Corrosion
Helpful Guides To the selection of acid-alkali-solvent proof
CONSTRUCTION MATERIALS
quite common for handling hot, strong acid, as in concentrators, and also for equipment where abrasion or erosion is involved. A large a m o u n t of d a t a is required to prepare a chart similar to Figure 1. Additional d a t a are required in acids above 6 0 % strength and temperatures around and above 300° F . in order to pin point the curves. T h e curves shown in Figure 1 m a y be slightly on the conservative side or in other words, they could be shifted slightly to the right. However, it is believed t h a t Figure 1 presents a p r e t t y fair picture of the situation.
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test
One of the difficulties involved in corrosion testing of lead concerns t h e protective coating t h a t fcvms on t h e specimens. How should t h e specimen be treated after exposure to the acid? To illustrate the point, lead will often show a gain in weight because of the coating. This indicates a negative corrosion rate which, of course, doesn't make much sense. This situation is somewhat similar to tests for oxidation resistance of metals a t high temperatures. To be strictly correct, t h e extent of corrosion should include all of t h e metal converted to corrosion product, which means t h a t t h e coating should be removed. This is sometimes done by dissolving t h e coating in hot ammonium acetate solutions or by scouring with an abrasive. Then we have the difficulty of some attack or removal of t h e base metal. A standard and recommended procedure consists of rubbing t h e specimen with a rubber stopper under flowing water from t h e t a p . This removes loose corrosion products, the assumption being t h a t t h e loose products would come off in service. If the specimen then shows a gain in weight, merely report it as zero corrosion. A good method for preparing t h e specimens prior to exposure is to clean t h e surface with an ink eraser and expose them to the acid shortly thereafter. Grinding or sanding causes flow and smearing of the soft lead.
lead
T h e d a t a used for constructing Figure 1 were obtained from corrosion tests on chemical lead which is the material ordinarily used, and should be used, for corrosion applications. This lead contains approximately 0.06% copper and a specified minimum of other impurities. I t is covered by ASTM specification B29-49 which calls for 0.04 to 0 . 8 % copper; 0.002 to 0.02% silver; 0 . 0 0 1 % zinc, maximum; 0.002% iron, maximum; 0 0 . 0 5 % bismuth, maximum; 0.002% maximum total for arsenic, antimony, and tin; and 99.90% lead. T h e ASTM specification for acid lead is essentially t h e same. High purity or corroding lead is less resistant t h a n chemical lead in hot acids in the range of concentrations shown in Figure 1. Pure lead has p i o r e r mechanical properties than chemical lead. Tellurium lead shows less corrosion than chemical lead in hot concentrated sulfuric acid but is equal or inferior to chemical lead for all other acid concentrations based on extensive laboratory tests and actual plant installations. T h e d a t a used to clot Figure 1 were obtained from simple immersion or static
INDUSTRIAL
AND
ENGINEERING
tests. Perhaps the most important factor that would change this picture is the effect of velocity or erosion. Lead is a very soft metal and is readily cut or abraded. Erosion or mechanical wear effects remove the protective coating and the exposed lead is rapidly attacked. This is particularly true in the stronger acids or at the higher temperatures. Figure 2 shows the effect of high velocity or erosion-corrosion of h a r d lead in 1 0 % sulfuric acid [W. A. Luce and M. G. Fontana, Corrosion, 5, 189-193 (1949)]. The static tests showed no weight loss b u t appréciable corrosion occurred and increased with temperature in t h e dynamic tests. For these reasons lead valves and pumps are n o t often used for handling sulfuric acid.
IOO DEGREES CENTIGRADE Figure 2
Hard
lead
Hard lead is often used when a material stronger t h a n chemical lead is required as in valves. Hard lead contains roughly 4 to 1 2 % antimony. However, t h e strength differential disappears a t high temperatures for long-time loads. For example, the creep strengths of hard or antimonial lead and chemical lead are about t h e same a t 190° F . The corrosion resistance of hard lead and chemical lead are both good in the more dilute acids. Chemical lead is usually superior under t h e more aggressive conditions. T h e writer knows of only one case where hard lead showed longer life t h a n chemical lead. This installation involved lead-lined steel tanks for handling a process liquor containing approximately 10% sulfuric acid a t moderate temperatures. T h e d a t a for Figure 1 were obtained from the Corrosion Research Laboratories a t T h e Ohio State University, T h e Xational Lead Co., the literature, and other sources. Correspondence concerning this column will he forwarded promptly if addressed to the author. c'(, Editor, INDUSTRIAL AND E N G I N E E R I N G C H E M I S -
TRY, 1155—16th St., N.W., WashinKton 6, 1). C.
CHEMISTRY
Vol. 43, No. 9
