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The Conquest of Corrosion
Page 66
Page 79
Page 80
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By Alan Smith SMELTEP metals corrode when exposed to the atmosphere, tending to revert to the state in which they exist geologically. The need for protecting metal parts of vehicles from corrosion has always existed, hut since Britain has be.ome the world's biggest exporter of motor vehicles, its importance has been greatly magnified. Manufacturers and research chemists are now devoting keen attention to the problem of preventing corrosion of vehicles and equipment during shipment anti in service under difficult conditions overseas.
Corrosion is an electrolytic process. Oxygen and a humid atmosphere together are necessary for the deterioration of metals which are derived from oxide ores, such as iron and aluminium. Oxygen, for instance, unites with iron to form an oxide(rust) through the medium of moisture as the electrolyte.
Chromium Plating
Metals such as chromium and nickel are not so susceptible to oxidizing as iron or aluminium Chromium' and nickel exist as sulphide ores and corrode when attacked by sulphur. As the incidence of sulphur in the atmosphere is naturally not as great as thatof oxygen, corrosion of chromium • or nickel is slow. For this, reason chromium and nickel plating are often used to protect vehicle components. particularly those in positions greatly exposed to corrosive elements, such as bumper _
bars. .
It is almost impossible to prevent corrosion entirely. All that can he done is to decelerate the process so as to preserve a metal component in usable condition for as long as its estimated working life. This protection is afforded by putting some kind of coating on the metal through which the atmosphere cannot penetrate easily. Different coatingsgive different degrees of porosity and protection:
The various comings available fall into a number of different categories. Some, such as plating or anodizing, are complete finishing processes in them a32 ' selves, whilst others, like phosphating. are pie-treatments which are finished by painting, oiling or varnishing.
When metal articles are in store or transit, treatments such as these are uneconomic and what is usually done temporarily to protect them is to cover them with oil.
Another method of protection is to cover metal parts with a tough skin of plastic which may be stripped off when • the parts are required for use. Metal objects treated in this way remain protected for many years.
Two such plastic coatings are made by Croda, Ltd., Croda House, Snaith, Goole. They are Crocell, which is applied by hot dip, and Lowerite, which is applied cold.
The manufacturer states that Crocell is not cheap, compared with other corrosion preventives, but for expensive precision-worked components the high degree of protection given and the amount of packing it saves nullify the initial cost.
Articles to be protected are immersed for five seconds in Croce!! at 160-185 degrees C. The thickness of the coating can be varied according to the temperature. One pound of Crocell will coat about 2.45 sq. ft. at a thickness of 0.06 in. The film solidifies within a few seconds of removal from the dipping tank.
Lowerites 1 and 2 are based on Crocell and contain solvents which make viscous compounds suitable for application by cold dipping. Thinners added to Lowerites yield a liquid suitable for application by brush or spray.
Lowerite films are heat-resistant and. like Crocell, exude oil to metal surfaces. Articles to be protected receive a film about 0.0055-in. thick at one dipping. A film of Lowerite 1 dries in 30-40 mins. and of Lowerite 2 in 5-10 nuns.
Much progress with plastic anti-corrosive coatings has been made in the U.S.A. Warships and military aircraft have been stored co mpletely "cocooned" by plastic films applied -by spray.
Another approach to the problem of protecting packaged components is the inclusion of an inhibitor such as Shell V.P.I. 260. This is a crystalline compound which vaporizes slowly and is soluble in water and alcohol. The vapour given off forms a thin film over a metal surface, making it passive and thereby protected against corrosion.
Shell V.P.1. (vapour-phase inhibitor) may be used as a powder, a solution, or embodied in a Plastic. The most simple application, however, is to boxes or paper for packing metal articles. Impregnated papers are made by R. A. Brand and Co., Ltd., St. James's Street, London, S.W.1, and Leonard Stace, Ltd., Swindon Road, Cheltenham.
Among the advantages claimed for V.P.I. are that it avoids greasing and degreasing, it has no abrasive action on soft metals, and it is applicable where grease or -oil is unsuitable, such as for instruments and other products containing non-metallic components.
Besides the many protectives on the market, there, are also preparations which remove corrosion.. products. To some extent, these removers act as preventives, enabling metal treated with them to be stored safely.
Crystalline Coating : The action of these. preparationsis. however, slightly different from that of phosphate pre-treatments, such as Bonderizing or Walterisation. These two processes protect the metal by changing the clean surface into a metallic phosphate, leaving a crystalline product which, in the case of Walierisation, is about 0.0004 in. thick.
De-rusting agents, on the other hand, are mild phosphoric acids which combine with corrosion products to make a metallic .phosphate surface. When applied to clean tin-rusted steel, a thin iron-phosphate material is still formed. The iron content of the surface is derived from the metal—not (as distinct from the other pre-treatments mentioned) being inherent to any great extent in the applied compound. Pre ventives applied by brush convert corrosion products into phosphates. If applied by dip, the corrosion products dissolve in the liquid.
Walterisation is a British process which was developed at. the beginning of the war to take the place of cadmium plating. Among its present users are numbers of leading industrial undertakings. Articles to be treated are first degreased and cleaned and then immersed in processing solution at 96-98 degrees C.
This gives a phosphate product. The atmosphere can, however, penetrate such a crystalline surface and for greater protection the fissures in it have to be filled. This can be done with paint. but for even greater protection the Walterisation Co., Ltd., Purley Way, Croydon, as do other concerns, offers a di-chromate dip.
This again can be followed by dipping in a shellac and alcohol dye and a dip in oil. The degree .
of protection afforded by the full process is claimed to be almost as good as that of cadmium plating, and was recognized as such by the Air Ministry. Articles fully processed resist corrosion for 1,300 hours under a salt spray. The cost of the full treatment is kd. to d. per sq. ft. or 1 lb. Weight of metal.
The company states that rinsing is unnecessary after Walterisation as there are no metals other than iron in the processing solution. With some other phosphate treatments, it says, rinsing is required to eliminate deleterious nonferrous metallic salts, Some phosphating processes may be applied to parts subject to sliding friction. It appears to have a value as an extreme-pressure lubricant. Both British and American manufacturers have evidence of the value of phosphate coatings in reducing wear of moving parts.
For surfaces which are too rusty for phosphating, the company has two solutions for the removal of corrosion products. These are R.S.1 and Deran. R.S.1 is applied by brush and converts rust into iron phosphates. Deran is applied by dip. Both leave surfaces pasSivated against corrosion and may be painted over. Deran costs 16s. 6d, per gallon; R.S.1 costs 27s. 3d. per gallon, in each case less bulk-purchase discount.
A widely used process in automobile production is Bonderizing, marketed by the Pyrene Co., Ltd., Brentford, Middx. This treatment covers metal surfaces with heavy-metal phosphate products. Different kinds of phosphate can be applied to different metals.
Bonderizing treatment can be applied either by dip or spray, and besides preventing corrosion, it retards the ageing of the organic finish. This process also provides better adhesion of the finish. When rust occurs on painted unphosphated surfaces, the dissolving iron or steel develops an alkali which seeps between the metal surface and the paint, destroying the adhesion over an ever-widening area.
Five Treatments
The Bonderizing immersion process consists of five treatments: cleaning, phosphating, rinsing, drying and paint ing. The phosphating application is done in Bonderizing solution at about 180 degrees F., the immersion time varying from three to six minutes. The minimum processing time can be ascertained by rubbing a Bonderized surface lightly with grade 000 steel wool. If the base metal cannot be revealed, processing is sufficient.
Rinsing is done in hot water and can he followed by a chromate dip. Bonderizing by spray is an automatic method for processing by conveyor.
It is possible to spot-weld through Spra-Bonderite surfaces. Components for assembly at distant factories can be carried in a Spra-Bonderized,statc, temporarily protected by an oil, and welded into assemblies at will.
Another Pyrene product is Preperite, which removes corrosion from iron, steel, aluminium and its alloys. This can be applied by brush or dip. An immersion solution of Preperite can continue in service indefinitely, with occasional replenishments of Preperite chemical. Before treatment for the removal of corrosion product s. degreasing is necessary,
After treatment by Preperite solution, work maY be rinsed by immersion or hosing: If this preparation be used as
passivation treatment for components which are to be stored or transported, they are not rinsed after the Preperite has been applied. Preperite is supplie4 itt 7-gallon carboys.
Jenolizing is another anti-corrosive treatment which is being increasingly adopted in industry. Jenolite liquid has a specific gravity of 1.28 and is soluble in water. It can be used to remove rust from iron and steel and scale caused by welding, brazing, milling and rolling The maker, Jenolite, Ltd., 43, Piazza Chambers, Covent Garden, London, W.C.2, claims that it does not attack metal to which it is applied and does not leave behind any harmful salts.
A phosphate surface is formed by the Jenolite process which passivates metal against corrosion. This coating also acts as a binding agent for a finish. The maker further claims that the Jenolite process forms extremely fine crystals which, together with certain additives. make it a preparation which will de oxidize and provide a key for a finish in one operation.
The treatment has been applied to many ex-W.D. vehicles which were reconditioned for civilian service. Vehicles were dismantled :Ind dipped in Jenolite P.S.V. to strip off paint and Jenolitc D.S.1 to remove scale. The standard Jenolite R.R.N.1 is not strong enough for de-scaling, being primarily a material for light rust removal.
Treatment in R.R.N.1 followed a wash after the descaling, to provide a key for a finish. After this treatment, parts were dried by hot air directed by fans. Before packing in knocked-down condition for overseas shipment, Austin commercial vehicles ha ve been Jenolized.
A cold-galvanizing method for preventing rust is the Glopane treatment. Glopane can be applied by brush, dip or spray, and it is not necessary for the surface to which it is to be applied to be chemically clean—it may even be rusty—but it must be wiped free of oil. This preparation, which is made by Clear Glass Prodocts, Ltd., Malvern Road, Southampton, is supplied in 7-1b. and 28-1b. tins. Thinner is also supplied if required. One pound of Glopane will cover 24 sq. ft. at a cost of 2s., says the manufacturer.
Fluids whico remove corrosion products and provide a phosphate binding. agent for a finish are manufactured by Sunbam Anti-Corrosives, Ltd.. 9a, Ladbroke Grove, London, W.1 l. Ferrockne No. 3 is a light rust remover, and Ferrociene No. 3A is similar, but contains agents for removing light oil. For the removal of large amounts of rust and grease, Ferroelene 3B is recommended and may be applied by brush or dip. Even stronger is Ferroclene MS, which, besides containing grease solvents, has wetting and emulsifying agents, giving it detergent as well as rust-removing properties. Another grade. 3T, has a high viscosity.
Lubricating Properties
This company also supplies protective oils for application to parts for storage or shipment. These are Ferromede 110 and 114. They contain dewatering agents. Ferromede 110 leaves a film which dries in 24-36 hrs. (or 30 mins. in an oven at 100 degrees C.) and may be painted over. The film left by 114 should be removed before finishing, but may be _etained during machining operations, as it has hibricating properties.
Croda, Ltd., supplies three protective finishes which are based on lanoline. These are Prevok, Bier° and Proil. Prevok is a general-purpose preventive that leaves a bard film which can be painted over.
Bier° is recommended for application to parts going for export which are likely to undergo handling, as it leaves a hard film like a lacquer. It can be painted over but, as it contains no resin, it has no lubricating properties and, if applied to working parts, must be removed when no longer needed. Proil, however, is a lubricant, but does not dry.
• Salvage from the Sea
Shell-Mex and B.P., Ltd., Shell-Mex House, Strand, W.C.2, conducted much research into rust-preventive fluids during the war, when problems of de-rusting aeroplane engines salvaged from the sea and protecting amphibious vehicles arose. Special dewatering agents were developed which are now embodied in the Shell Ensis range of protectives. Shell Ensis fluids 252, 254 and 256 give various periods of protection, and Ensis fluid 260 dries and leaves a tough film.
Shell Ensis oil 152 is a rust-preventive lubricant for working parts liable to corrode while standing. Ensis oil 452 is made for the protection of engines not likely to be used for long periods. It can be used as a normal lubricant during running-in. Ensis compound 352 gives a heavy greasy film designed to protect parts to be stored in the open.
Manchester Oil Refinery (Sales), Ltd., 2, Mosley Street, Manchester, 2, offers a range of protective liquids, among them being AC30, which is recommended for the protection of internalcombustion engines during transport. It consists of a highly refined mineral lubricant containing an anti-corrosive chemical. For exposed parts, the company has produced AC50.
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