The second of a series of columns summarizing corrosion data in chart form presents temperatures and concentrations for the corrosion of lead by sulfuric acid
L
is widely used in industry for handling sulfuric acid. When sulfuric acid is mentioned, many persons automatically ihink of lead as the associated corrosion-resistant material. I n other wor?s, this is a natural “hand and glove” combination. L e a l resists sulfuric acid very well except for the strong acids. Lead and steel are relatively cheap materials and it is fortunate that 0110 of these materials complements the other. If the acid is too strong for lead, steel will resist it; if too weak for steel, lead can take over. A condensed and streamlined picture of the corrosion of steel by sulfuric acid as a function of temperature was presented in chart form in this column last month. A similar picture for lead, using the same ranges of corrosion rates, is shown here as Figure 1. The curves in this figure are less complicated than the ones for steel because the dips in the curves are not present. Figure 1 does not show concentrations EAD
below 50% because the corrosion resistance of lead is very good at temperatures including boiling. Some waters are corrosive to lead but this chart is primarily concerned with sulfuric acid solutions. Lead depends on the formation of a protective film of presumably lead sulfate for its corrosion resistance to sulfuric acid. The solubility of lead sulfate increases as the concentration of acid is increased in the stronger acid range, and accordingly corrosion increases. Figure 1 shows t h a t as the temperature and/or concentration is increased, the corrosion resistance of lead decreases. Concentration of acid is an important factor. Lead dissolves quite rapidly in the very strong acids-98% and above at room temperature. Fuming acid attacks lead rapidly. The corrosion resistance of lead to cnncentrated sulfuric acid a t elevated temperatures is poor. Refercnce to the chart for steel in last month’s column shows that the same situation exists for steel. This represents a gap or set of conditions
that cannot be handled 11y steel o r lt,ad. In i:tct, nonc of t h e commercinl , mct:il$ unri :~lloysi. corrosion rc-sistnrit t o hot, strong ucid except high silicon iron (Duriron) and noble metals. This is the reason why Duriron is so widely used for heating surfaces in equipment for concentrating sulfuric acid. Duriron is somewhat deficient in mechanical properties and will not withstand severe mechanical and thermal abuse. Occasionally, lead is used in strong acid a t high temprratures with fairly frequent replacement because it is the economical material to use. When heat transfer is not, a prohlem, the lead is often prot,ected by acidproof brick. Steel tanks with an intermediate lining of lead and a n inner lining o f brick are (Continued on page 106 A )
0 PER GENT SULFURIC ACID Figure 1
September 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
105 A
Corrosion quite common for handling hot, strong acid, as in concentrators, and also for equi ment where abrasion or erosion is invorved A large amount of d a t a is required t o prepare a chart similar to Figure 1. Additional d a t a are required in acids above 60% strength and temperatures around and above 300' F. in order to pin point the curves. The curves shown in Figure 1 may be slightly on the conservative side or in other words, they could be shifted sli htlv to the right. However, it is believe5 t h a t Figure 1 presents a pretty fair picture of the situation. Corroebn teete
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One of the difficulties involved in corrosion testing of lead concerns the protective coating t h a t fo,mb o n the specimens. How should the specimen be treated after exposure to t h e 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 at high temperatures. To be strictly correct, the extent of corrosion should include all of the metal converted to corrosion product, which means t h a t the coating should be removed. This is sometimes done by dissolving the coating in hot ammonium acetate solutions or by scourin with a n abrasive. Then we have the di&culty of some attack or removal of t h e base metal. A standard and recommended procedure consists of rubbing the specimen with a rubber stopper under flowing water from the tap. This removes loose corrosion roducts, the assumption being t h a t tge 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 the surface with a n ink eraser and expose them to the acid shortly thereafter. Grinding or sanding causes flow and smearing of the soft lead.
ChemZCal lead
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I06 A
The 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. It is covered by ASTM specification B29-49 which calls for 0.04 to 0.8% copper; 0.002 to 0.02% .silver; 0,001% zinc, maximum; 0.00270 iron, maximum; 00.05'% bismuth, maximum; 0.00270 maxi-mum total for arsenic, antimony, and tin; and 99.90% lead. The ASTM specification for acid lead is essentially the same. High purity or corroding lead is less resistant than chemical lead in hot acids in the range of concentrations shown in Figure 1. Pure laad has plorer 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 o n extensive laboratory tests and actual plant installations. The d a t a used to plot Figure 1 were obtained from simple immersion or static
tcsts. Perhaw the most imriortant factor that would change this picture is the effect of velocitv or erosion. Lead is a very soft metal and is readily c u t o r abraded. Erosion or mechanical w'air effects remove the protective coating and thc exposed lead is rapidly attacked. This is particularly true in the stronger acids or at the higher tem eratures. Figure 2 shows the effect of higg velocity or erosion-corrosion of hard lead in 10% sulfuric acid [W. A. Luce and M. G. Fontana, Cwrosion, 5, 189-193. (1949) 1. The static tests showed no weight loss but appreciable corrosion occurred and increased with temperature in the dynamic tests. For these reasons lei~tl valves and umps are not often used for handling suPfuric acid.
*OIl-m-l
XaSTATIC TEST
0
DEGREES CENTIGRADE
Figure 2
H a r d bad Hard lead is often used when a materi;il stronger than chemical lead is required as in valves. Hard lead contains roughly 4 to 12y0 antimony. However, t h e strength differential disappears at high temperatures for long-time loads. For example, t h e creep strengths of hurt1 or antimonial lead and chemical lead arc. about the same a t 190' F. The corrosion resistance of hard lead and chemical lead are both good in th(. more dilute acids. Chemical lead is usually superi3r under the more aggressivc conditions. The writer knows of only one case where hard lead showed longer life than chemical lead. This installatioii involved lead-lined steel tanks for handling a process liquar containing approximat.rly 1Oyosulfuric acid at moderate tempvratures. The data for Figure I were ohtaiiwtl from the Corrosion Rescarch 1,abolxtoriw a t The Ohio State Universit,y, The SatioiuI Lead Co., the litsraturc, and other WUYWS. Correspondence concerning thix coliinin will t i c forwarded promptly if addressed to the author. r : , Editor, INDUSTRIAL A N D ~ N G I N & E R I S GCHY.\llsT R Y , ll55-16th St., N.W., U'ashinRton 6 . T). ('.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 43, No. 9
