Materials Performance

MAY 2015

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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11 NACE INTERNATIONAL: VOL. 54, NO. 5 MATERIALS PERFORMANCE MAY 2015 Continued on page 12 acids, quaternary amines, etc. In many cases, a combination of chemical types is used. It all depends on the application. An injection quill equipped with an atomizer is the typical application method. This re- quires a special type of setup to do it right. A: Be careful when selecting a test method. In your case, you want a chemical that will partition from the hydrocarbon phase into the water phase. We have used a test procedure that involved contacting a 50/50 hydrocarbon/ water mixture with inhibitor. After agita- tion and settling, you drain of the water and then use it for the test. Tis will ensure that the inhibitor partitions to the water phase. In many cases, the wrong inhibitor was selected and too much chemical stayed in the oil phase, resulting in severe bottom-of-pipe corrosion. In gas lines, gunking is a major concern. Tere is a test to evaluate the efect of solvent evaporation and the potential to leave a sticky gunk in the line. Tere are several companies that do this ty pe of testing. Materials of construction for mixed acids Q: What are the recommended materials of construction for nitrating mixed acid streams such as the following examples? • Nitric acid (HNO 3 ) (30 to 50% concentration) + sulfuric acid (H 2 SO 4 ) (50 to 70% concentration) • HNO 3 (50 to 85% concentration) + H 2 SO 4 (15 to 50% concentration) There is no water in either of these products. I believe Ty pe 304L stainless steel (SS) (UNS S30403) and Ty pe 316L SS (UNS S31603) have been used. I would also like to know what the preferred material of construction is and the expected longevity of using these ty pes of SS for storing or transporting these two products. A: Te corrosion of SS in HNO 3 depends on the concentration of the two acids and, of course, the tempera- ture. Te corrosion rate of Ty pe 300 series SS would vary within the wide range of compositions that you quote, depending also on temperature. For example, in your second range of compositions, Ty pe 316L SS corrodes ~0.04 mm/y in 80% HNO 3 / 20% H 2 SO 4 at 70 °C, and 0.20 mm/y in 53% HNO 3 /47% H 2 SO 4 at 90 °C. Tere is also likely to be increased attack in the vapor phase of these mixed acids. In the vapor phase of the two acids mentioned above, the corrosion rates were determined to be ~3 and 10 mm/y, respectively. In many mixed acid applications, Ty pe 300 series SS are used successfully, with the Ty pe 304L SS grades being preferred for mixtures with bigger concentrations of HNO 3 . You would need to better def ine your range of acid

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