Corrosion weight-loss

Equation (2-38) is valid for every region of the surface. In this case only weight loss corrosion is possible and not localized corrosion. Figure 2-5 shows total and partial current densities of a mixed electrode. In free corrosion 7 = 0. The free corrosion potential lies between the equilibrium potentials of the partial reactions and U Q, and corresponds in this case to the rest potential. Deviations from the rest potential are called polarization voltage or polarization. At the rest potential = ly l, which is the corrosion rate in free corrosion. With anodic polarization resulting from positive total current densities, the potential becomes more positive and the corrosion rate greater. This effect is known as anodic enhancement of corrosion. For a quantitative view, it is unfortunately often overlooked that neither the corrosion rate nor its increase corresponds to anodic total current density unless the cathodic partial current is negligibly small. Quantitative forecasts are possible only if the Jq U) curve is known. 

Two areas of passivity are located in Fig. 2-2 where Fe has a very low corrosion rate. In contrast to cathodically protected metals in groups I and II, the corrosion rate of anodically passivated metals in groups III and IV cannot in principle be zero. In most cases the systems belong to group IV where intensified weight loss corrosion or local corrosion occurs when U > U" There are only a few metals belonging to group III e.g., Ti, Zr [44] and A1 in neutral waters free of halides. 

Weight loss corrosion of passive metals (e.g., CrNi stainless steels in acids) (cathodic protection in acids is not practicable see Fig. 2-14) ... 

I Ualloy ferrous materials Neutral waters, saline and soil solutions (25°C) <-0.53 <-0.85 Protection against weight loss corrosion Fig. 2-9 [29-34] (with film formation is more positive)... 

AI, A1 alloys Cold water Protection against weight loss corrosion and pitting corrosion [36,39,42]... 

Inhibitor Concentration of Inhibitor (mol/1) Weight loss Corrosion mg/25cm2. Potential day mV vs. H2 ... 

B Ferric Sulfate-50% Sulfuric Acid boiling 120 h Chromium carbide Weight-loss/corrosion rate... 

Method A B Ferric Sulfate 50% Sulfuric Acid 23% Sulfuric 1.2% Hydrochloric 1% Ferric Chloride 1% Cupric Chloride boiling boiling 24 to 120 h 24 h Ctirbides and intermetallic phases Ctirbides and intermetallic phases Alloys C-276, C-22, 59 Weight-loss/corrosion rate and/or microscopic examination Weight-loss/corrosion rate and/or microscopic examination... 

Weight loss corrosion rates, which represent an average of corrosion over the test period, are useless from a predictive point of view, but are often used in conjunction with other measurements for quality assessments. Corrosion kinetics can be measured in different ways. Most favored are electrochemical techniques. They are, however, contrary to common belief, indirect techniques and must be properly calibrated and interpreted to be useful. If corrosion products are soluble in solution (as, for instance, iron carbonate), the buildup of such in solution can be used to monitor how corrosion progresses. Hydrogen, a byproduct of anaerobic corrosion, can also be used to monitor kinetics. Less common, but equally direct, are methods that use the removal of radioactivity from irradiated surfaces. Kinetic measurements have also been carried out with electrical resistance probes. As a general principle, no one method is in itself without some problems and should, therefore, always... 

TABLE 3—Corrosion inhibition measurements in 17 % HCl at 200 F on various steels with three different inhibitors. R = weight loss corrosion rate over integrated LPR measurement. = same for first two hours of exposure, same for 2 to 24 h, ii = same for average of total exposure time. Average 24-h corrosion rates in mpy, P indicates coupon pitted or PP pitted severely. 

Weight loss corrosion This type of corrosion occurs most commonly in oil and gas production due to an electrochemical reaction between metal and the corrodents in the environment. 

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