Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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A number of methods are available for the production of metallic coat-

ings.

The most widely used are either electro-plating or dipping the articles

to be coated into a bath of the molten metal. Other methods involve

spraying or volatilising the protective metal on to the surface whilst mech-

anical 'cladding' is also used.

21.81 'Cladding' is used chiefly in the production of 'clad' sheet and

is employed prior to the final manufacturing stages of a component. The

base metal is sandwiched between pieces of the coating metal and the

sandwich is then rolled to the required thickness. In some cases the film

of protective metal can be sprayed on to the base surface and then rolled

on. 'Alclad' is duralumin coated with pure aluminium (up to 0.01 mm thick

depending upon the overall thickness of the stock) but other high-strength

alloys of aluminium, which have poor corrosion resistance because of their

multi-phase microstructures, are also clad. Mild steel clad with stainless

steel is also available but it is important to note that with all forms of

cladding damage to the protective skin can take place during fabrication.

21.82 Hot-dip Metal Coating Tin and zinc are the metals most often

used to produce metallic coatings in this manner, though use is also made

of aluminium. Ideally the coating should adhere to the base surface by

solid solution between the two. In some cases however, brittle intermetallic

compounds are formed at or near the interface such as FeSn2, FeSn, or

Sn in the case of hot-dip tinning.

(a) Hot-dip Tinning. Tinplate is principally used in the manufacture of

cans for packaging a wide variety of foodstuffs. The non-toxic properties

of tin, combined with its good corrosion-resistance and the ease with which

tin-coated articles can be soldered, make it a useful metal for coating

articles which come into contact with all types of food.

The mild-steel sheet used for the manufacture of tinplate is usually about

0.25 mm thick, and, after being annealed, it is freed from all traces of

oxide by pickling in either dilute sulphuric acid or dilute hydrochloric acid.

The tinning unit comprises a thermostatically controlled vessel of molten

tin in which is submerged a system of guides and rollers for conducting the

pickled sheets down through a layer of molten flux into the tin and then

upwards and out of the tin through a layer of palm oil. As they pass

through the palm oil, the sheets are subjected to a squeegeeing action by

tinned-steel rollers, which serve to control the final thickness of coating on

the sheets.

After leaving the tinning machine, the tinned sheets are cooled and then

cleaned, usually by washing them in an alkaline detergent. Finally, they

are polished in either bran or wood meal.

Other iron and steel items of kitchenware such as mincing machines and

mixers, are coated by hot-dipping after fabrication. After being cleaned,

degreased and pickled, the component is immersed in a pot of molten tin,

on the surface of which is a layer of flux. If the operations have been

correctly carried out a brilliant finish is obtained which requires little in

the way of after-work.

(b) Hot-dip Galvanising. The term galvanising is derived from the name

of Luigi Galvani, the Italian physiologist, whose classic observations of

dead frogs, in 1786, led to Volta's development of the electric cell. In 1829

Faraday observed that when zinc and iron, in contact with each other,

were exposed to the air in the presence of a salt solution the iron did not

rust; whereas if the zinc were removed the iron rusted rapidly. Faraday

attributed this to 'the effect of chemical action ... in exciting electricity'.

In 1836 a Frenchman, Sorel, took out a patent for a process of coatmg

steel with zinc by hot-dipping, and named the process galvanising. The

word was used to indicate the sacrificial electro-chemical protection

afforded iron by the zinc.

Before being galvanised, the surface of steel must be thoroughly cleaned,

either by shot blasting or, more generally, by pickling in dilute sulphuric

acid. In order to prevent contamination of the molten zinc by iron, the

work is then washed to remove all traces of iron salts formed by pickling.

It is necessary, however, to protect the surface of the work from fresh

oxidation, so it is immersed in a flux bath consisting of a solution of zine

chloride and ammonium chloride. This flux is usually dried on to the sur-

face in an oven which is provided with an adequate draught and working

at a low temperature, to prevent interaction between the flux and the iron

surface.

The work, with its protective flux coating, is then immersed in the molten

zinc bath. The flux coating peels away to form a layer on the surrounding

bath surface, which in turn is protected from oxidation; and the surface of

the work is left clean so that it is immediately wetted by the molten zinc.

The thickness of the coating produced depends largely upon the tempera-

ture of the bath and the'rate at which the work is withdrawn, but good-

quality galvanised articles should carry between 0.3 and 0.6 kg/m

of zinc

on the surface. Thinner and more flexible coatings are obtained by using

a zinc bath containing about 0.2% aluminium. This limits the formation of

intermetallic compounds of iron and zinc on the steel surface.

In addition to corrugated iron for roofing, the large number of galvanised

products includes buckets, barbed wire, water tanks, marine fittings, bolts,

and fencing fittings, agricultural equipment and window frames. Many

galvanised products such as dust bins have been replaced by plastics mould-

ings whilst aluminium is becoming increasingly popular for the manufac-

ture of window frames.

tion of iron and steel surfaces under a wide variety of trade names, the

best-known of which is 'Aludip'. In all of these processes the surface must

be cleaned by grit blasting, de-greasing and/or pickling. It is then carefully

dried before immersion in a bath of molten aluminium at about 700

generally through a layer of molten flux. In some processes steel sheet is

heated in an atmosphere of hydrogen at 1000

C prior to dipping in order

to reduce any oxide films present, but in addition to the extra cost of this

treatment the quality of the product is often less satisfactory.

In some methods, particularly in the USA, the aluminium-dipped mild-

steel sheet is then rolled to give a better surface finish. This product is used

to replace tinplate for some containers.

(d) Hot-dip Coating with ZinclAluminium Alloys. These relatively new

commercial processes are said to give a much more corrosion-resistant

coating on steel sheet, strip, wire and tube than is obtained by conventional

hot-dip galvanising. The high-aluminium coatings Galvalume and

Zincalume use an alloy containing 55Al-45Zn and small amounts of sili-

con; whilst Galfan coatings are based on 95Zn-5Al (with very small

additions of 'rare earth' metals). Though Galfan coating is less corrosion

resistant than those of the 55Al-45Zn alloys it has better formability and

retains paint more effectively.

21.83 Coating by Means of a Spray of Molten Metal Metal spraying

consists in projecting so-called 'atomised' particles of molten metal from

a special pistol on to a suitably prepared surface. Surface preparation

usually involves blasting the surface with an abrasive; steel grit having

replaced sharp silica sand for this purpose because of health hazards

involved when the latter is used. It is essential that the surface is rough-

ened, rather than peened or polished by the grit in order that good

adhesion shall take place. Hence attention to the condition of the grit is

important. The metals most commonly used for spraying are zinc and

aluminium, though coatings of tin, lead, cadmium, copper, silver and stain-

less steel can be so deposited.

Early metal spraying processes, originally invented by Dr. Schoop, used

an oxyacetylene flame to carry particles of metal on to the work surface.

The metal was injected into the flame as a powder. Later modifications of

the Schoop process used electrical melting of the metal. An arc was struck

between two zinc wires in the pistol and the molten zinc so produced

was 'atomised' and carried forward in a blast of compressed air. Flame

impingement methods are now the most common, using a specially

designed flame gun in conjunction with either a continuous rod or a hopper

feeding powder to the high-velocity flame. The temperature is generally

sufficient to melt the metal, but not necessarily so, and in either case the

small particles are projected with high energy at the work surface.

The most important factor is rigid control of the flame conditions. If

too high a temperature or too oxidising flame conditions prevail then the

particles may be oxidised before they reach the work surface. Nevertheless

it is a satisfactory method for structural steel provided that flame conditions

are controlled.

Metal spraying has wide application in view of its portability and flexi-

bility; thus, large structures, such as storage tanks, pylons and bridges, can

be sprayed on site. Notable recent examples include the Forth Road Bridge

and the Volta River Bridge (Ghana), both of which were zinc coated using

modern developments of the Schoop process.

21.84 Sherardising is a cementation process, similar in many respects

to carburising, in that zinc is made to combine with an iron or steel surface

by heating the work with zinc dust at a temperature below the melting

point of zinc. The first patent for this process was taken out in London in

1901 by Sherard Cowper-Coles, after whom the process is named. Much

thinner coatings can be obtained by sherardising than are possible with

hot-dip galvanising, and generally about 0.15 kg/m

of zinc is used. More-

over, a more even film is obtained, thus making possible the treatment

of such articles as nuts and bolts, the threaded portions of which would

undoubtedly become clogged by hot-dipping.

As in all other coating processes, the surface of the work must first

be properly prepared. This usually involves degreasing, followed by acid

pickling in cold 50% hydrochloric acid or in hot 10% sulphuric acid. Shot

blasting is also used, particularly in the case of iron castings, to remove

graphite and core sand.

The work is packed into mild-steel drums along with some zinc

powder. The drums are heated to 370

C and rotated slowly so that the

tumbling action brings all the components into contact with the powdered

zinc.

'Calorising' is a process in which steel components are coated with alu-

minium by a similar method. The components are treated in a heated

rotating cylinder containing a mixture of aluminium and aluminium oxide

powders. It is used principally to provide a protective coating for heat-

treatment pots, ladles and furnace components. Nickel alloys may be pro-

tected from attack by sulphur gases by aluminising them. The components

are packed into a heat-resistant box along with aluminium powder. Air

is then purged from the box by flushing it with argon or nitrogen. The

tightly-sealed box is then heated for several hours to promote aluminisation

of the surface. 'Chromising' is a similar process involving chromium coat-

ing of a mild steel surface in which corrosion resistance comparable with

that of a 12% chromium steel can be achieved.

21.85 Electro-plating The formation of metal coatings by electro-

deposition is well known, and a wide variety of metals can be thus used,

including copper, nickel, chromium, cadmium, gold and silver. Tin and

zinc can also be electro-deposited, and a coating thus formed has advan-

tages over one produced by hot-dipping in respect of flexibility, uniformity

and control of thickness of film.

The usual degreasing and pickling processes must be applied to the

surface before any attempt is made at electro-plating it. Most instances of

peeling and blistering in the case of chromium-plating are due to inad-

equate preparation of the surface. The author has seen specimens in which

the layer of chromium had been deposited on to oxide scale, with the

inevitable peeling of the chromium as a result.

In the actual process of electro-plating the article to be plated is made

the cathode in an electrolytic cell. Sometimes the metal to be deposited is

contained, as a soluble salt, in the electrolyte, in which case the anode is

a non-reactive conductor, such as stainless steel, lead or carbon. In most

cases,

however, the anode consists of a plate of the pure metal which is

being deposited, whilst the electrolyte will contain a salt or salts of the

same metal. Then, the anode gradually dissolves and maintains the concen-

tration of the metal in the electrolyte as it is deposited on to the articles

forming the cathode.

The conditions under which deposition takes place are very important,

so that the cell voltage, the current density (measured in amperes per

square metre of cathode surface), the ratio of anode area to cathode area

and the time of deposition, as well as the composition and temperature of

the electrolyte, must all be strictly controlled if a uniform adherent and

nonporous film is to be obtained.

21.86 Vacuum Coating and 'Ion Plating' In vacuum-coating pro-

cesses the work pieces are contained in a vessel from which air is evacuated.

The coating metal is electrically heated within the vessel so that, because

of the near vacuum conditions, it vaporises at low temperatures and will

then condense on any cold surfaces with which it makes contact. Head-light

reflectors and mirrors of all types are coated in this way.

With both vacuum coating and electroplating complex shapes are diffi-

cult to coat uniformly and to overcome this a technique called ion plating'

was introduced. Here the article to be plated is made the cathode in a

vessel containing low-pressure argon (Fig. 21.17). 'Glow discharge' occurs

in the region of the cathode due to ionisation of the argon and the Ar

ions are attracted to the cathode, bombarding it and having a cleaning

effect on the surface. This is known as 'sputter cleaning' or 'ion scrubbing'.

Metal atoms are present as vapour emitted from the tungsten foil 'boat'

and about 1% of these ionise. These M

ions are immediately accelerated

towards the work-piece comprising the cathode with which they collide.

The kinetic energy of the ions is dissipated as heat so that good adhesion

is provided because of the tendency of the plating ions to diffuse into the

substrate. By adequate control of the evaporation source alloy coatings

can be applied as well as pure metals.

Fig.

21.17 The principles of 'ion-plating'.

Protection by Oxide Coatings

21.90 In some instances the film of oxide which forms on the surface of

a metal is very dense and closely adherent. It will then protect the metal

surface beneath from oxidation. Stainless steels owe their resistance to

corrosion to the presence of a high proportion of chromium, which is one

working

vessel

(containing

argon at

2N/m*)

insulation

high

voltage

d.c.

glow

discharge

component

vacuum pump

coating metal

tungsten

foil 'boat

high-current

power supply

of these elements that form oxide films impervious to oxygen. The blueing

of ordinary carbon steel by heating it in air produces an oxide film of such

a nature that it affords partial protection from corrosion.

21.91 Anodising Reference has already been made (21.21) to the

protection afforded aluminium by the natural film of oxide which forms

on its surface. Anodic oxidation, or anodising, is an electrolytic process

for thickening this oxide film. This process may be applied for several

reasons, such as to provide a key for painting, to provide an insulating

coating for an electrical conductor or to provide a surface which may be

dyed, as well as to increase the resistance of aluminium to corrosion.

Before being anodised the surface of the article must be chemically

clean. Preliminary treatment involves sand-blasting, scratch brushing or

barrel polishing, according to the nature of the component. This is followed

by degreasing in either the liquid or vapour of trichlorethylene; or by

electrolytic cleaning. The latter process is used mainly for highly polished

surfaces. The article is made the cathode in an electrolytic bath comprising

a 1% solution of sodium hydroxide, and treatment is continued for about

half a minute at a voltage of 14 and a current density of 550 A/m

. The

mechanism of this cleaning process is largely one of flotation of the film of

grease by hydrogen bubbles given off at the surface of the article which

comprises the cathode.

In the actual anodising operation which follows, the aluminium article

to be treated is made the anode in an electrolyte containing either chromic,

sulphuric or oxalic acid; the cathode being a plate of lead or stainless steel.

When an electric current is passed, oxygen is formed at the anode and

immediately combines with the aluminium surface of the article. The layer

of oxide thus formed grows outwards from the surface of the aluminium.

The normal thickness of a satisfactory anodic film produced commercially

varies between 0.007 and 0.015 mm, and a film having a thickness within

these limits would be formed by anodising a component in a 15% sulphuric

acid solution at 20

C for about thirty minutes, using a current density of

about 100 A/m

at a cell voltage of 15. A longer period of treatment

produces a thicker, but soft and spongy, film which would be unsatisfactory

in service. The thickness of the natural film produced on an aluminium

surface by exposure to air at normal temperatures is of the order of

0.000 013 mm.

The surface produced by anodising can be dyed by immersion in either

hot or cold baths of

dyestuff,

but whether or not the film is dyed, it should

always be sealed. The anodic coating is somewhat porous as it leaves

the electrolytic tank and is easily stained, but sealing renders the coating

impermeable. A simple, but quite effective, sealing process consists in

treatment with hot or boiling water, which renders the coating non-

absorptive without any visible change in appearance. Other sealing pro-

cesses include treatment in solutions of 1% nickel or cobalt acetates at

100

followed by boiling in water, the application of linseed oil or lanolin

to the surface or the application, while the article is warm, of a mixture

containing equal parts of turpentine, oleic acid and stearic acid.

21.92 Beryllium Oxide Coatings The addition of small amounts of

beryllium to aluminium-base alloys causes a coating of beryllium oxide,

BeO,

to form on the surface. This is even more protective than the natural

skin of aluminium oxide, Al

Ch, described above. Unfortunately beryllium

is an extremely expensive metal. Nevertheless 0.1-0.3% beryllium is added

to some high-quality aluminium alloy castings for aerospace purposes.

In addition to providing the protective BeO skin beryllium acts as a

'scavenger' of oxygen and nitrogen in the melt. It also refines the structure

of any iron-based intermetallic compounds which may be present, replac-

ing the coarse needles with small equi-axed crystals.

Protection

Other Non-metallic Coatings

21.100 Coatings of this type usually offer only a limited protection against

corrosion and are, more often than not, used only as a base for painting.

21.101 Phosphating A number of commercial processes fall under

this heading, but in all of them a coating of phosphate is produced on the

surface of steel or zinc-base alloys by treating them in or with a solution

containing phosphoric acid and, generally, a metallic phosphate. The phos-

phate film which forms on the surface is usually grey in colour, about

0.0005 mm thick and offers only limited protection against corrosion but

since its surface is rough it provides an excellent 'key' for paint, varnish or

lacquer.

In automobile body work it also limits any filiform or scab corrosion

resulting from stone chipping by thus preventing any 'crevice corrosion'

(21.51) beneath the paint film.

In the automobile industry the spot-welded car body (termed the 'body

in white' at this stage) is usually given a preliminary degreasing treatment.

The phosphate coating is then achieved by using a phosphoric acid spray

in order that the hydrogen so liberated is flushed away:

Fe 4- 2H

-> Fe(H

)

+ H

The reaction will then proceed to completion:

3Fe(H

)

-> Fe

(PO

)

+ 4H

Iron phosphate, Fe

(PO

)

, forms as an insoluble grey adherent crystalline

coating on the surface being treated.

A number of proprietary processes based on phosphating have been

introduced since 1903. Some readers may have used the preparation Kurust

which is available for the small-scale treatment of rusted steelwork, princi-

pally the bodywork of decrepit—though not necessarily ancient—motor

cars.

21.102

Chromating Chromate coatings are produced on magnesium-

base alloys, and on zinc and its alloys, by immersing the articles in a bath

containing potassium bichromate along with various other additions. The

colour of the films varies with the bath and alloy, from yellow to grey and

black. With steel a chromate film has little permanence but it is sometimes

used following phosphating or zinc-coating of steel and as an additive to

zinc-based primer paints.

21.103 Electrophoresis This process—also known as 'electro-

painting'—is used mainly to apply undercoat paint to steel which has

already been phosphated. The chemistry of the process is fairly involved,

but briefly, the paint bath consists of colloidal particles of the appropriate

'resin' suspended in a water solution which contains an electrolyte. DC

electrodes one of which is the article being painted, are immersed in the

bath.

The resin particles, constituting the paint, acquire a charge by adsorbing

ions from the electrolyte so that they are then attracted towards the appro-

priate electrode. Sometimes the ions are provided by one of the reagents

in the paint bath. For example, if a colloid is kept in suspension by soap

solution the negatively-charged ion of the soap (the 'fatty acid' radical)

may be adsorbed by the colloid particles giving them a negative charge so

that the combined particle is attracted to the anode which will be the article

being painted.

This method produces a very uniform paint film on both flat surfaces

and sharp edges. As soon as the colloidal particle is deposited it insulates

that portion of the surface and so other charged particles then 'seek out'

bare areas which are still charged. For this reason a very complete and

uniform film can be deposited and if an ammeter is included in the circuit

it will read zero when coating is complete. The fact that the paint is carried

in a water solution reduces fire risks but at the same time the resultant film

is dense because a phenomenon known as electro-osmosis causes molecules

of the liquid to migrate away from the electrode so that the paint film

becomes denser. The process is of obvious value in the motor industry

where complete bodies can be coated by dipping into an electrophoretic

bath.

21.104 The 'Elphal' Process is similar in principle but is used to coat

steel strip with aluminium. Here finely-divided aluminium is carried in a

water/alcohol bath which also contains an electrolyte. The aluminium par-

ticles are not ions and neither this process nor electrophoresis generally

should be confused in any way with electroplating. In this case aluminium

particles become charged by attachment to ions from the electrolyte and

consequently migrate to the steel surface (electrode) where they adhere

loosely. The solvent is removed by heating and the coating then compacted

by cold rolling. Finally the aluminium film is sintered on to the surface of

the steel by heat treatment.

21.105 Surface Protection at High Temperatures The drive towards

higher efficiency and performance means that engineering environments

are becoming increasingly hostile. The first line of defence against deterio-

ration of any component is its surface. Unfortunately many materials which

are designed for strength do not offer a high resistance to corrosion and

wear, especially at high temperatures. This is a major problem in the design

of gas turbine engines, particularly in respect of the blades.

Many problems

wear

and

corrosion

in gas

turbines have been solved

by using plasma-sprayed coatings

on the

surface.

The

plasma torch

(Fig.

21.18) uses

the

energy

in a

thermally ionised

gas to

propel partially molten

fine powder particles

on to a

prepared surface

that they adhere

impact.

A DC arc is

initiated between

the

electrodes

by a

high

EMF. The

design

of the

torch

such that

the

energy involved

concentrated into

very small region

that temperatures

up to 30

000

can be

achieved.

the heated argon expands rapidly

the

particles

are

accelerated

very high

velocities

(up to 600 m/s). In

order

prevent oxidation

of the

particles

the torch

usually fitted with

inert

gas

shield.

Fig.

21.18 A

typical plasma torch.

Most

of the

coating materials

are

based

aluminium

and

chromium,

both

which form protective films

tenacious refractory oxide. Such

surface films

are

effectively self-sealing

and

since

the

ionic mobilities

(21.21)

chromium

and

aluminium within

the

film

are low,

oxidation

rates beneath

the

film

are

also very

low. The

surface tenacity

chromia/

alumina film

increased

by the

addition

small amounts

of the

'rare-

earth' metal yttrium.

These materials

are

designated MCrAlY coatings (Table

21.2)

where

either nickel

cobalt

(or a

mixture

both)

and are

used

for

turbine

corrosion protection

advanced military

and

civil aero

gas

turbines.

Table

21.2

Some UCAR* UCrAIY Alloys.

Composition

(%)

UCAR designation

Co Ni Mo Cr Al Y

LCO-7

63 — — 23 13 0.6

LCO-22

39 32 — 21 7.5 0.5

LCO-37

44 23 — 30 3 0.5

LCO-34

0.5 67 0.5 20 11 0.5

*UCAR

is a

trade mark

of the

Union Carbide Corporation

PLASMA

FLAME

COPPER ANODE

ARGON

TUNGSTEN CATHODE

COOLING

WATER

POWDER

WORKPIECE

Cathodic Protection

21.110 Steel structure will corrode more rapidly if it becomes anodic to

its environment. Thus the hull of a ship can be expected to rust more

quickly at the stern since it is anodic to the nearby manganese bronze

propellers. Moreover salt water acts as a strong electrolyte since it contains

large concentrations of Cl" as well as OH" ions. The hull can be sacrificially

protected in the region of the propellers by bolting slabs of zinc or mag-

nesium to the hull. Since these slabs are more strongly anodic than is the

steel hull towards the propellers, then they naturally replace the steel hull

as anode. The hull then assumes the role of 'external circuit' in the galvanic

system so produced, so that it transmits electrons without suffering ionis-

ation (Fig. 21.19).

21.111 Similarly an iron or steel pipe or storage tank situated below

ground can be sacrificially protected by burying a slab of zinc or magnesium

adjacent to it. Alternatively an external EMF may be used to render the

steel structure passive to its surroundings (Fig. 21.20) That is the EMF is

applied in the direction Y indicated on the Pourbaix diagram (Fig. 21.8).

The inert graphite electrode is made the anode by virtue of the applied

EMF and so the steel tank becomes cathodic to it. Obviously the value of

the EMF necessary to provide passivity will depend on the pH of the soil

as well as other factors, and the electrical output will need to be controlled

by a feed-back system which automatically adjusts the EMF (E

) required

from the transformer/rectifier system. E

can be monitored by a high-

resistance voltmeter connected to a reference electrode. Where mains elec-

tricity—and the necessary qualified maintenance back-up—are available

this system will be cheaper to operate than any electro-chemical source, ie

sacrificial anodes. This is obvious thermodynamically since the metals of

which the sacrificial anodes are made have to be deposited electrolytically

in the first place.

21.112

It is also more satisfactory to use a power-impressed EMF to

Fig.

21.19

Cathodic

pro-

tection

of a

ship's

hull.

Fig.

21.20

Cathodic protection

of a

buried steel tank

means

of an

'impressed'

EMF and a

non-consumable

graphite anode.

transformer/rectifier

d.c.

output

a c.

mains

graphite

anode

steel tank

(cathode)

soil

high-resistance

voltmeter

reference

electrode

hull

zinc slab

sea water