Carrosion
Julw 1950
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Corrosion of steel decreases and lead increases when wet ting agents are added to dilute acid and basic solutions
bw Mars 6. Fontana of the effect of wetting agents on the erosioncorrosion and corrosion of steel and lead by solutions in a p H range of about 3 to 9 yielded interesting results. The rate of attack on the steel was decreased but in the case of lead it was increased. The writer is not aware of any actual applications where wetting agents by themselves are used to inhibit corrosion of steel or increase the rate of solution of lead but these possibilities might be considered in special cases. Closed or recirculating steel systems would be an example of the former and dissolving lead to produce lead acetate might be an example of the latter. Specific study of each individual case and special attention with regard to pitting would be required. Erosion-corrosion and equipment to study this form of attack, were described previously in this column [IND. ENG. CHEM.,39,87A, 88A (1947) I. Erosion-corrosion is a combination of corrosion and mechanical wear effects such as liquids moving at substantial velocities, solids in suspension, turbulence, and impingement. I n many cases the protective surfaces of metals and alloys are destroyed and very rapid and unexpected deterioration occurs. The equipment briefly consists of a 30-gallon, glass-lined tank, a Chlorimet 3 pump, neoprene hoses, and a nonmetallic housing which contains a 4-inch diameter test specimen disk. The specimen rotates in this housing and the housing assembly is somewhat similar to a centrifugal pump except that the “impeller” is a flat disk. Heaters and temperature controlling and recording instruments are part of the apparatus. The Chlorimet 3 pump recirculates the solution in the tank. The solution is pumped into the housing assembly in such a manner that the liquid is forced pabt the face of the disk under a close STUDY
clearance. High velocities and erosioncorrosion conditions are obtained. The tank samples mentioned herein consist of ordinary corrosion test coupons suspended in glass cradles in the glass-lined tank. These specimens are subjected to low velocity flow or the swirling motion that obtains because of recirculation of the liquid by the pump. The stagnant or static specimens are placed in an Erlenmeyer flask containing the same solution as the large tank. This flask is placed in the liquid in the large tank so that this liquid acts as a temperature bath. No motion of liquid exists in these flask tests. Three conditions are thus obtainednamely, high velocity for the disk specimen, low velocity for the tank samples, and static conditions in the suspended flask. All tests are run overnight or from 18 to 20 hours. Surface preparation of the specimens consists of finishing on 120 emery cloth and washing in acetone.
Steel Table I shows the results obtained from tests designed to determine the effect of wetting agents on the corrosion of plain low carbon steel by distilled water a t 50’ C. The pH was lowered through the addition of sulfuric acid and raised by means of sodium hydroxide. Aerosol, an ionic material, was added in all cases except for one wherein Sterox-SK, a nonionic material, was used. The volume of solution in the tank for these tests was approximately 80 liters or 20 gallons. The Aerosol was added as a 10% solution. The erosion-corrosion of steel is greatly reduced as a result of adding the wetting agent. At a pH of about 8 the rate of attack is reduced from 250 mils per year to zero. The former is, of 1000. course, a high rate of corrosion. At a pH I 5ML. OF 10% AEROSOL apprP of 6 the rate of deterioration is reduced NO AEROSOL AD= 0 -DISU although it is quite low without the ada 9 eoodition agent. At a pH of 4,the rate is re* pH ADJUSTED WITH duced approximately fourfold, but the h,sO* AND NAOH attack is still substantial with the Aerosol present. The Sterox-SK addition be9 600. si haved about the same as Aerosol in the erosion-corrosion tests. As little as 5 ml. Y \ I of Aerosol stifled attack in the tests run a t $ 400 about pH 8. I I I I I I \ I 1 I The wetting agents decreased the attack on the tank specimens and the stagnant specimens except a t p H 6 where variable effects were observed. The results obtained were somewhat surprising in that it was felt that reducPH PH ing the surface tension of the liquid Figare 1. Corrosion Rate of Chemically Pure Lead 1)s. pH of Distilled should provide (Continued on page 66 A ) Water at 50’ C.
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3
I \ I
65 A
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I
Corrosion more “intimate” contact between the metal and the corrosive and, therefore, higher corrosion. The opposite” effect which was actually observed can be explained by the fact that the addition of iierosol resulted in the formation of R dense, adherent, and magnetic film on the steel. This film is apparently ferrosoferric oxide (Fe30a),and it protects the metal and stifles attack. Cardwell and Eilers reported on the use of wetting agents in conjunction nith acid inhibitors [Im. ENG.CHEM.,40, 1951-6 (1948)l. They shoived that the addition of netting agents in acid solutions, containing organic inhibitors, reduces corrosion of steel. The amount of reduction of corrosion depends upon the initial wetting of the metal by the acid-inhibitor solution. The lower the degree of wetting tlir greater the reduction of corrosion because of wetting agent additions. A partial explanation of the phenomenon i5 that the increased n-etting due to the addition agent reduce5 the size of the I~ydrogenbubbles, thus decreasing pitting CGI’rmion.
TABLEI. EFFECT OF U-ETTINGAGENTSo s CORROSIOS OF 1020 STEELISDISTILLED WATERAT 50” C.
$.%J:;
( p H adjusted with S a O H or H Y S O ~ ) Surface ~ ~Corrosion ~ Rate, lIils/Year ~ i ~StagDynes pH per Cm., E-C Tank nant Initial Final 50° C. disk samples tests
8.8
6.8 B 6 6 . 5
8.5 8.5 8.5 8.5 8.1 6.1
6.5 7.5
6.1
6.05 4.1 4.0
7.4 5.9 6.8 6.0 6.0 6.0
58.1 56.8 48.9 40.6 30.6 36:3 30.7
4.8
4.7
33:1
250
48
8
n
5
0 6 5 4 8
0 0
n
8 5 1
0 300 80
9
7 0 0 85 49 80 105 21
~
~
Wetting Acent Added, 111. Sone Aerosol 5
so
50
12 9
200 400 None 200 50 (Sterox-SR) L4erosol)
18 9
Sone 200 (Aerosol)
4
Lead -4series of tests was run on pure lead and an effect opposite to that observed for steel mas found. The results for the lead tests are shown in Figure 1. Note the tremendous increase in erosion-corrosion a t pH 4 as a result of the Aerosol addition. I n general, the rate of attack was increased for all specimens although the effect was less pronounced a t the highest pH. The corrosion product on the samples in solutions ithout Aerosol consisted of the typical gray film often found on lead. However, when Aerosol was present, the lead specimens were covered with a sticky gray substance which did not dissolve readily in acetone but dissolved quickly in dilute hydrochloric acid. This sticky filni apparently resulted from reaction hetween the lead and the Aerosol. This film is not protective. The writer conducted a brief investigation several years ago on chemical lead in this connection. This lead showed little erosion-corrosion in hot tap water but was destructively attacked when an organic wetting agent was added. A rate of erosion-corrosion of several thousand mils per year was 01)served. The data reported are part of a thesis submitted by Howard B. Bomberger in partial fulfillment of the requirements for a master’s degree in the Department of Metallurgy a t The Ohio State University. 66h
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