Coatings Technology Handbook Episode 3 Part 5 doc

101.1.1.2 Flame/Plasma Spray Although all the flame and plasma spray processes project liquid metal droplets through the air to the substrate, each starts with a solid metal wire or soli

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101-2 Coatings Technology Handbook, Third Edition

zinc oxide surface as the zinc layer becomes thinner The white zinc oxide has substantial water solubility and washes away in rain The zinc surface is often painted to give protection against these losses

101.1.1.2 Flame/Plasma Spray

Although all the flame and plasma spray processes project liquid metal droplets through the air to the substrate, each starts with a solid metal wire or solid molten powder The solid is taken into a device which heats it to the molten state, breaks it into microscopic droplets, and propels the droplets at the substrate Longo edited a reference book on this topic.1

The “flame” or “thermal” spray uses a modified oxyacetylene torch as the heat source After the flame has been adjusted to its hottest, additional compressed air is blown into the flame The wire or powder

is then fed in via a funnel, and the blast of liquid metal particles is pointed at the substrate The “plasma” spray uses an electric arc as the heat source to melt the wire or feed powder into the compressed airstream The distance between the molten metal’s origin and the substrate will determine the type of coating obtained The close approach of the nozzle means that most particles will hit the substrate surface as a liquid Many will stick, and transfer their heat to the substrate, but some particles will bounce off If the nozzle is further away, the smaller liquid particles will solidify and bounce off the substrate, while the larger particles will still stick If the nozzle is too far from the substrate, none of the particles will stick because they will have cooled too much Losses from bounce-off or from premature cooling will be on the order of 25% of the weight of the metal sprayed

The metals most commonly sprayed include copper, zinc, iron, and aluminum Alloys and even metal oxides can also be sprayed, provided there is enough heat in the flame to make the particle soften Since the sprayed metal is of lower density than the solid metal, there is occluded air, and even porosity However, adhesion to most substrates is good, and is often assured by cleaning and roughening the surface (e.g., sandblasting) The inorganic coating may be built up enough to be machined, and may be strong enough to be a bearing surface

101.1.1.3 Other Liquid Metal Coatings

Any solid can be dipped into a molten metal

101.1.2 Solid Metal Processes

101.1.2.1 Sherrodizing

An item of steel or iron is put in a drum with powdered zinc and steel balls, and the drum is rotated The zinc powder is essentially hammered onto the surface of the steel/iron item as individual spots The length of time in the rotating drum, the amount of zinc powder, the number of steel balls, and the number of items to be treated all govern the actual amount of zinc that ends up on the surface of the item to be treated The zinc coating on the iron/steel acts as a corrosion (rust) inhibitive protection for the item

101.1.2.2 “Detaclad” Process

The construction of metal laminates, such as the coinage products now used by the U.S Treasury, is a simple process involving a layer of metal applied to another metal surface by the force of an explosion

on one surface If silver is needed as an outer surface over copper, the explosive is on the side away from

101.1.3 Vapor Processes

101.1.3.1 Vacuum Evaporation

The simplest of the vapor processes is vacuum evaporation The items to be coated are put on racks that circle a central set of trays formed from electrical resistance heaters A bell jar is lowered over the whole array, and sealed to be pumped down to 1 mm Hg of vacuum The resistance heaters are fired off to evaporate the metal powder or slug in the tray, and the individual atoms of metal fly off in a straight

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the copper in the setup, as shown in Figure 101.1

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101.1.4 Plating

101.1.4.1 Electroplating

A good place to start the investigation of plating processes is an annual handbook supplied by a trade magazine publisher.16 Plating on plastics has been reviewed by Saubestre17 and by Muller and associ-ates.18 Springer and associates19 described details of polymer treatments20,21 and morphology22 as impor-tant in electroplating polymer surfaces Safrenek23 detailed the properties of electroplated metals and alloys A zinc–nickel alloy electroplated onto steel is reputed to have better corrosion properties than galvanized steel.24

101.1.4.2 Electroless Plating

The handbook cited earlier is a good reference for electroless plating as well.16 The idea is simply to put

a metal solution on a surface, and let it deposit the zero valent metal on the surface because of an added chemical It turns out not to be quite so simple, because addition of the reducing agent to a solution does not guarantee that the metal will deposit on the surface, and it does not mean that the deposit will adhere if deposited Hence, there are sequences of “washes” and “activators” that prepare the substrate

to accept the final plating formulation In the case of plating on plastics, several oxidative techniques (chromic acid solutions, plasma etching, nitric acid washes, etc.) are used to prepare a surface One process uses three deposition steps, with clean water rinses between, to end up with copper on plastic The three solutions are tin chloride, palladium chloride, and a formulated copper solution that has reducing agent along with rate modifier and surface modifier chemicals as well Rigorous rinsing between the metal solutions is needed to assure that the last solution does not become a slurry of colloidal copper, because of “dragout” or contamination by preceding metal ions The electroless plating deposits are continuous, conductive, and bright, but are thin — micrometers in thickness

101.2 Coating on Metals

Coatings for metals may be divided into two classes by main function: those that are decorative, and those that are protective That is not to say that a protective coating cannot be decorative, but that the coating’s main function is protective, while it is chosen to be decorative as an additional option

101.2.1 Decoration

The decorative aspect of coatings lies in several features that are dealt with elsewhere in this book Among those aspects are color, gloss/flatness, and texture Each entails specific approaches the formulator uses

to attain the desired appearance, mainly involving choices of vehicle (the adhesive that sticks the pigment particles together and then to a substrate), pigments, the ratio of pigments to vehicle, and (occasionally) the additive that confers a certain property when pigment or vehicle cannot

Yet another aspect of decoration is pattern If and when there are multiple colors on a surface, the shape described at the color interfaces can be important The camouflage paints strive to have what appear to be random splotches, because a regular geometric pattern (especially a straight line) attracts

smears, and lines simulating natural mineral or rock formation surfaces And the pattern is a main point

in signage or artwork

101.2.2 Protection

The metal substrate is generally thermodynamically unstable and is easily converted to the more ther-modynamically stable oxide However, the protection provided to the metal substrate is often designed

to guard against sorts of attack other than chemical oxidation Mechanical damage may aid the oxidation,

by providing sites for the oxidant to work Electrical exposure or damage can ease the chemical degra-dation So, work on protecting a metal substrate must include consideration of the insults or attacks to

the eye to a potential target The multicolor paints (described in Chapter 87) have the dots, splotches,

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Metal Coatings 101-5

be expected In the instance of corrosion, the protection may aim to prohibit contact of the coating with oxygen or water — each a necessary element of the corrosion reaction The aim may also be to prohibit contact with corrosion catalysts — salts, acids or alkalis Schweitzer25 reviewed corrosion and protective schemes, and one unit of the educational booklets from the Federation of Societies for Coating Technology deals with corrosion protection26a while another covers corrosion and surface protection for painting.26b Wilmhurst22 described aqueous maintenance paints for corrosion protection in Australia, while Campbell and Flynn27 did so for the United States and Poluzzi28 for Italy A Golden Gate Society for Coatings Technology Technical Committee study29 showed the waterborne systems have come to equal the solvent-borne coatings for corrosion protection in aggressive environments (i.e., the Golden Gate Bridge)

At the National Coatings Center, we divide a “pitchfork” diagram (Figure 101.2) to describe the three major corrosion protection schemes The main emphasis was on improving ways to give corrosion protection through combinations of some of the corrosion protection schemes

Examples of the barrier coatings are fairly straightforward As noted earlier, the zinc in galvanizing is initially a barrier, and any impervious coating is a barrier, be it wax, asphalt, coal tar, polyethylene, or whatever There are selective barriers that aim to protect against corrosion by blocking a specific corrosive element The wax, polyethylene, coal tar, or asphalt is specifically aimed at prevention of water permeation

to the metal surface, as water is a specific corrosive substance Other coatings (PVDC, Barex-type nitriles, etc.) aim to prohibit oxygen permeation In both cases, the coatings are diffusion barriers, and the key

is to have a coating that dissolves as little as possible of the permeant within the coating, and also inhibits permeation A highly polar coating material is always more water permeable than the nonpolar material, because water dissolves so well in the polar material that the polar material acts as a pipeline rather than

a barrier It is a chuckle to hear of silicones or acrylates described as waterproofings, but you have to understand the implied statement that they protect against liquid water, while water vapor goes through them 100 to 1000 times faster than it would go through a hydrocarbon barrier Munger30 reviewed protective coatings for corrosion prevention

The electrochemical protection schemes are bound up in converting the iron or steel into a cathode, since the corrosion reaction is an anodic oxidation of the zero-valent iron metal to ionic forms (usually ferrous) The easiest way is to simply contact the metal with something that is oxidized more easily (zinc, magnesium, aluminum, etc.) and let the iron be the cathode while the other metal is corroded as the sacrificial anode Indeed, there is a substantial market in bars of magnesium or zinc that are attached to iron (pipelines, underground tankage) to act as sacrificial anodes We already noted that galvanized iron with the zinc surface damaged to penetrate to the iron is still electrochemically protecting the iron The

“zinc-rich” paints (having about 85% by weight of zinc metal powder and 15% binder) are also cathodic protectors of steel But the sacrificial anode does send its corrosion products into the surroundings In

a tidal area, the sacrificial anode specified by some regulatory agencies feeds metal ions into the under-ground water while it is protecting the underunder-ground storage tank

FIGURE 101.2 Corrosion protection schemes.

Corrosion Protection

Inhibition

Electrochemical Inhibition

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An alternative in cathodic protection is to impress a current onto the potentially corroding metal to make it cathodic Here a battery, or a step-down transformer is used with another power source This technique is used on shipboard for iron hulled vessels, and the battery can be recharged when needed Some pipelines are cathodically protected with impressed current The advantage is that there are no ions lost to the surroundings, hence subsequent contamination of water

The chemical inhibitors are many and varied items of commerce and may work in many ways Most are sold because they have been shown to work, though no mechanism is proposed Some of the surfactant-type materials (oleyl sarcosine, for instance) may only add a physical barrier by adsorbing to the surface and blocking approach of oxygen, water, or catalytic ion Other materials may adsorb on the metal and act as a pH modifier or buffer, as an amine would inhibit acid-catalyzed attack Some inhibitors modify the electromotive potential at which corrosion occurs and are said to have “passivated” the surface Most of the chemical inhibitors are low molecular weight compounds and can be washed away or otherwise rendered ineffective by chemical attacks or reactions They are effective over short periods of time, and can be stabilized against erosions by formulation to some degree For instance, there are oils into which corrosion inhibitors are formulated

There are corrosion-inhibitive pigments Seldom is there discussion of the mechanism by which these chemical inhibitive pigments work, and such characterization could extend their utility It has been shown and corrosion studies by the technical committees of the Northwest, New England, and Golden Gate Societies for Coatings Technology) Indeed, Nadim Ghanem (American University of Cairo) told of a basic lead carbonate formulation study in which the solvent-borne vinyl binder gave no corrosion protection, while a waterborne vinyl gave excellent protection His hypothesis was that the coating needs

to be permeable to water to get the lead ions to migrate to where they need to be to act as corrosion protectors That may have also been the message in the Los Angeles Society for Coatings Technology work with aminosilane-treated talcs in latex formulations Though many demonstrations of the effec-tiveness of the corrosion-inhibitive pigments exist, the mechanisms should be more thoroughly described

to aid the formulator

References

1 F N Longo, Ed., Thermal Spray Coatings: New Materials, Processes and Applications, American Society for Metals Conference Book Metals Park, OH: ASM, 1985

2 R T Sorg, Prod Finish., July, 72 (1978)

3 Anon., Ind Finish., January, 24 (1978)

4 British Patent 1,369,056; Chem Abstr., 83, 116382u

5 M Rogers, Plast Technol., November, 20 (1982)

6 R H Hochman et al., Eds., Ion Implantation and Plasma Assisted Processes, American Society for Metals Conference Book Metals Park, OH: ASM, 1988

7 Fujitsu Ltd., Japanese Patent 8,089,833 December 28, 1978; Chem Abstr 5 39579m

8 A K Sharma et al., J Appl Polym Sci., 26, 2197 (1981)

9 R Athey et al., J Coatings Technol., 57(726), 71 (July 1985)

10 A Morikana and Y Asano, J Appl Polym Sci., 27, 2139 (1982)

11 C Arnold, Jr., et al., J Appl Polym Sci., 27, 821 (1982)

12 M R Havens et al., J Appl Polym Sci 22(10), 2793 (1978)

13 D T Clark and M Z Abraham, J Polym Sci., Polym Chem Ed 19, 2129 (1981)

14 N Inagaki et al., J Polym Sci., Polym Lett Ed., 19, 335 (1981)

15 H A Beale, Ind Res Dev., July, 135 (1981)

16 M Murphy, Ed., Metal Finishing Handbook, Hackensack, NJ

17 E B Saubestre, in Modern Electroplating,3rd ed., F Lowenheim, Ed New York; Wiley-Interscience, Chapter 28, 1974

18 G Muller et al., Plating on Plastics Surrey, England: Portcullis Press

that nominally corrosion-inhibitive pigments do not work in some formulations (see SSPC literature

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Metal Coatings 101-7

19 J Springer et al., Angew Makromol Chem., 89, 81 (1980)

22 A G Wilmhurst, Aust Oil Colour Chem Assoc., November, 12 (1978)

23 W H Safranek, The Properties of Electrodeposited Metals and Alloys American Electroplaters and Surface Finishers Society, 1986

24 S A Watson, Nickel 4(1), 8 (September 1988)

25 P A Schweitzer, Ed., Corrosion and Corrosion Protection Handbook New York, Dekker, 1983

26 (a) Unit 27, Anticorrosive Barriers and Inhibitive Primers Philadelphia: Federation of Societies for Coating Technology, 1979 (b) Unit 26, Corrosion and the Preparation of Metallic Surfaces for Painting Philadelphia: FSCT, 1979

27 D Campbell and R W Flynn, Am Paint Coatings J., March 6, 55 (1978)

28 A Poluzzi, 14th Annual FATIPEC Congress Proceedings, 1978, p 61

29 R Athey et al., J Coatings Technol., 57(726), 71 (July 1985)

30 C H Munger, Corrosion Prevention by Protective Coatings National Association Engineering, 1984

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102 Corrosion and Its Control by Coatings

102.1 Introduction 102-1

102.2 Coatings 102-6 References 102-9

102.1 Introduction

102.1.1 Energy Transfer

The metallic state is in most metals an unstable condition resulting from the smelting operation, in which energy is imported by the ore as the metal is derived After extraction, most metals undergo a slow deterioration process during which they shed this energy and return to a more stable condition in which they are combined with some element of their environment, such as an oxide, a sulfide, or some other corrosion product This energy conversion process is known as corrosion

102.1.2 The Electrochemical Nature of Corrosion

Corrosion is most usually driven by some electrochemical inhomogeneity in the metal or its environment

In this process, different areas of the metal, having different levels of free energy and therefore different corrosion potentials, become the electrodes of an electrochemical cell in contact with a common electro-lyte.1 − 5 The electrochemical couples are set up with areas of more active electrochemical potential acting the anodes as metal dissolves into the electrolyte as ions, so releasing electrons, which pass through the metal to the adjacent cathode areas where they react with the environment This flow of electricity, the electron passage from anode to cathode, and the accompanying charge transfer back through the elec-trolyte from cathode to anode, make up the corrosion current The rate of the current flow, i.e., the magnitude of the corrosion current (I) that develops, is a measure of the amount of degradation and is related to the potential difference (V) between the anodic and cathodic sites by Ohm’s law:

(102.1) where R is the total resistance of the cell

I V R

=

Clive H Hare

Coating System Design, Inc.

Energy Transfer • The Electrochemical Nature of Corrosion

Depassivation • Area Effects Corrosion Control by Coatings • Barrier Coatings • Inhibitive

• Electrode Reactions • Polarization • Electrode Film Breakdown and Depolarization • Passivation and

Coatings • Zinc-Rich Coatings

as anodes of the cell, while more passive areas act as cathodes (Figure 102.1) Corrosion takes place at

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Corrosion and Its Control by Coatings 102-3

where n is the valency of the metal

In the case of iron, this equation becomes

The exact nature of the reaction at the surface of the cathode (in which electrons released in anodic dissolution are, in turn, consumed) depends upon the nature of the environment Under neutral and alkaline conditions, the reaction involves oxygen and proceeds

Under acidic conditions, if oxygen is present, the reaction may proceed

Where oxygen is not available, hydrogen gas may form under acidic conditions:

Migration of the oxidative product (Mn+) from the anode and the reduction product (OH–) from the cathode occurs until they combine to form the oxide, which precipitates In the case of steel this may be Fe(OH)2, ferrous hydroxide, or, depending upon the nature of the environment, one of several precursor products, such as ferrous hydroxy chloride in salt water Ferrous products are readily soluble, and this favors migration, so that oxide formations are not intimately associated with the anode but are loosely adherent and porous Given sufficient oxygen, a second oxidation reaction will occur in steel corrosion, which converts the divalent ion to the trivalent ferric state,

The solubility of the trivalent corrosion product is much less than that of the ferrous product Under normal circumstances, however, where the secondary oxidative process occurs gradually after the ferrous ions have migrated away from the anode, the corrosion product is no more tightly adherent than is the ferrous product from which it is formed, and films of rust, hydrated ferric oxide (Fe2O3 × Η2Ο), are usually loose and crumbly

TABLE 102.1 Electrode Potentials Electrode Reaction Potential (volts)

Active

Noble (Passive)

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103 Marine Coatings

Industry

Bibliography 103-2

Marine coatings are special-purpose coatings that are supplied to the shipbuilding and repair, offshore, and pleasure craft markets The products used are diverse and unique and are formulated for severe climatic and immersion conditions

As a result of these conditions, the coatings used must have maximum resistance properties to salt spray, constant seawater, and in the case of tankers, a broad range of chemicals For these reasons, a substantial volume of products sold today are two-component epoxy primers, intermediate high builds, and tank linings

Above-the-waterline finishes are still predominantly single-package alkyds or acrylics on commercial ships and offshore platforms This is due to the subsequent ease of maintenance required Similarly, single-pack alkyds, urethane-alkyds, and silicone-alkyds are predominant in the pleasure craft market,

at least for hulls up to the 30- to 35-foot class

Larger pleasure crafts are still painted with the single-pack finishes, but many such craft (yachts) are coated today with two-part aliphatic polyurethanes to achieve the best in gloss and gloss retention, abrasion resistance, and long-term durability

The use of two-component products, whether applied to a ship’s tanks or a yacht’s topside, requires more professional applicators to achieve the best result Such applicators must be familiar with multiple spray application equipment from the simple siphon cup to the sophisticated twin-feed heated airless spray Whether coating deep-sea ships, offshore platforms, or pleasure craft, one unique characteristic of the marine coatings industry is the need to protect the underwater surfaces from the attachment and growth

of marine fouling organisms These are living animals, algae or slime, that will adhere, colonize, and grow rapidly if not controlled through the use of antifouling coatings

Antifouling paints are unique to this industry and make up approximately 50% of the total volume

of coatings used By their nature, in order to mitigate fouling attachment, antifouling paints contain biocides, which are registered with the U.S Environmental Protection Agency (EPA) as pesticides under the Federal Insecticide, Fungicide and Rodenticide Act

Subsequently, all antifouling paints must be both federally registered with the EPA and registered with the state EPA in which they are sold

This unique class of product is expensive to develop, test, and register and thus is expensive for the customer Most antifouling paints contain rosin (gum or wood) as part of the vehicle and a copper compound−cuprous oxide being the most common — as the biocide

Some antifoulings are based on organotin-copolymer resins, which are biocidally active polymers along with a copper compound These are generally the best-in-class for complete fouling control

Jack Hickey

International Paint Company

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104 Decorative Surface Protection Products

104.1 Introduction 104-1 104.2 History 104-1 104.3 Products 104-2

104.4 Process of Manufacturing 104-5

104.5 Applications 104-7 References 104-8

104.1 Introduction

Decorative surface protection products, as the term suggests, are products that provide both decoration and protection to a wide variety of surfaces These products are available to consumers in a variety of colors, decorative designs, and surface finishes They are attractive, durable, washable, waterproof, and resistant to stains from food, beverages, and common household items They must be conformable to a wide variety of surfaces, requiring no tools, water, or paste to be applied to any plain surface

In most cases, they have informative and instructional carriers traditionally known as printed release liners, which are affixed by a flexible pressure-sensitive adhesive to a base sheet, also known as a primary substrate or a face stock In some cases, the release liner has been eliminated by using an embossed primary substrate Alternatively, the primary substrate may have been coated on one side with a release coating and on the other side with a low tack adhesive.1 These products are known as self-wound adhesive coated decorative sheets They are made in solid colors, decorative prints, and textured woodgrains

104.2 History

Pressure-sensitive adhesive usage in making decorative surface protection products goes back to the early 1950s Similar types of vinyl film were used by consumers on products such as printed and laminated tablecloths and printed draperies The idea of printed film led to the making of decorative surface protection products by a low tack, pressure-sensitive adhesive on printed film

Jaykumar (Jay) J Shah

Decora

Primary Substrate or Base Sheet • Pigmented Coating for Pressure-Sensitive Adhesives

Application of Release Coatings • Adhesive Coating Decorative Printing • Release Paper • Release Coatings •

Application Process • Finishing

The construction of decorative products is shown in Figure 104.1

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