This paper describes a new aspect on evaluation of hydrogen embrittlement susceptibility and its mechanism of austenitic stainless steels and pure titanium on the basis of the results already reported, which were mainly obtained by using a constant load method under an open circuit condition. In addition, a criterion to judge whether the failure of the materials takes place by stress corrosion cracking (SCC) or hydrogen embrittlement (HE) is also described for the constant load method.
Notch tensile strengths (NTSs) of circumferentially notched round bar specimens of high strength steels were measured after hydrogen precharging using SSRT, and power law relationship between NTS and hydrogen concentration was found. The occurrence of delayed fracture during constant load test of atmospherically corroded specimens corresponded to the balance between critical hydrogen concentration for delayed fracture (HC) and hydrogen concentration from the environment (HE). This fact suggests that the delayed fracture of actual high strength bolt also occurs when HE exceeds HC. Delayed fracture properties of high strength steels were successfully evaluated using SSRT after atmospheric corrosion to take into consideration the hydrogen entry caused by atmospheric corrosion. The ultrahigh strength steel containing hydrogen traps possesses relatively high HC. However, the hydrogen traps resulted in an increase of HE, so that the resistance to delayed fracture was not improved. Hydrogen entry under cyclic corrosion test monitored by electrochemical hydrogen permeation test indicated an increase of hydrogen entry with time probably because of enhancement of hydrogen entry caused by the changes of pH of inner rust layer and potential. It is suggested that the “delay” of delayed fracture is the time required for the enhancement of the hydrogen entry efficiency.
Hydrogen embrittlement phenomena and recent developments of low alloy steel high strength Oil Country Tubular Goods (OCTG) and linepipes are reviewed. In oil and gas fields, Sulfide Stress Cracking (SSC), which is caused by hydrogen absorption from H2S containing (sour) environments and applied tensile stress, is a major problem for high strength OCTG. Various metallurgical improvements had been tried to improve SSC resistance of high strength OCTG. In 1990's, 110 ksi (YS 758 MPa) grade sour resistant OCTG was developed by grain refinement and reduction in segregation elements such as P, S and Mn. In 2000's, 125 ksi (YS 862 MPa) grade sour resistant high strength OCTG was developed by inclusion refinement, decrease in dislocation density and control of carbides morphologies. Hydrogen Induced Cracking (HIC), which occurs in both high strength and low strength steels without applied stress, is a major problem for linepipes. HIC occurs and propagates along elongated inclusions such as MnS or center segregated hardened portion of steel plate. 65 ksi (YS 448 MPa) grade sour resistant linepipes are widely commercialized by inclusion spheroidization and prevention of segregation through Thermo Mechanical Controlled Process (TMCP). Furthermore, 70 ksi (YS 483 MPa) grade sour resistant linepipes are developed by a progress in above mentioned techniques recently.
Low Carbon-13%Cr martensitic stainless steels have been widely used for line pipe application because of their high strength and excellent corrosion resistance in corrosive conditions. Recently, both laboratory and field experiences related to cracking near fusion line of these steels weld joints in hot acid environments have been published. In this paper, SCC (Stress Corrosion Cracking) mechanism near fusion line of low C-13%Cr welded joints is discussed. Especially, initiation process is focused. Mainly electrochemical measurement using solution flow type micro-droplet cell and surface analysis of weld joints are conducted. In the evaluation of electrochemical behavior of low C-13%Cr welded joints with and without PWHT (Post Welded Heat Treatment) by the use of a solution flow type micro-droplet cell, PWHT leads to more noble and stable potential in HAZ (Heat Affected Zone) compared to as-welded condition. In the HAZ portion of the as-welded joint, Cr depleted layer was detected under welding scale with the use of GDS (Glow Discharge Spectrometer). PWHT was confirmed to eliminate Cr depleted layer under welding scale. This can be an initiation of SCC near girth welded joint in hot acid environment. Finally, mechanism of SCC initiation near fusion line of as-welded joint was proposed. Localized corrosion would start at Cr depleted layer under welding oxide scale. This dissolution was also accelerated by a galvanic effect due to a large cathode area of base metal.
Many kinds of corrosion testing methods are being used to evaluate corrosion resistance of coated steel sheets for automobiles. In this study, perforation corrosion behavior of Zn coated steel sheets analyzed in various corrosion testing was compared with that of actual vehicles used in North America. It was found that the influence of weight and type of coating on perforation corrosion behavior changed depend on the conditions of combined cyclic corrosion tests. In order to establish practicability of CCTs, our study focused on corrosion rate ratio of steel to Zn coating in the perforation corrosion process to introduce a new index of PCI (Perforation Corrosion Index). The corrosion behavior in CCTs and actual vehicles can be correlated using the PCI values and perforation corrosion resistance of zinc-alloy coated steel in actual vehicles can be predicted based on the tendency of corrosion resistance as a function of PCI. It is also important to make the lapped specimen with adequate clearance and bare area so that the corrosion mechanism in CCTs be equivalent with that in actual environment. It is expected that coated steel sheets will be developed appropriately and efficiently applying this corrosion prediction method.