Oxidizer concentration effect corrosion rate

Production of differential aeration cell. A scatter of individual barnacles on a stainless steel surface creates oxygen concentration cells. The formation of biofilm generates several critical conditions for corrosion initiation. Uncovered areas will have free access to oxygen and act as cathodes, while the covered zones act as anodes. Underdeposit corrosion (crevice corrosion) or pitting can occur. Depending on the oxidizing capacity of the bacteria and the chloride ion concentration, the corrosion rate can be accelerated. However, the presence of a biofilm does not necessarily mean that there will always be a significant effect on corrosion. (Dexter)5... 

Duncan and Frankenthal report on the effect of pH on the corrosion rate of gold in sulphate solutions in terms of the polarization curves. It was found that the rate of anodic dissolution is independent of pH in such solutions and that the rate controlling mechanism for anodic film formation and oxygen evolution are the same. For the open circuit behaviour of ferric oxide films on a gold substrate in sodium chloride solutions containing low iron concentration it is found that the film oxide is readily transformed to a lower oxidation state with a Fe /Fe ratio corresponding to that of magnetite . 

For all these reasons an increase in chromium concentration may be the best solution to increase the sulphidation resistance of iron-aluminium alloys. The present work has shown that chromium has a very beneficial effect on the sulphidation resistance in oxygen deficient gases. Such improvement of sulphidation resistance in H2S environments has also been found in nickel-aluminium alloys, where the corrosion rate decreased significantly, when 8 wt% chromium were added [22], Whether chromium increases the rate of alumina formation or contributes to the formation of protective spinell type oxides Fe(Cr,Al)203 cannot be decided from the available results. It is also possible that chromium catalyses the transition from a less to a more protective type of A1203. It was for example found that chromium enhances the transition from 0-Al2O3 to a-Al203 in Ni-Al at temperatures above 1000°C [30],... 

Both the H2S concentration over the range of 0-1.0 v/o of the CGA gas and the temperature controlled the measured corrosion rates. Figure 1 illustrates the effect of temperature on corrosion rates of several alloys and coatings in the CGA gas containing 1 v/o H2S. Alloys AISI 309, AISI 310, and IN-800 demonstrate a clear temperature dependence of total oxidation-corrosion in 1000 hr. The 309 alloy had a scatter band of 5 to 125 mils total metal loss for four specimens at 1650°F. This is typical of borderline alloys that undergo time-dependent transitions to accelerated corrosion rates. Total corrosion of aluminized 310 and 800 was relatively unaffected by temperature over the range of 1500°-1800°F for 1000 hr exposures. 

The effect of H2S concentration on total corrosion of selected alloys in 1000 hr at 1800°F is illustrated in Fig. 2. A variety of different oxidation-corrosion behaviors were observed. Ferritic alloys, like AISI 446, generally showed increased corrosion rate with decreasing H2S concentration, whereas 300 series austenitics typified by AISI 310 generally exhibited maxima at both 0.1 and 1.0 v/o H2S. IN-800 had high corrosion only above 0.5 v/o H2S. Aluminized AISI 310 and IN-800, IN-671, and several high-chromium alloys did not indicate a strong dependence of H2S concentration in 1000 hr total corrosion. Cobalt-base alloys also generally performed as shown for the aluminized AISI 310 and IN-671 specimen. 

Zirconium has corrosion rates of less than 0.127 mm/year. Zirconium is totally immune to attack by hydrochloric acid at all concentrations and at temperatures well above boiling. Aeration has no effect, but oxidizing agents such as cupric or ferric ions may cause pitting. Zirconium also has excellent corrosion resistance to hydrobromic and hydroiodic acid. 

The crucial SO2 effect in zinc atmospheric corrosion has been corroborated through a large number of studies [4, 5, 11-13]. When the critical relative humidity is reached, SO2 adsorbed in the humidity layer that forms on zinc is transformed into sulfite and, subsequently, into sulfate. The hmnidity layer is acidified by the oxidation of SO2 to sulfate, which greatly accelerates the rate of corrosion. Haynie and Upham [14] proposed a simple linear function to correlate SO2 concentration with the corrosion rate of zinc. In subsequent investigations these equations have been adjusted and the eontribution of other variables has been analyzed [15-18]. 

Figure 9-7 shows the iron concentration of test solutions as a function of the HEDP concentration at the end of gravimetric measurements (24 h) (Kalman et al., 1994). The iron content was determined separately in dissolved and precipitated (rust) form. The variation of total iron content is in accordance with the corrosion rate of carbon steel, showing a minimum curve. At lower HEDP concentrations, high amounts of iron exist in the form of rust, indicating a corrosion protection effect of the oxide layer on the carbon steel surface. At higher concentrations (c> 10 M), HEDP keeps the total amount of iron in the solution phase. These results indicate that complex formation between HEDP and iron ions has an important effect on the inhibition effect of HEDP. The... 

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