Acceptable corrosion rates

Extensive field experience has shown the 50 Cr/50 Ni and 60 Cr/40 Ni alloys to offer the best answer to controlling fuel oil ash corrosion. Type 446 stainless steel also shows acceptable corrosion rates but must be used judiciously due to its low strength at elevated temperatures and weldability. Since components of 50 Cr/50 Ni in contact with vanadium-sodium fuel ash melts still suffer high corrosion rates, they should be designed to minimize the amount of surface area available where ash may accumulate. 

Also Dow Fire and Explosion Index (1987) considers corrosion risks, but the penalties are given through unacceptable corrosion rates. Design standards also include advice of acceptable corrosion rates (Uhlig and Reviev, 1985). 

Despite the problems in specifying and quantifying measurable and acceptable corrosion rates, some starting point is nevertheless called for. 

Onuki et al. have screened a number of materials in an acid mixture of H2SO4 (50 wt%) and HI (0.1 wt%) at temperatures up to 120°C (table 4.7). This simulates the composition of the upper liquid phase. They found Ta, Zr, Pb, and quartz glass to be corrosion resistant in this acid complex, whereas common construction material such as stainless steel and Hastelloy did not possess acceptable corrosion rates. PFA (Teflon) also showed satisfactory corrosion performance, but I2 absorption by perfluoroalkoxy or PFA has been observed, which raises questions about its longterm viability. 

In phosphoric acid, common ferritic Cr steels have very limited resistance and only show acceptable corrosion rates at... 

The corrosion rates determined in the various tests will vary greatly depending on the alloy being evaluated. Acceptance corrosion rates are not specified in the ASTM test procedures. They must be agreed upon by the supplier and user. Examples of typical acceptable corrosion rates determined for various alloys in ASTM G 28, Practice A are listed in Table 2 [3]. Due to test and material variations, the maximum acceptable rate will be somewhat higher than this nomin2d number. 

Use of these tests requires work and communication between user and producer to establish the maximum acceptable corrosion rate. In one example of this process for austenitic stainless steels, hundreds of specimens representing several product forms were evaluated in ASTM A 262 tests to compare actual corrosion rates with the specified maximum rate [13]. 

For some chemietd processes, low levels of metal ion contaminants may edter the product and/or chemical reactions. One source of contamination could be the corrosion reaction. Even if a material of construction has an acceptable corrosion rate to meet mechanical integrity requirements, low levels of met2d ions introduced into the system by corrosion may not be acceptable. 

Use 1 mpy as the acceptable rate for product purity. The acceptable corrosion rate is usually determined by purity requirements. Thus, for greater accuracy, back-calculate the maximum allowable corrosion rate from the maximum amount of metallic ion contamination allowed in the product. Estimate the avEiilable area of metal in the process equipment emd piping that contributes to corrosion and factor it into the corrosion rate. 

On a tonnage basis carbon and low-alloy steels are the most widely used engineering materials. An acceptable corrosion rate for... 

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