Oil refinery hydrotreating and hydrodesulfurization are high temperature processes which can cause sensitization and/or reduced ductility in some materials of construction, while the presence of sulfur and other impurities in these processes can lead to various corrosion mechanisms. Alloy 825 (UNS N08825) is often used in this demanding application. The effects of long term elevated temperature exposure and welding on the mechanical properties, microstructure and corrosion resistance of this nickel base alloy have been investigated.
Austenitic stainless steels and high nickel alloys are used in refinery and petrochemical applications where superior corrosion resistance and high temperature strength are required. INCOLOY alloy 825 (UNS N08825) has been used in these types of applications for decades and continues to see an increase in usage. Resistance to corrosion plays an important role in selecting construction materials. Temperature, in turn, plays an important role in the severity of many corrosive environments. Materials intended to resist high temperature hydrogen sulfide in refineries include a range from carbon steel to high nickel alloys. The chemical compositions of the materials discussed in this paper are listed in Table 1. Severe problem areas for this type of corrosion include applications such as heater tubes and transfer lines, piping exchangers and air coolers. These applications require a combination of tube, pipe, plate, sheet, bar, forging stock, and welding products.
Carbon steel is commonly used for temperatures up to about 500°F(260°C) , and then materials such as 5Cr-0.5Mo (UNS K41545)and 9Cr– 1Mo (UNS K90941) are substituted to provide better resistance to hydrogen sulfide attack.1,2 Even these materials suffer from heavy corrosion and metal wastage as temperatures, pressures and sulfide concentrations are increased. The environmental mechanism involves the reaction of the sulfide with the iron in the steels to form iron sulfide. This gives rise to heavy metal wastage and scaling.3 The scaling causes two detrimental results. First is the reduction of material thickness; and second is the fouling and plugging of lines from the material which spalled off and entered the process stream, McCoy4 reported the effects of this type of corrosion on 9Cr-lMo and showed the increased corrosion resistance of the austenitic stainless steels and nickel alloys. The results listed in Table 2 show that the austenitic stainless steels have a substantial reduction in corrosion rate as compared to 9Cr-lMo Steel. Alloy 800 (UNS N08800) has a slight increase in corrosion resistance over the austenitic stainless steels in this service.
The polythionic acid (PTA) and chloride stress corrosion cracking (SCC) resistance of several stainless steels and high nickel alloys were examined. Sheet and heavy section plate and welding products used to join these materials were evaluated for their resistance to these conditions . A long time aging study was also conducted on alloy 825 unwelded tubing to determine effects on hardness, microstructure and corrosion properties.
During the evolution of hydrotreating and hydrodesulfurization processes a number of steels and alloys have been used with varying degrees of success.
