Hydrogen Entry into an AISI 4135 High Strength Steel in Tribocorrosion Environment

Abstract

High strength steels are susceptible to hydrogen embrittlement which decreases long-term reliability. Suspension coil springs for automobile are made of high strength steel and have high tensile strength. The end turns of springs are considered to be suffered from both corrosion and wear and the synergistic effect of them, tribocorrosion. To examine the possible enhancement of hydrogen entry by tribocorrosion, we recently investigated hydrogen entry in a simulated tribocorrosion environment, and found that the amount of entered hydrogen in tribocorrosion environment was larger than that in simple corrosion environment in a NaCl solution. For further understanding of the role of wear and corrosion on hydrogen entry into steels, hydrogen entry caused by tribocorrosion and in corrosion in a low pH solution has been investigated in this study. The effects of low pH solution, corrosion and wear on the amount of hydrogen entry will be discussed in our presentation. Tribocorrosion test was conducted using samples of an AISI 4135 with 1300 MPa of tensile strength with 20 mm diameter and 5 mm thickness. In the tribocorrosion test, pin-on-disk type wear apparatus equipped with a cell containing corrosive solutions was used and the sample was worn by a SiC ball with 10 mm diameter in a NaCl solution or a NaCl+H2SO4 solution for 48 h. The rotation diameter of SiC ball was 10 mm and the rotation speed was 150 rpm. The tests were conducted at room temperature. For comparison, corrosion test without wear was also conducted by samples immersion in the NaCl solution or the NaCl+H2SO4solution. Changes in pH of solution were measured during the test. After tribocorrosion test and corrosion test, the amount of hydrogen in the sample was measured by means of hydrogen thermal desorption analysis. The result of hydrogen thermal desorption analysis showed that the amount of entered hydrogen in the worn sample was lower than that of the immersed sample in the acidic NaCl+H2SO4 solution, though the amount of entered hydrogen in the worn sample was larger than that of the immersed sample in the neutral NaCl solution as mentioned above. This is because that pH of the acidic NaCl+H2SO4 solution during the tribocorrosion test increased with time.