involve a number of possible measures that either improve electrochemical stability of metal surface or restrict access of corrosive media to surface and along metal/coating interface. Common preventive strategies are summarised below, that have been discussed elsewhere[1, 2].

Corrosion prevention strategies

•Conversion coatings

•Cathodic protection

-Galvanising

-Zinc dust primers

•Sealing of pores

-Multi-layer coatings

•Barrier effect

•Dielectric properties of a coating

•Reduced moisture and gas uptake

•Interception of corrosive agents

-Active anions

-Ion exchange pigments

•Improvement of substrate adhesion

-Adhesive primers

-Silane/siloxane coatings

•Increase of alkalinity

-Cement coatings

The metallic state is defined as a feature of solid matter, where atoms are located, densely packed, at the sites of a crystal lattice, and one or more of their electrons (bonding electrons) released to be freely distributed across the entire macroscopic crystal[3]. Therefore, in principle all metals are prone to corrosion due to free availability of their electrons at the outer surfaces of the metal crystal.

One driving force for corrosion is the ease of electrons to be released, or in other terms, of the metal to be oxidised. This depends on the kind of metal, and is usually characterised by the metal’s electrochemical potential. The chemical reactions that take place during atmospheric corrosion can be resolved into a pair of red-ox reactions, shown in Equations 2.1 and 2.2.

Figure 2.1: (a) Atmospheric corrosion at a punctured coating over a zinc substrate; (b) Formation of a barrier layer by precipitation of zinc oxide (native oxide barrier)

Equation 2.1

Zn → Zn2++ 2 e- Anodic reaction

Equation 2.2

1/2 O2+H2 O+2 e-→2 (OH)- Cathodic reaction

The metal serves as the anode, i.e. the source of free electrons which are released upon the dissolution of metal atoms. These are oxidised to metal cations (e.g. Zn2+) in the first reaction, and the corresponding equivalent amount of electrons (e–) is made available for the second reaction. In case of atmospheric corrosion, these electrons pass through the interface into the surrounding solution, where they are consumed for the reduction of oxygen. Figure 2.1 shows a schematic view of this situation[4, 5].

This description, in fact, leads directly to the major strategic principle of any anticorrosive coating: If either of the reaction branches is slowed down effectively, the onset and development of corrosion can be largely retarded. Corrosion inhibition (passivation) hence can be achieved by the formation of a physical barrier that either insulates electrically, i.e. prevents the transition of electrons from the metal to surrounding electrolyte, or mechanically, i.e. blocks the direct access of electrolyte or atmospheric oxygen to the metal surface.

2.1.2Metal oxide formation

A passivation or conversion coating, therefore, usually involves formation of a dense coating of metal oxide, in order to display this barrier effect. The metal oxide, most often incorporating dissolved ions of the substrate metal itself, precipitates on the substrate surface and forms a physical barrier. This barrier can only be penetrated by oxygen or ions through diffusion, thereby slowing down the access of these materials to the base metal.

An effect of metal oxide coatings is sometimes also described in terms of a band model. Many oxides of transition and higher main group metals have semiconductor properties. For example, zinc oxide (ZnO) displays characteristics of an n-semiconductor, which means that electrons in the valence band of the oxide can be promoted to the conducting band by relatively low energy input. The energy gap (i.e. the distance of energy levels between the valence and conducting bands) is a characteristic feature for each substance. The band gap of magnesium oxide, MgO, for example, is much wider, which makes the compound an insulator. Oxides of alloys, like the species MgZn2, display energy gap values between those observed for oxides of the pure elements. Additionally, defects in the oxide coating result in a reduction of the energy gap. A better resistance of an oxide coating against corrosion is obtained with the widening of the energy gap. Though MgZn2 is the prevailing component in the new Zn-Mg galvanising coatings, the situation in technical Zn-Mg coatings is complicated by the additional presence of aluminium. The effect of the various corrosion products in this model is also not yet entirely understood[6].

One approach for a rationalisation of this effect is the respective stability of corrosion products (oxides and hydroxides) in alkaline conditions, as occur under the regime of oxygen reduction. MgO and Mg(OH)2 are insoluble in alkalis, and therefore form stable barrier layers. Consequently, zero-corrosion current densities are observed for lower (more negative) electrode potentials and higher alkalinities, the more Mg is present in the overall metallic coating composition (cf. Figure 2.2)[7]. In coherence, micrographic and photo electron spectroscopic (XPS) studies have shown mainly Zn corrosion products in defective areas with high dissolution, while areas with a low corrosion aspect were found to be covered by an MgO/Mg(OH)2 film.

Native oxide coatings of this kind develop on zinc (Zn), aluminium (Al), magnesium (Mg), or chromium (Cr), sometimes also incorporating carbonate ions from atmospheric carbon dioxide (cf. Chapter 3.3.3.1).

Most oxides and hydroxides are instable when contacted with chloride containing electrolytes. Chloride can replace oxygen in the crystal lattice, so that mixed oxy/hydroxy chlorides are formed that are known as intermediates in the dissolution process which often leads to local (pitting) corrosion. Hydrotalcite pigments are reported to act as chloride scavengers by virtue of their ion-exchange properties[8]. Calcium compounds are also known to intercept residual chloride in coatings for polyalkylene packaging foils[9]. A similar effect can be expected from calcium compounds in organic coatings for metals.

Source: Elsevier Ltd. [7]

2.1.3Cathodic protection

Cathodic protection can also be made a function of the primer. Zinc pigmented primers have been used as anti-corrosion barrier for various artisan and industrial coating applications, in particular in heavy machinery, architecture and industrial construction, bridge engineering, water engineering, shipbuilding, etc. For these purposes, primer coats are used with gauges of 50 µm and more[10–13]. The usefulness of zinc-rich primers is often attributed to the cathodic protection provided by zinc particles to otherwise unprotected steel surfaces. Zinc dust primers have also been reported as protective coatings under powder coatings.

As the authors did not observe any influence of the zinc content in various primer formulations, they conclude that the major contributions to the improved performance are made by the mechanical barrier effect and the adhesion promotion that is brought about by the extra primer layer[14].

The principle of cathodic protection has also been used in corrosion protection primers (CPP) that are applied on sheet for car manufacture[15, 16]. By their use, improved corrosion protection is achieved particularly in critical areas of a car body, like seams, box sections or flanges, where a pretreatment is geometrically difficult to apply and may therefore be faulty, or the cathodic electro-dip primer is only insufficiently deposited due to shielding effects.

2.1.4Passivation and conversion coating

Native oxide layers may serve well as efficient anticorrosive barriers for bare bulk metal, but from the perspective of technical surfaces, it is often desired to create a more uniform, controlled passivation layer, to better preserve the surface aspect and features. To this end, it is necessary to remove the native oxide film (cf. Chapter 3.3.3.1), and replace it by a similar film under controlled conditions, with improved features, for instance, uniform thickness, lower porosity, better transparency, or higher electrical resistance.

Equation 2.3

(CrO4)2-+ 8 H++ 3 e-→ Cr3++ 4 H2 O Cathodic reaction

Underneath a coating, the situation is more complicated. It is important to provide a good adhesion of the organic coating by means of a conversion pretreatment. A good conversion pretreatment has to form chemically bonded layers that are insoluble in water under changing pH conditions. Under a mild atmosphere, the reduction of oxygen tends to result in an increase of the pH, as hydroxyl (OH–) ions are released in the course of reaction (Equation 2.2). While these ions may be consumed again by ongoing precipitation of oxides, locally and temporarily high alkalinity may occur.

The long-lasting corrosion resistance provided by chromate passivation is also due to the so-called self-healing effect. This is usually attributed to residual chromate ions that are incorporated in the oxide layer. In case of mechanical damage of the passivation layer that penetrates through the underlying substrate metal these chromate ions are available for a quick passivation reaction at the newly exposed metallic location. Other “active anions” that may also serve as strong oxidants for metals, like the oxo-anions ferrate, (per)manganate, molybdate, tungstate, or vanadate, are considered as possible direct replacements for chromate[18]. For example, Patent WO 0036182[19] describes a pretreatment for aluminium and its alloys, based on an alkaline ferrate (VI) solution and additional oxo-anions like molybdate. The patent is valid in several European states, but no commercial application is known yet.

2.2Design of organic coating systems

2.2.1Diffusion barrier features – humidity uptake and electrolyte permeation

While cross-linking of a coating renders a macroscopically intact and closed network with the required mechanical and chemical features, on a molecular scale, any polymer film will remain penetrable to some extent by gases and electrolytes, due to diffusion and migration. Increasing the film thickness is one possible measure to improve on this situation. For instance, in coil coating, primer coatings are normally used at 5 µm dry-film thickness. For high anticorrosive requirements, e.g. certain appliances and marine architectural applications, coil primer coats of up to 30 µm are common.

The uptake of water or moisture is considered an important feature of organic coatings, as it is linked with changes in the density and dielectric properties of the polymer film. Absorption and incorporation of water molecules will lead to physical swelling (volume expansion) and results in a softening of the coating surface. Moreover, as water is an electrolyte carrier, its presence is related with the ease of dissolved ions to penetrate through a coating, where they may accumulate and, finally, cause degradation of the polymer film and corrosion of the metal substrate[20].

Water uptake is influenced by chemical and physical properties of the coating, and therefore it is determined by the molecular design of the polymer. Typical coating resins like epoxies or polyurethanes display a modest water uptake in humid atmosphere. However, the rate of incorporation is strongly accelerated when approaching the dew point. In principle, polar (hydrophilic) polymers will allow better wetting and hence also a higher water absorption[21].

The elasticity of a coating, on the other hand, plays a role in the diffusion transport of molecules through the interstices of the polymer film, which can be interpreted as the relative mobility of polymer moieties[22]. While this property changes with temperature, usually showing a jolt from the rigid to the elastic regime at the glass transition temperature (Tg), it will also be affected when the film is plasticised by incorporation of water[23]. The design of the polymer therefore includes the task of reconciling disparate features like flexibility (formability) vs. curing temperature vs. diffusion barrier properties, or water miscibility vs. chemical resistance vs. adhesion vs. wetting.

Another strategy involves the use of nanoscale particles like titanium dioxide (TiO2) preparations, or natural or synthetic mineral pigments like clays and hydrotalcites. The latter two contain agglomerates of nanosized crystalline platelets that can be separated from the agglomerate and distributed in the bulk coating. The idea is that these platelets can form an additional barrier to penetrating media, as they could extend the diffusion path for any corrosive ion or media through the coating, cf. Figure 2.3. The observation ofthe expected effect was reported for oxygen, carbon dioxide and water vapour[24].

Figure 2.3: Schematic view of the extension of the diffusion path by a nano-platelet filled coating; according to Lewis[1]

2.2.2Active pigments

Additives are also used to obtain a higher anticorrosive performance. Often active pigments are incorporated that interfere with penetrating corrosive agents (cf. also Chapter 4.3). Here again, chromates, in particular strontium chromate, and dichromates have been used, but due to their toxicity and pollution control requirements, are being replaced by successors that are free from hexavalent chromium (chromate) or entirely chromium-free, e.g. zinc phosphates.

Mineral or synthetically generated particles like hydrotalcites or zeolites can also be designed as carriers to host e.g. anticorrosive ions like molybdates or vanadates. Such carrier concepts are considered to be particularly useful, because they enable retarded release of the active agents that thus remain available for longer periods, instead of quickly bleeding out of a coating, depriving it of its properties. The release might even be controlled by the conditions of the surrounding medium, e.g. pH value, temperature or the presence of (corrosive) counter ions, so that the active ions are only available on demand (cf. Chapter 6.5.5)[25].

Other suitable anticorrosive additives comprise ion-exchange pigments like calcium-modified silicates that release calcium ions (Ca2+) in the presence of acids[26] thereby reducing the corrosivity of the electrolyte, and precipitating corrosive ions like chlorides or sulphates. A number of products are commercially available.

Condensed conductive polymers like polyaniline (PAni) have been around for a number of years[27]. They have been shown to suppress delamination on steel. The mechanism of PAni is attributed mainly to an increase of the electrochemical potential of the substrate (ennoblement) thereby obstructing oxygen reduction[28]. PAni is rendered conductive by the formation of its protonated salts, and the reverse deprotonation process is triggered by the potential decrease during the delamination reaction. If counter-anions were suitably selected as to inhibit corrosion, they might be truly released on demand with the onset of a corrosion reaction. However, a controversial discussion is going on around this point. It has been commented that PAni can be useful when added in low amounts, but can be detrimental to corrosion protection in other conditions[29-31].

2.3Function of individual coating layers

Industrial organic coating systems consist of a conversion coating and a single- or multi-layered paint coating. Any coating is designed to combine a set of varying features appropriate for the final use[32, 33]. In many cases, two or three-coat paint systems are used that comprise a primer layer, an optional intermediate coating and a topcoat. In the manufacture of cars, multi-coat systems are used. An electrocoat paint is applied as primer, followed by a filler, then the coloured basecoat and finally a clearcoat.

Features provided by an organic coating system on metals can be summarised as follows (cf. also Figure 2.4)

•Colour, gloss, structure

•Corrosion resistance

•Chemical resistance

•Adhesion

-Substrate adhesion (fingerprints, condensation, etc.)

-Intercoat adhesion

•Elasticity, formability

•Wear and scratch resistance

•Soil and stain resistance

•Heat resistance

•UV resistance

Figure 2.4: Schematic of a typical multi-coat paint system and functions of individual layers

The conversion coating is necessary to obtain the primary adhesion to the substrate and a sufficient electronic anticorrosive barrier (cf. Chapters 2.1 and 8). Usually, the primer provides an additional part of the corrosion resistance and flexibility, while the topcoat is used to give aesthetic appearance (colour, gloss) and mechanical and chemical resistance.

Primers therefore form a physical barrier that prevents access of moisture, water, electrolytes and reactive gases (e.g. oxygen) to the metal surface. This is achieved by choice of the resin chemistry. Resins that are commonly used in primers comprise acrylates, polyesters, epoxies and polyurethanes. For example, acrylates are used in primers for polyvinyl chloride (PVC) plastisol coatings. Polyesters usually display good adhesion to the substrate, and provide good corrosion resistance. Polyurethanes are known for their flexibility that allows deformation, resistance against mechanical wear, and a low sensitivity towards thermal stress, particularly by low temperatures in the environment. Chemical resistance is brought on by cross-linking of single polymer strands, which is also enabled by the suitable formulation of the paint.

Intermediate coatings are used to build up film thickness, and mediate the distribution of mechanical stress. They often contain platelet pigments, like micaceous iron oxide, in order to extend the diffusion path for intruding corrosive species (cf. Chapter 2.2.1).

Applications in particulary demanding corrosive environments may require a topcoat containing such a pigmentation which may restrict the obtainable colour and the level of gloss. It is also important that the topcoat is non-transparent to UV radiation.

2.4Literature

[1]Lewis, O. D., A Study of the Influence of Nanofiller Additives on the Performance of Waterborne Primer Coatings, PhD Thesis, Loughborough Univ., Loughborough 2008, p. 78

[2]Vogelsang, J. A., Selbstheilende Beschichtungssysteme – Ein Überblick (Self-Healing Coating Systems…), Schiff und Hafen (8) 2009, pp. 28 ff

[3]Barrow, G. M., Physikalische Chemie (Physical Chemistry), vol. 3, 3rd ed., Bohmann-Vieweg, Vienna 1977, pp. 203 ff

[4]Meuthen, B., Jandel, A.-S., Coil Coating, 2nd ed., Vieweg, Wiesbaden 2008, p. 89 ff

[5]Fafilek, G., Kronberger, H., Thermodynamik, metallische Werkstoffe und Korrosion (Thermodynamics, Metallic Construction Materials and Corrosion), Experimental Exercises Script, Exercise TK, Inst. Chem. Technologies and Analytics, TU Vienna 2005, p. 6, www.tuwien.ac.at/echem/education/158076/pcvt_lu_TK_skriptum.pdf (15-03-2010; 18:42 h)

[6]Giza, M., In-situ Spectroscopic and Kelvin Probe Studies of the Modification of Solid Surfaces in Low Temperature Plasmas, Diss., University of Paderborn, 2008

[7]Hausbrand, R., Stratmann, M., Rohwerder, M., Corrosion of Zinc-Magnesium Coatings: Mechanism of Paint Delamination, Corr. Sci. 51, 2009, pp. 2107 ff

[8]Mahajanam, S. V. P., Buchheit, R. G., Characterization of Inhibitor Release from Zn-Al-[V10O28]6-Hydrotalcite Pigments and Corrosion Protection from Hydrotalcite-Pigmented Epoxy Coatings, Corrosion 64, 2008, pp. 230 ff

[9]Crass, G., Janocha, S., Bothe, L. (inv.), Biaxially oriented multilayered polyolefin film which is printable and of which both sides are sealable, its production and use, European Patent EP 0263963, Hoechst AG 1987

[10]Meyer, K., Schutz von Stahlkonstruktionen, Beispiele aus der Eisenindustrie und dem Bergbau (Protection of Steel Constructions…), Fette, Seifen, Anstrichmittel 69, 1967, pp. 90 ff

[11]Schröder, T., Erfahrungen mit Schutzbeschichtungen an Stahlwasserbauten der Wasser- und Schiffahrtsverwaltung im Bereich der Nord- und Ostsee (Experiences with Protective Coatings on Hydraulic Structures…), Materials and Corrosion 23, 1972, pp. 993 ff

[12]anon., Kanalüberführung Leine (Canal Flyover Leine River), Wasser- und Schifffahrtsverwaltung des Bundes – Neubauamt Hannover (Federal Waterways and Shipping Administration), Hannover 2009; www.nba-hannover.wsv.de/baumassnahmen/abgeschlossene_baumassnahmen/neubau_leinequerung.html (15-03-2010; 18:45 h)

[13]Vogelsang [2] p. 30

[14]Schütz, A., Kaiser, W.-D., Kein Rasten gegen das Rosten (No Resting versus Rusting), FARBE UND LACK 110, 2004, pp. 26 ff

[15]Meuthen, Jandel [4] p. 75 ff

[16]Schinzel, M., Advanced Corrosion Protection of Automotive Body Sheet – A Challenge for Coil Coating, ECCA Autumn Congress Brussels, Proc., European Coil Coating Association, Brussels 2009

[17]Weast, R. C., Astle, M. J. (ed.), Handbook of Chemistry and Physics, 60th ed., CRC Press, Boca Raton 1981, p. D-155

[18]Kendig, M. W., Buchheit, R. G., Corrosion Inhibition of Aluminum and Aluminum Alloys by Soluble Chromates, Chromate Coatings, and Chromate-Free Coatings, Corrosion 59, 2003, pp. 379 ff

[19]Minevski, Z., Eylem, C., Maxey, J. (inv.), Ferrate Conversion Coatings for Metal Substrates, WO 0036182, Lynntech Inc., 2001

[20]Goldschmidt, A., Streitberger, H.-J., Basics of Coating Technology, 2nd Edition, BASF Coatings AG, Münster 2007, p. 447

[21] Öchsner, W. P., Bergk, B., Fischer, E., Gaszner, K., Sorptionsisothermen für Wasser in organischen Beschichtungen und deren Einfluss auf die Beschichtungseigenschaften (Sorption Isotherms for Water in Organic Coatings…), FARBE UND LACK 111, 2005, pp. 42 ff

[22]Lewis [1] p. 87

[23]Goldschmidt, Streitberger [20] p. 401

[24]Lewis [1] p. 34

[25]Mahajanam, Buchheit [8] p. 231

[26]Vogelsang [2] p. 29

[27]Wessling, B., Corrosion prevention with an organic metal (polyaniline): Surface ennobling, passivation, corrosion test results, Materials and Corrosion 47, 1996, pp. 439 ff

[28]Holness, R. J., Williams, G, Worsley, D. A., McMurray, H. N., Polyaniline Inhibition of Corrosion-Driven Organic Coating Cathodic Delamination on Iron, J. Electrochem. Soc. 152, 2005, pp. B73 ff

[29]Rohwerder, M., Intelligent Corrosion Protection by Conducting Polymers, Smart Coatings II, ACS Symposium Ser. 1002, Am. Chem. Soc. 2009, pp. 274 ff

[30]Meine, D., Korrosionsschutz der Zukunft? (Future Corrosion Protection?), Conf. Report, Europ. Coatings. Conf., Vincentz Network 2007, FARBE UND LACK 113, 2007, pp. 33 f

[31]Saji, V. S., Thomas, J., Nanomaterials for corrosion control, Current Science 92, 2007, pp. 51 ff

[32]Kittel, H., Streitberger, H.-J. (ed.), Lehrbuch der Lacke und Beschichtungen (Coursebook of Paints and Coatings), Vol. 6, 2nd ed., Hirzel, Stuttgart 2008, pp. 485 ff

[33]Meuthen, Jandel [4] pp. 51, 79 ff

3Surface preparation

Jörg Sander

3.1Industrial cleaning

3.1.1Importance of cleaning process

Anticorrosive metal coatings require a clean and well-prepared metal surface, if a long-lasting product with an aesthetic appeal and high durability is to be obtained. The efficiency of the surface preparation determines the endurance of any conversion, anodic, galvanic or organic coating between weeks and years. Quite often however, the difference between a clean and a contaminated surface does not become apparent by visual inspection. Thorough monitoring of the cleaning[1, 2] stage is therefore mandatory in order to ensure that both a high-class pretreatment and final coating are achieved.

Mechanical cleaning like brushing, grinding, shot or grit-blasting etc. can serve for a basic level of surface preparation. Much more often, chemical cleaning is a necessity. Several application methods are available for industrial cleaning operations, like immersion, spraying, or vapour cleaning. Prior to painting, metal surfaces are most efficiently and economically cleaned by aqueous spray cleaners, but the choice is made according to the size and shape of the work pieces and the type of contamination.

In many cases, e.g. job-coater facilities, mixed-metal car body manufacture or coil coating, a mix of different substrates must be cleaned and pretreated. Most industrial cleaners therefore must apply on different substrates. Aqueous cleaner solutions are used on steel, galvanised steel, aluminium, but also on die-cast alloys (Al, Zn, Mg), other non-ferrous metals and – in the automotive and appliance industries – prephosphated sheet.

3.1.2Contaminants

Industrial cleaning deals with a great variety of soils and contaminants.

Rust, scale, residues, e.g. of old paints, galvanising layers, faulty conversion or anodic coatings, usually require mechanical precleaning or pickling.

Oils and fats usually originate from the generation process of the unmanufactured or semi-finished material. They stem from protective oils (temporary corrosion protection, often containing additional organic inhibitors, e.g. triethanol amine) or metal working fluids (drilling, cutting, stamping, drawing oils etc.).

Polishing agents like stearic or paraffinic waxes, usually contain additional grinding compounds like finely dispersed silica, lime, corundum, colloidal clay minerals etc.

Sweat or fingerprints contain fats, fatty acids, proteins etc.

Fines and metal abrasions originate from rolling, grinding and cutting processes, in particular also from forming (deep-drawing, flanging).

Pigment soil, graphite and carbonised oil often appear after assembly processes (grinding, joining, welding) or arise from annealing processes e.g. during coil production.

Figure 3.1: Schematic: Sessile drop showing the interfacial tension acting at the wetting frontThe angle μ between the tension vectors σ for the solid-liquid (s, l) and the liquid-gas (l, g) interfaces is measured by microscopic inspection.

3.1.3Surface energy and tension

“Chemical cleanliness” is often mistaken for a water-break-free surface, i.e. the complete wettability of a metal. While water-break-free rinsing gives a rough indication of the presence or absence of oils, fats etc., it depends on the surface tension of contaminating substances (e.g. self-emulsifying oils) and thickness of the water film, and can be affected by evaporation of water or the presence of surfactants. Native oxide layers may not result in particularly poor surface cleanliness, but they usually inhibit any further surface reaction that is necessary to build up a proper conversion coating prior to painting.

Surface tension occurs at any interface between non-miscible phases. It is brought about by differences of attractive forces that apply to molecules inside vs. between phases. This effect rules the form of droplets, micelles and foam bubbles, and it determines the adsorption of gases to solids, as it brings on capillarity phenomena like wetting and spreading of liquids on a metal surface[3]. The wettability of a solid by a liquid is characterised by the contact angle q that liquid makes on the solid. The mathematical description is given by Young’s equation:

Equation 3.1:

where: Θ is the contact angle of a sessile drop on a solid surface, σsrepresent the tension vectors for the solid-liquid (s, l), solid-gas (s, g) and the liquid-gas (l, g) interfaces (cf. Figure 3.1).

Table 3.1: Contaminants

Oils, fats, waxes

Fines, abrasives

Pigments

Rolling, casting oils, emulsifiers

Grinding, cutting, tool fines

Graphite, molybdenum disulphide

Corrosion protective oil

Grinding compounds

Drawing, pressing lubricants, soaps

Carbonised oil, welding, brazing residues

Drilling, cutting fluids

Waxes

Sweat, fingerprints

The surface tension of liquids is usually measured by capillarity or sessile drop methods. For the latter, the contact angle, i.e. the angle that is enclosed by the tension vectors for the solid-liquid and liquid-gas interfaces (cf. Figure 3.1), is measured by microscopic inspection. Table 3.2 gives some exemplary values[4]. Contact angles <90° (i.e. cos q > 0) indicate wetting, angles >90° (cos q < 0) repulsion situations. Accordingly, good wetting is obtained when the interfacial tension between solid and liquid exceeds the tension at the solid-gas boundary. If the following process requires the surface to be wet by aqueous media, the interfacial tension should exceed 72 mN/m, which is the surface tension of water vs. air[5].

To determine the surface tension of a solid surface, the contact angles of droplets of different liquids with known surface tensions are measured and the resulting overall surface tensions calculated[6]. A simple method for measuring surface tension uses prefabricated test inks[7].

Figure 3.2: Phase diagram for carbon dioxide (CO2) 0.1 MPa ≈ 1 atmosphere (Illustration according to Kukova[9])

3.2Mechanical cleaning

Mechanical cleaning is necessary when thick scale or rust layers must be removed. For steel surfaces, this is quite often supported by acid pickling. Mechanical cleaning, however, does not give sufficiently clean surfaces for subsequent galvanising or conversion coating. Apart from brushing or grinding, shot or grit-blasting is often used to remove coarse contaminations like rust, scale, carbonised oil or old paint. Grit-blasting uses various abrasive particles (e.g. sand, slag, corundum, glass or plastic beads) that are shot at the target by pressurised air. For delicate surfaces, dry ice (solid CO2) has become a familiar abrasive that is used to remove fats and oils, glues, paints and inks, corrosion products etc. Dry ice sublimates, i.e. evaporates directly from solid state, at a temperature of -78 °C at atmospheric pressure. Dry ice blasting therefore generates less waste than other grit-blasting processes. Figure 3.2 depicts the phase diagram of carbon dioxide, showing the solid, liquid and gaseous states[8, 9].

Table 3.2: Surface tension of liquids at different temperatures (in mN/m)

a) at 0°C; b) molten at 970°C; c) molten at 1080°C; d) molten at 452°C; cf. Barrow [4]

In cases when larger engineered constructions have to be cleaned on the site of their erection, the ultra-high pressure water jet technique has gained importance, to avoid dust and soil in the vicinity of the construction ground. Water pressures of 1,700 to 2,100 bar are sufficient to remove non-adhering rust and salt contaminations[10, 11].

Some discussion has been focused on supercritical CO2 (scCO2). The supercritical state is achieved when a real gas is heated and pressurised beyond its critical temperature and pressure. For CO2, the respective data are 31.04 °C and 7.38 MPa. Beyond this point, there is no physical difference between the liquid and gaseous form, and gas obtains fluid and solvent features that enable its use as an efficient extractant. scCO2 is used for high purity extraction of natural substances (e.g. decaffeination) and other food processing applications[12]. Further applications of scCO2 comprise extractions of fragrances[13], synthesis and polymer production and processing (octene hydroformylation, fluorinated polymer and polycarbonate production) and powder production. scCO2 has some potential use as fluid for cleaning of small metallic work pieces that can be handled in an enclosed, high-pressure cleaning tank[14].

Finally, ultrasonic cleaning[15] employs the conversion of DC voltage into mechanical waves above the audible frequency range (>20 kHz) by a piezo-electric generator. The vibrations create local vacua and overpressure in rapid succession (cavitation) which literally blast off soil from a contaminated surface. Ultrasonic cleaning achieves a high degree of efficiency at a very fast rate. It has been used on a wide variety of work pieces of different materials like glass, ceramics, or metal, regardless of dimension and shape.

3.3Chemical cleaning

3.3.1Plasma and corona processes

For finer cleaning that achieves a higher degree of cleanliness, plasma[16] treatments have gained interest. Plasma is generated by ionisation of vapour, and is sometimes called the fourth aggregate state of matter. Being accelerated against a solid surface, the energy of the plasma is transferred to this surface and contaminants are chemically destroyed and removed. Plasma cleaning is now available in atmospheric operations, saving expensive vacuum technique that used to be necessary. The equipment that is available nowadays can operate at a speed of several hundreds of mm/min, and plasma source arrays can span up to two metres width.

Atmospheric zero-potential plasma cleaning is technically used for applications where composite materials made from plastics, metal, glass and even ceramics are to be treated. The plasma beam makes no distinction between materials. Plasma cleaning has proven sufficient for removal of neat oil residues in stamping of painted lids on jam jars or air conditioning equipment parts in cars and trucks. The treated surfaces are sufficiently susceptible for labelling adhesives or printing inks. Applications on bare metal surfaces include a small aluminium coil coating operation and riveting in aircraft industry, where former solvent cleaning processes could be successfully replaced[17].

Plasma cleaning has been combined with deposition of materials from the gas or plasma phase, to form thin protective, in particular soil-repellent coatings[18]. High-voltage corona discharge treatment is commonly used to clean and activate plastics surfaces, especially of polyalkylenes[19]. It has also been used to clean and deposit thin coatings on metal[20].

3.3.2Solvent cleaning

Fatty, oily and wax contaminations can be removed by immersion of work pieces into organic solvents or by degreasing in condensing solvent vapour. Halogenated hydrocarbons have been used to this end due to their inflammability and solubilising power towards most fatty contaminations. To remove pigment and soils, solvent cleaning must be supported by mechanical means, e.g. brushing or ultrasonic treatment. Soil that adheres by chemisorption, oxides and metal soaps cannot be removed by solvent cleaning[21]. Furthermore, the use of solvents is subject to legal regulation, i.e. the EU Directive 2008/1/ EC[22], commonly referred to as the Integrated Pollution Prevention and Control (IPPC) Directive. This directive and related regulations, e.g. on emission of volatile organic compounds (VOCs)[23], restrict emission levels of volatiles and the use of certain solvents that pose a health hazard, i.e. due to carcinogenity or mutagenity. Hermetically closed cleaning systems are therefore mandatory.

3.3.3Chemistry of aqueous cleaners

3.3.3.1Mechanism: alkalinity, saponification and metal dissolution

In most metal coating lines, aqueous immersion or spray processes are used to clean the metal surface, and remove oil, solid contaminations and superficial scale. Typical aqueous cleaners contain alkalis for saponification of fats and oils, and for pickling of the metal surface, builders (e.g. phosphate, silicate) to disperse solid dirt particles in the solution after their removal from the surface, and surfactants to quickly wet the metal surface and to emulsify fatty and oily contaminations in a cleaner bath. Moreover, additives like defoamers etc. may be present.

The alkalinity of the cleaner is selected according to the degree of ageing and contamination of the feedstock. The alkalinity will change during the course of reaction, mainly due to consumption of the alkali by saponification and metal dissolution. However, ageing effects caused by entrapment of carbon dioxide (carbonation, see Equation 3.2) from the air must also be considered, as they leave less “free” alkalinity available for the main reaction paths (cf. Equations 3.3 to 3.8). The alkalinity therefore must be monitored manually or automatically, preferably by titration methods.

Equation 3.2:

2 OH-+ CO2→ (CO3)2-+ 2 H2O Carbonation reaction

The reactions of the alkaline cleaning process on galvanised surfaces were studied using an electro-chemical quartz crystal micro-balance (ECQM) array[24]. The principle of this method is that the oscillation of a piezo-electric quartz wafer is proportional to the wafer’s thickness. A galvanic-coated quartz wafer will therefore change its oscillating frequency in response to any chemical reaction that takes place on its galvanised surface.

Dissolution results in reduction of thickness, hence accelerated oscillation, while coating formation is supposed to cause an increase of the wafer thickness, thereby slowing down its motion. The sensitivity of the method allows monitoring of thickness changes on the atomic scale, so that molecular layers can be detected while forming, recording mass and charge simultaneously and in-situ. The schematic presentation of the experimental set-up of the ECQM for registration is given in Figure 3.3. Experimental results were obtained with a zinc-coated, special quartz crystal, that was mounted in a flow cell and then subjected to cleaner solutions at various flow rates, temperatures etc. Two different reactions can be identified when this experiment is carried out in different atmospheres, i.e.

Stage A: Dissolution of the native oxide/carbonate/hydrate layer (chemical)

Equation 3.3:

ZnO + 2 OH- + H2O → [Zn (OH)4]2-

Equation 3.4:

Zn (OH)2 + 2 OH- → [Zn (OH)4]2-

Equation 3.5:

Zn5 (OH)6 (CO3)2 + 14 OH- → 5 [Zn (OH)4]2-+ 2 (CO3)2-

Stage B: Dissolution of metallic zinc and zinc hydroxide layer formation (electrochemical)

Equation 3.6:

Zn + 4 OH- → [Zn (OH)4]2- + 2 e- Anodic reaction

Equation 3.7:

O2 + 2 H2O + 4 e- → 4 (OH)- Cathodic reaction

In an aerated solution, both electrochemical reactions are possible. However, in the absence of oxygen, the cathodic reaction is inhibited, so that only chemical dissolution is observed.

While the chemical reaction is largely unaffected by process conditions, Zn dissolution is also profoundly influenced by aeration, alkalinity, “age” of the cleaning solution and kinetic effects. Bath ageing thus can be interpreted as the build-up of dissolved zinc ions and carbon dioxide absorption from the air. The results are illustrated in Figures 3.4 and 3.5.

Figure 3.3: Experimental set-up: electro-chemical quartz micro-balance (ECQM). Source:[24]W.E.: Working electrode; C.E.: Counter electrode

On aluminium surfaces, similar reactions take place. For this substrate, an acidic rinse is recommended after alkaline cleaning, to remove compounds of alloying elements that are otherwise insoluble. As aluminium is amphoteric, which means it is also dissolved in acids, acid cleaning is common practice whenever a low contamination level allows (coil, can).

Figure 3.4: ECQM simulation: dissolution of zinc surfaces in alkaline cleaner solution with increasing age.

Source:[24]

Figure 3.5: ECQM simulation: dissolution of zinc surfaces in alkaline cleaner solution with different aeration.

Source:[24]

3.3.3.2Ingredients of aqueous cleaners

3.3.3.2.1General considerations

Major functions of an aqueous cleaner[25] comprise its ability to emulsify liquid contaminations, deflocculate solid soils and prevent their redeposition. These features are ruled by the interfacial tension between solution and soil, thus directly depending on the wetting power of the solution.

In general, fats, oils and waxes can be saponified, and the soap that is generated acts as a surfactant and emulsifier. Most industrial cleaners therefore are alkaline, in order to bring on the required saponification reaction (cf. Equation 3.8). Strong alkaline cleaners often contain caustic soda (sodium hydroxide), in case of liquid products also potassium hydroxide due to its higher solubility. To reduce the alkaline attack, carbonate may be present in the formulation.

Equation 3.8:

Especially when used for metal mixes, alkaline cleaners may also contain silicates which display a high alkalinity themselves, however form a thin silica layer, once the bare metal is exposed, thereby inhibiting excessive pickling reaction.

Figure 3.6: Schematic: removal of an oil droplet by surfactant action

For wrought and rolled alloyed aluminium (Mg, Mn containing alloys), alkaline cleaners usually contain complexants to help solubilise these elements. However, an acid rinse is often necessary to remove alkali-insoluble alloying elements and their oxides and hydroxides (e.g. MnO). In alkaline and acidic aqueous cleaner formulations, the task of dispersing solid contaminants is frequently taken by builder ingredients, i.e. salts. To remove fat, oil, and often entrapped solid contaminations from the metal surface, the aqueous products also contain surface-active substances (surfactants). Table 3.3 summarises the common ingredients of aqueous cleaners.

Table 3.3: Ingredients of industrial cleaners

Acidic cleaners

Alkaline cleaners

Neutral cleaners

Phosphoric acid, dihydrogen phosphate (Al also: sulphuric acid)

Alkali hydroxides

Surfactants, solubilisers, inhibitors

Phosphates, carbonates

Fluorides, fluoro complexes

Silicates

Surfactants, solubilisers

Surfactants, solubilisers

Complexants, sequestrants

Inhibitors

3.3.3.2.2Surfactants

Surfactants act by virtue of their amphiphilic nature, i.e. they assemble at the interface of non-miscible phases, self-aligning according to the polarity of the two phases and to their own dipolar character. They can displace oily matter (and any solid that is entrapped in it) from a metal surface. Figure 3.6 illustrates this process.

The amphiphilic character of surfactants is caused by the molecular structure, having a polar moiety bonded to an unpolar, often longer hydrocarbon chain. Depending on the type of polar functionality, surfactants are classified as anionic, cationic or non-ionic. Figure 3.7 illustrates examples for the three surfactant classes.

Important anionic surfactant groups are salts of fatty acids (soaps), sulphuric acid semi-esters of long-chain fatty alcohols or alkyl benzene sulphonates. Historically, tetrapropylene benzene sulphonate (TPBS) played a particularly dominant role as a widely used surfactant in detergents. However, TPBS had to be replaced, as it was particularly poorly degradable in the environment. Cationic species are e.g. quaternary alkyl group containing ammonium bases. Typical representatives of the non-ionic classes are polyethoxylated alcohols or esters.

Figure 3.7: Exemplary formulas of surfactants