by W. Ε. Kemp Koppers Co., Inc.
I CORROSION A
W O R K B O O K
F E A T U R E
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Product Fallout —A Serious Corrosion Problem Protecting structural steel work is a problem in areas experiencing con tinuous fallout of corrosive materials. Serious thought should be given to the corrosion problem in the design stage I HE corrosion of structural steel, piping, tanks, stairways, electric con duits, and the like may not only inter fere with plant operation but lead to exorbitant maintenance costs. Rarely is a complete corrosion survey made on the exterior environment before a plant is built. Much time and effort are spent investigating potential in-process corrosion problems and making ade quate provision for these (but no con sideration is given to the corrosive environment in which the plant may be built or the effect of atmospheric by-products produced during plant operation on its exterior equipment and structure). It is not always pos sible to provide adequate corrosion protection after the plant has been put in service because of the impossibility of obtaining satisfactory surface prepara tion under fallout conditions. Fallout areas occur in coke and steel plants, particularly those adjacent to, or downwind from, coke quenching towers where all exterior steel surfaces are subject to continuous fallout of moisture, ammonium chloride, sulfurous acid, and corrosive dusts as well as traces of oil. Similar conditions exist in many chemical plants where chlorination or sulfonation processes arc employed and acidic and alkaline by-products are car ried into the atmosphere. The corrosion rate of carbon steel in these areas reaches fantastic proportions and may exceed 0.5 inch per year. For example, even heavy galvanized electric conduit and guard fencing fails through pitting at tack in less than a year. Where exterior corrosive conditions have not been recognized prior to plant construction and operation or where the initial selection of a protective coating was inadequate, repair and maintenance of a coating are difficult unless fallout on the surface can be prevented during the repainting operation. Corrosion prod ucts are trapped beneath the protective coating and the coating fails from beneath
Photograph shows serious fallout condition which can cause corrosion
because of pressure of ferric corrosion products beneath the film. Experience has shown that the failure of paint films and protective coatings in fallout areas (assuming an original pinhole or holiday-free surface) usually starts from microscopic cracks in the protective coating. These occur from natural hardening of the paint film
Performance of a protective coal ing in a fallout area depends on : • • • •
Surface preparation Film thickness Adhesion of organic film Elasticity or ductility of the organic film • Film resistance to the environ ment
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through polymerization or loss of plasticizers and concomitant reduction in duc tility as well as the stresses of temperature cycles or the normal movement of the structure. Failure is usually not from actual destruction of the coating by corrosive agents, although this is a contributing factor. In a heavy industrial environment the metal surface should be sandblasted to white or at least gray metal correspond ing to grades BSa3 and BSa2 on the cleaning scale of the Corrosion Com mittee of the Royal Swedish Academy of Engineering Sciences. If the surface has been contaminated, particularly with chlorides, it may be necessary to wet down the freshly sandblasted sur face with water and reblast the following day. This second blasting proceeds rapidly and will effectively remove bur ied corrosion products which have been brought to the surface overnight. In ORKBOOK
F E A T U R E S
75 A
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CORROSION
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other instances where it is impossible to blast, or where the surface cannot be completely protected against fallout during the painting schedule, an inhibitive primer (preferably of the alcoholic type) is useful. The film thickness of the protective coating should be at least 20 mils, preferably applied in two or more coats, and the coated surface holiday-detected by means of a low voltage, sponge-type electrical holiday detector. The elasticity or ductility of the protective coating should be such that a 20-mil cured film on 28-gage steel can be bent 180° around a 'A-inch mandrel without cracking. In areas of heavy contamination it may also be advisable to reinforce the coating by embedding a glass membrane between the first and second or second and third coats of the protective coating. The adhesion of the protective coaling to sandblasted steel must be good initially and should not be changed after immersion of the coating in distilled water. Finally, the coating system must have an inherent resistance to attack by the corrosion products expected to be encountered. The area surrounding or downwind from a coke plant or a coke quencher tower is typical of fallout areas, in which the corrosion rate for mild carbon steel is as high as 800 mils per year. The photograph on page 75 A shows a source of corrosion producing fallout. The photograph above illustrates the condition of uncoated ] 4-inch mild steel plate after less than a year's exposure. It was situated 200 feet downwind from the fallout source. The conduit in the third photograph was originally galvanized and uncoated. The larger pipe was coated after sandblasting in the field with an industrial grade chlorinated
Feature
No coating—no steel. This uncoated '/•i-inch mild steel p l a t e was situated 2 0 0 feet downwind from fallout source. Exposure time was only one y e a r
rubber base maintenance paint to a total thickness of 7 mils. Koppers' experience with cold applied coatings and tapes of all types in film thicknesses less than 1 5 mils in equivalent fallout areas has been poor. The ductility of the top coat in these areas is most important and performance is generally better from coatings which have and maintain elastic films. For this reason the coal tar—urethane coatings usually exhibit superior perform-
Photograph shows attack on a horizontal pipeline with adjacent conduit piping in about 14 months. Note perforation of the upper surface of the conduit 76 A
INDUSTRIAL AND ENGINEERING CHEMISTRY
ance over epoxy coatings, all other things being equal. Where the steel can be coated before installation, best performance has been achieved by sandblasting to a BSa3 finish, followed by two coats of a coal tar-urethane chemically cured coating and one top coat of a coal tar-polyacrylonitrile emulsion coating to a total thickness of 25 to 35 mils. In all instances performance and life of the base coat are improved by top coating with a coal tar base emulsion. Field welds and new construction in fallout areas are sandblasted, with provision for protection from fallout contamination. They are then immediately coated with a coal tar-chemically cured coating of the epoxy or urethane type with provision for speeding up the cure time by resistance heating, if necessary. An easily maintained elastic top coat of the coal tar-acrylonitrile emulsion type is usually applied to provide further protection from the effects of progressive hardening of the chemically cured coating. This top coat is important where abrasion or excessive movement of the steel surface is anticipated. For equipment or structures which have been erected without consideration of possible exterior corrosion and must be subsequently protected in areas subject to continuous contamination, and blasting is not permissible or practical, a chemically cured coating is worthless. Under these conditions a program of continuous coating maintenance must be anticipated and programmed, because all coatings applied under these circumstances will suffer premature failure by attack from underneath. The lowest maintenance, and therefore costs, will be achieved through a program including flame cleaning or chipping and/or power wire brushing, application of a fast-drying inhibitive primer which may be of the alcoholic type, followed by immediate application of a coal tar cutback top coated with two coats of a coal tar emulsion of the elastic type. The final thickness should be 35 to 45 mils. Most important, recognize that a problem of exterior corrosion and corrosion fallout exists—through a good corrosion survey before the plant or equipment is built—and plan your protective coating system accordingly.
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