Materials Performance

SEP 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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24 SEPTEMBER 2018 W W W.MATERIALSPERFORMANCE.COM C L A S S I C oxide from cement and mud sys- tems. Thus the pH would be be- tween six and nine for carbonate systems and about 12 in calcium oxide systems. Solutions in this pH range do not produce severe gen- eral corrosion of steel, but pitting may be observed when oxygen or sul fide is present. Oxygen is pres- ent when wells are first completed or might be supplied later by forma- t ion waters near t he surface. In most cases, however, sulfide ap- pea rs to be t he major cause of corrosion. In some laboratory systems the average rate of metal loss over the ent ire surface was less t ha n ex- pected. In some sulfide systems, electrical re sistance meter data in- dicated decreas ing resistance across the coil. There fore, correlation be- tween corrosion rates and polariza- tion data were not obtainable for all systems, a nd major emphasis is placed on the empirical correlations for t he t wo solut ion s dis c ussed below. S o d i u m A c e t a t e - S o d i u m Su l f ide -Hyd ra zi ne Solut ion : A brine solution containing 100 grams of sodium chlo ride, 50 grams of so- dium acetate and approximately one gram of sodium sulfide per liter was adjusted to a pH of six with ac e t ic ac id. Hyd ra z i n e s u l fat e (NH 2 NH 2 ·H 2 SO 4 ) was added to re- move ox yge n a nd t he s olut ion placed in a stoppered test vessel. Any oxygen that might normally have entered the solution during the test period is presumed to have been removed by reaction with ex- cess hy drazine sulfate. After tests on this solution were completed, the pH was measured as 8.9, and the sulfide ion content determined as 1.3 × 10 –2 molar. Electrical resis- tance data taken to study the effect of applied current on corrosion rate are plotted in Figure 3 and polar- ization curves (silver-silver sulfide reference) are shown in Figure 4. The unprotected corrosion rate shows a slight increase with time, indicative of pitting, with an aver- age rate of about 15 mils per year (mpy). Weight-loss data in this solu- tion showed an average rate of 2.6 mpy with pitting during a 48-day test. A minimum protective current o f 0 . 3 5 m i l l i a m p e r e (m a) i s indicated. The in it ial polarizat ion curve (Cur ve No. 1, Fig ure 4) was ob- tained by holding each current in- crement until t he potential shift was less than one millivolt in five minutes. The curve does not show a pr o n o u nc e d br e a k a lt ho ug h a straight line seg ment is observed from 0.34 to 0.6 ma. For Curve No. 2 in Figure 4, each current increment was held until the potential was es- sent ially constant, and t he total elapsed t i me was 38 days. Th is curve has a shape approximate to Curve C in Figure 2 and to some f ield c ur ves obtai ned wit h slow polar ization. The protective current ap pears at the point of inflection of the curve. Calcium Carbonate-Hydrochlo- ric Acid-Sodium Sulfide-Hydra- zine Solu tion: Eleven grams of cal- cium carbo nate were suspended in one liter of brine solution contain- ing 100 grams of sodium chloride, and 18 ml of re agent grade hydro- chloric acid (12 normal) were added. After a short period of standing, the pH of the so lution was 8.7 with ex- cess solid cal cium carbonate re- maining. The pH did not change during the test, and the conditions were approximately the same as the final conditions of the sodium ace- tate-sodium sulfide-hydrazine so- lut ion, except for t he bu f fer i ng a g e n t . We i g h t- l o s s s p e c i m e n s showed average rates of 7 mpy and 3 mpy during a 48-day test. The rate was apparently dependent on the amount of pitting taking place. Electrical resistance data in Fig- ure 5 show an initial rate of about 19 mpy, presumably due to general pit ting, followed by a decrease in rate a f ter t he f i rst pola r i zat ion curve. The indicated protective cur- rent is again 0.35 ma. Figure 5—Effect of applied current on cor- rosion rate in calcium carbonate-sodium sulfide-hydrazine solution, pH 8.7.

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