Sulfur corrosion tests, high-temperature

Other than ex situ testing based on half-cell systems, in situ tests in full cells are conducted to further understand the durability issues for Pt-alloy based catalysts. Comparison of the results from different test systems can yield much useful information. Investigating the influence of carbon support corrosion, Pt dissolution, and sintering, Arico et al. (2008) evaluated the stability of Pt/C and Pt-Co/C in both a gas-fed sulfuric acid electrolyte half-cell at 75°C and a PEM fuel cell at 130°C. Results in sulfuric acid revealed the Pt-Co alloy to be more stable than Pt after cycling and more sensitive to carbon support corrosion. In high-temperature PEM fuel cell testing, Pt-Co/C showed smaller sintering effects than Pt/C. 

Free, or corrosive, sulfur in an appreciable amount could result in corrosive action on the metallic components of an appliance. Corrosive action is of particular significance in the case of pressure burner vaporizing tubes that operate at high temperatures. The usual test applied in this connection is the corrosion (copper strip) test (ASTM D-130, ASTM D-849, IP 154). 

Materials classes that were tested included ceramics, nickel-based and cobalt-based alloys, refractory metals and alloys, reactive metals and alloys, noble metals and alloys, and high-temperature polymers, a total of 26 materials. Test periods varied between 37.5 and 47.5 hours. None of the materials was found to be suitable for all test conditions, and most exhibited moderate (equivalent to between 10 and 200 mil per year) to severe (>2()0 mil per year) corrosion. Titanium and titanium alloys (Nb/Ti and Ti-21S) exhibited the best performance, showing only slight corrosion in the presence of excess sodium hydroxide. Under acidic conditions, titanium showed increased rates of corrosion, apparently from attack by sulfuric acid and hydrochloric acid. Both localized pitting and wall thinning were observed. 

Based on the results of corrosion tests in a sulfuric acid solution, low-temperature nitrided samples showed higher corrosion resistance than those nitrided at high temperature in a 5 wt % sulfuric acid solution. In addition, as shown in Figure 7.1.15, the interfacial contact resistance (ICR) of nitrided SUS316 showed a lower value than that of an un-nitrided sample. 

Coal ash corrosion is a widespread problem for superheater and reheater tubes in coal fired power plants that bum high-sulfur coals. The accelerated corrosion is caused by liquid sulfates on the surface of the metal beneath an over-lying ash deposit. Coal ash corrosion is very severe between 540 and 740°C (1000°F and 1364°F) because of the formation of molten alkali iron-trisulfate. Considerable work has been done to predict corrosion rates based on the nature of the coal (its sulfur and ash content). This was accomplished by the exposure of various alloys to synthetic ash mixtures and synthetic flue gases. The corrosion rates of various alloys were repotted in the form of iso-corrosion curves for various sulfur dioxide, alkali sulfiite, and temperature combinations. An equation was developed to predict corrosion rates for selected alloys from details of the nature of ash by analyzing deposits removed from steam generator tubes and from test probes installed in a boiler [33]. Then laboratory tests were conducted using coupons of various tdloys coated with synthetic coal ash that was exposed to simulated combustion gas atmospheres. 

T e corrosion reaches a maximum at about 80% acid concentration. Above that concentration, attack decreases rapidly until at 98% it becomes mild, less than 5 mpy. In other laboratory tests, fuming acids containing 101, 103, 107 and 115% sulfuric acid caused moderate attack of 3003 alloy at ambient temperature. Aluminum alloy heat exchangers, piping and tanks have been used to handle sulfuric acid in 98% concentrations and at temperatures as high as 200°C (392"F). See also Ref (1) p. 145, (2) p. 811,(3) pp. 22. 41. (4) pp. 18, 19, 29. 30, 31, 34, 74. 96. 97, (7) p. 177. 

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