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by Kenneth Tator, Kenneth Tator Associates
CORROSION A
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Sacrificial Metals as a Base for Organic Coatings In the long run, it is cheaper to use a combination of galvanic and organic coatings than either one alone | T HAS long been known that sacri ficial metals, such as zinc, aluminum, and magnesium, will protect steel from corrosion when in electrical contact with the steel and both metals are in mutual contact with a corrod ing electrolyte. I t has been known for an equally long time too, that organic coatings can last as long as 10 years in even the most severe ex posures, provided proper inspection and spot repair arc done. However, only recently has using a combination of these two methods of corrosion protection been appreciated for severe chemical service where the galvanic coatings alone would pro vide but short steel protection. Even in these atmospheres this "short" time is measured in months, and de laying coating deterioration for months is a real advance. The C o m b i n a t i o n
Sacrificial metal coatings on steel will dissipate themselves uselessly by normal corrosion of the sacrificial metal itself, and very rapidly in severe exposures. But if the sacri ficial metal coating is overcoated with an organic coating, such dissipation of metal weight and thickness cannot occur, and the full amount of sacri ficial metal will be reserved for its prime purpose of protecting steel from corrosion at the time of need. On the other hand, organic coat ings fail in severe exposures from the expanding, prying ferric corrosion products which occur at breaks in the coating to expose base steel. Corrosion products of the sacrificial metals are light and powdery with no prying action, so that if discon tinuities develop in organic coatings over a sacrificial metal base, pro gressive coating deterioration is ex
ceedingly slow. This would be true even in mechanical injury through both the coating and the sacrificial metal layer exposing the base steel, as even in this case the sacrificial metal will corrode preferentially, preventing the steel from developing its corrosion product. The principal advantage of this delaying action is to reduce the rigidity of inspection and spot repair required to obtain maximum life from organic coating protection. Costs a n d Useful Lives
The protective life of a properlyselected and applied organic coating system, maintained always as a con tinuous barrier over the metal base, is limited only by physical and chem ical breakdown within the coating material itself. These breakdowns are usually through progressive overoxidation or polymerization, or plasticizer volatilization, which embrittle the coating system. Even in the most
severe exposures this coating break down will not occur in less than 10 years, and depending upon coating type might last longer. Coating integrity beyond 10 years, however, is unpredictable. For this reason, a 10-vear coating; life is used in these computations. Periodic inspection and spot repair of protective coating systems are always recommended in the use of each organic system. Cost: 3.5 cents a year for systems without sacrificial metal base, and half this amount for those with sacrificial metal base. The frequency of such repairs is much reduced with a base sacrificial metal. While these preventive procedures always maintain organic coatings in continuous protective barrier over the underlying metal for its life period of 10 years, during this period the surface will become unsightly from surface soilage. Consequently, in these computations we have allowed for a brush clean up and a freshening coat of paint at the end of 5 years to
PRECAUTIONS WORTH
NOTING
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Galvanic coatings are effective only when in electrical contact with the steel to be protected. Hence their a p p l i c a t i o n must be preceded b y pickling or blasting the clean steel.
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The galvanic coating should be at least 3 mils thick.
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O i l y residues on the surfaces of newly g a l v a n i z e d steel should be removed, either by short weathering or solvent wash, b e f o r e coating with the organic system.
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As all sacrificial metals are necessarily reactive regardless of their method of a p p l i c a t i o n , the prime of the organic system must b e one known to be a suitable g a l v a n i z e d primer. Otherwise h y d r o g e n blistering and ultimate peeling of the organic will result.
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CORROSION
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A Workbook
Feature
CUMULATIVE PROTECTION COSTS
W H A T ARE THE COSTS?
SACRIFICIAL COATING ONLY ORGANIC SYSTEM ONLY ORGANIC OVER SACRIFICIAL
Three-coat organic application
6 9 cents
Sacrificial metal a l o n e . . 4 4 cents Organic and sacrificial metal 8 9 cents
improve appearance. Cost: 20 cents per square foot. At the end of the 10-year period, assumed to be the physical life expectancy of the coating itself, the old failing coating system must be removed and replaced. I n this coating replacement operation, it is assumed that surfaces protected with sacrificial metal base will be somewhat easier to clean than those not so protected. Therefore, in the latter case, the cost of such renewal has been reduced by one half of the original surface preparation cost, to 57 cents per square foot. Galvanic metal coatings by themselves in these same chemical i n dustrial exposures vary in life from 1 to 5 years, and an average life of 2.5 years will be assumed. On the basis of these costs and life assumptions, cost of protection by sacrificial metal coatings alone would be $3.52 for a 20-year period ; $2.48
YEARS OF PROTECTION
for organic coating systems alone; and $2.20 for organic coating systems over a sacrificial metal base. In spite of the higher initial cost of the organic system over sacrificial base, this system will prove less expensive than straight sacrificial metal coatings after 5 years of service, and less expensive than the organic coating system alone after 10 years of service. These cost economies are i n fact an added dividend. I f such cost advantages did not exist, the combination of organic coatings with sacrificial metal base would be completely justified by the lessened (in time and rigorousness) attention required of the coating system after its initial application. This coating system combination, therefore, is suggested where present methods of protection require too frequent attention at present, or where coated areas will be made inaccessible for preventive maintenance attention by subsequent building enclosures or equipment placement. Applications
W a t e r tank in severe chemical fume a r e a , metallized with aluminum, and over coated with vinyl system. Five years service to need spot repair as opposed to two years with organic system alone 78 A
A good case history to illustrate the assumptions in the cost projections just cited is the example of a large inorganic and organic chemical manufacturing plant in West Virginia which 4 years ago conducted a plant scale study of sacrificial metal coatings as paint bases. A t that time an elevated water tank in the direct path of a cooling tower and hydrochloric acid fumes was blasted, metallized with aluminum, and protected with a vinyl system. I n the third year it received its first and onlv
INDUSTRIAL AND ENGINEERING CHEMISTRY
spot repair, at a cost well within 1.8 cents per square foot, which figure was used as an annual cost of spot repair in our cost analysis. Selected production areas within the plant were variously coated with galvanize, metallized zinc and aluminum, and zinc-rich coatings as bas; for vinyl systems. These applications are now also 4 years old and within this time have needed and received no more than two instances of spot touch-up, the cost of each touch-up being well within the 1.8 cents per square foot figure used in the cost analysis. This experience indicates that our cost analyses, while favorable for this combination, are conservative. The cost advantage of using sacrificial base for organic coating systems may turn out to be appreciably greater than that shown. In a salt-producing plant on the Gulf Coast, the life of unprotected galvanized and zinc-rich coatings has been found to be 7 to 12 months. The life of a vinyl system without preventive maintenance is less than 2 years. But with vinyl over both galvanized and zinc-rich coatings, service life has been over 3 years without any repair attention or evidences of failure within this time.
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