Heat recovery systems are used in the manufacture of sulfuric acid. Depending on the process available, sulfuric acid at concentrations of 93 to 99.5 weight% can be used. The trend towards higher levels of efficiency in the recovery of heat in sulfuric acid manufacturing is leading to higher temperatures in the absorption systems and hence to more severe demands on the corrosion resistance of metallic materials. Although there are several engineering and material concepts available, from a corrosion science and engineering point of view, it is still a challenge to better understand the corrosion mechanism and the role of the alloying elements in stainless steels and NiCrMo alloys. This paper contains an extended literature review covering numerous aspects of the complex corrosion system that we are faced with in such applications. Further on, the paper also addresses corrosion testing results, which may help to understand some observations people experienced in the past in this complicated matter. Finally, the paper also contains a short section covering the state of the art review of the most common materials for this technology.
In terms of quantity consumed, sulfuric acid is the most important industrial chemical. Sulfuric acid is the basis for a large number of chemical products and for many years its output was considered to be an index of a country's industrial importance. The acid is produced mainly in the concentrated form, that is to say at concentrations of 93% 1 or more. As sulfuric acid is only slightly corrosive at low temperatures and flow velocities, the use of unalloyed steel or cast iron materials for its transportation, storage and handling - these materials are used for this purpose almost to the exclusion of others - leads to few problems of corrosion.
In recent years new sulfuric acid handling techniques have been developed for the processes which ease the burden on the environment (desulfurization of combustion gases and the working-up of spent acid are examples) and for processes in which heat formed in production of the acid is recovered. Where the environment is concerned the concentrations of the acid are nearly always well below 90%. Hence the present study is devoted principally to the recovery of heat in the manufacture of sulfuric acid, where higher concentrations are encountered. The acid temperatures at which economic recovery of the energy formed in the various exothermic reactions is possible from absorber and dryer circuits, for example - are considerably higher than those at which sulfuric acid is stored. The resultant increase in the acid's corrosivity necessitates the use of stainless steels and nickel-based alloys. The trend towards even higher levels of efficiency has led to higher temperatures in absorption systems, thus placing correspondingly severer demands on the corrosion resistance of materials. All large manufacturers plant for the production or handling of sulfuric acid have devoted their attention to this problem and devised a number of solutions.
Although numerous heat recovery systems have been installed in existing sulfuric acid plants or incorporated at the outset in new plants, and despite the existence of individual publications on the problems connected with materials [1-11], the corrosion mechanism of stainless steels and nickel alloys in sulfuric acid has not yet been adequately explained.
The purpose of this study is to investigate the corrosion behavior of a number of FeCrNi(Mo) and NiCr(Mo) alloys in static and flowing sulfuric acid at concentrations of 90% to 100% at temperatures up to 150 o C. The results of the investigation will be evaluated to see whether the difficult problems i
