Determination of Corrosion Rates in Hot, Concentrated Sulfuric Acid

• Published in: Corrosion (Houston, Tex.) Tex.), 2003-04, Vol.59 (4), p.363-370
• Main Authors: Rodda, J.R., Ives, M.B.
• Format: Article
• Language: English
• Subjects: Applied sciences; Corrosion; Corrosion environments; Corrosion products; Corrosion rate; Exact sciences and technology; Metals. Metallurgy; Stainless steel; Stainless steels; Sulfuric acid; Sulphuric acid
• ISSN: 0010-9312; 1938-159X
• DOI: 10.5006/1.3277569

ABSTRACTThe commercial success of the application of stainless steels (SS) in sulfuric acid (H2SO4) production has been achieved through the progressive development of new steel grades by specialty steel producers. Evaluating the performance of new steels in the laboratory, in advance of their commercial application, requires accurate and relevant measurements. There have been cases where laboratory corrosion rate determinations predicted successful use of certain alloys, but costly "plant scale" experiments sub(1) UNS numbers are listed in Metals and Alloys in the Unified Numbering System, published by the Society of Automotive Engineers (SAE) and cosponsored by ASTM.sequently proved that they were inaccurate. Unfortunately, most of these negative experiences fail to get published and the mistakes are repeated. One example is the use of UNS S31008(1) SS in the H2SO4 plant absorber acid system, involving H2SO4 in the range from ~98.2% to 99.5% H2SO4, with temperatures from ~80°C in the conventional acid plant, up to 200°C in heat recovery applications. For this application, corrosion rate tests were conducted by sealing small coupons of SS alloys in small volumes of plant acid, which was contaminated with corrosion products, and heating these for a week in a controlled temperature furnace.1-2 Those results, as shown by this work, are skewed significantly by the corrosion products. Regrettably, the results of these tests have slipped into reference publications without any real consideration as to the method used to obtain them. The measurement of corrosion rates of (nickelcontaining) SS in hot, concentrated H2SO4 is complicated further by a characteristic potential cycling of the alloys when left at "open-circuit" conditions.3 This obviates the use of electrochemical measurements to determine corrosion rates so that weight loss becomes the most reliable method. The alloy producers and plant designers also find it more expedient to use simple weight loss as a readily demonstrable means of qualifying new alloys. Any laboratory program for determination of corrosion rate in H2SO4, and indeed in any other corrosive chemical, needs to consider the influence of several factors on corrosion rates. For metals that exhibit active-passive behavior, where the potential