|Content Type||Conference Paper|
|Title||Effect of Test Solution Compositions on Corrosion Resistance of 13Cr Materials in a Little Amount of H2S Environment|
|Authors||Hisashi Amaya and Masakatsu Ueda, Sumitomo Metal Industries Ltd.|
|Source||CORROSION 99, April 25 - 30, 1999 , San Antonio, Tx|
|Copyright||1999. NACE International|
|Keywords||Martensitic stainless steel, Hydrogen sulfide, Corrosion resistance, pH, Buffer solution.|
The effect of the buffer solution composition on the corrosion resistance of super 13Cr martensitic stainless steels is studied. It is widely recognized that the SSC susceptibility of 13Cr stainless steels depends on the H2S partial pressure and pH in the solution. To control the pH value, buffered solutions are defined in NACE TMO177-96 (solution B) and EFC publications to simulate the actual field conditions in the laboratory tests under normal pressure. However, with some modifications of the buffer agents, there were some cases that the discoloring of the specimen was observed and the pitting occurred with the larger corrosion rate in some test conditions. Therefore, the corrosion resistance of super 13Cr stainless steels are considered to be influenced by the composition of the buffer solution. That is, the corrosion resistance of the steel depend on the concentration of CH~COONa/CHqCOOH included in the test solution as the buffer agent, even in the same pH. The super 13Cr steel was not passivated in the buffer solution including 0.4 wt?40 CH~COONa adjusted pH 3.5 with CHqCOOH recommended in NACE TMO177. Cr was detected in the solution, and it is considered that the CH~COO- ion enhanced the dissolution of the steel since it can form the complex ion with Cr and therefore it prevents the surface of the steels from passivation. It is suggested that the buffer solution with 0.04 W% CH~COONa with CH~COOH is appropriate solution for evaluating the corrosion resistance of super 13Cr steels because it has enough buffer power during the corrosion test, and has no effect on forming of the passivation film.
The field application of martensitic stainless steel containing 13?40Cr (AISI 420, hereafter referred to as conventional 13Cr) has been increasing due to its good corrosion resistance in COZ (sweet) environment . However, the SSC resistance of the conventional 13Cr stainless steel is insufficient in the COZ environment with HZS,and also it has the limitation of the localized corrosion resistance in C02 environment at elevated temperatures .
Recently, a number of new super 13Cr martensitic stainless steels with the higher resistance to general and localized corrosion in COZ environment at elevated temperatures and to SSC in the environment with a little amount of H$ at room temperature have been proposed by some researchers [3-6]. The super 13Cr steel has the stable passivation film even in the CO, environment with a little amount of HIS [6-7], while the conventional 13Cr can not be passivated if HZS is included in the corrosion environment . Because of these superior properties, the super 13Cr stainless steel has been widely applied for OCTG  and for flow line materials .
It is widely recognized that the SSC susceptibility of 13Cr stainless steels depends on the H2S partial pressure and pH in the solution. For this reason, the application limit of 13Cr steels is discussed in the test condition of pH-HzS combination [11-12]. To control the pH value, buffered solutions are defined in NACE TM0177-96 (solution B)  and EFC Publications  to simulate the actual field conditions in the laboratory tests under normal pressure, as summarized in Table 1. The acidification by C02/H# gas can be achieved in the pressurized test with autoclave, however, it is difficult to obtain the appropriate pH in the normal pressure test condition. Therefore, the buffered solution is applied in the normal pressure test to simulate the pH of the pressurized condition.
However, with the some modifications of the buffer agents, there were some cases that the discoloring of the specimen was observed and the pitting occurred with the larger corrosion rate in
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