Corrosion Behavior Of Precipitated Hardened Stainless Steel In HCl Solution

At present,precipitation hardening stainless steel are widely used in various industries such as petrochemical,nuclear power plant,aerospace,etc.,and are generally used to manufacture key materials such as oil fracturing pumps,nuclear power plant pressurized water reactor valves,etc.However,the environment including acid corrosion and stress corrosion is harsh in practical applications.Therefore,it has higher performance requirements for stainless steel.In this paper,the polarization corrosion test and electrochemical impedance spectroscopy test were used to study the electrochemical corrosion characteristics of 15-5PH and 17-4PH stainless steels.Scanning electron microscope,transmission electron microscopy and X-ray photoelectron spectroscopy were used to analyze the matrix structure and rust layer morphology.The stress distribution of the C-ring was simulated by Abaqus,and the C-ring stress corrosion experiment was carried out with scanning electron microscope to analyze the stress-corrosion characteristics of 15-5PH and 17-4PH stainless steels.Research indicates:The 15-5PH stainless steel was soaked in HCl solution to observe the electrochemical reaction.As the increase of the solution concentration,the corrosion potential(E_corr)decreased,the corrosion current density(I_corr)increased,and the rust layer resistance(R_r)and charge transfer resistance(R_ct)gradually decreased.So that the corrosion resistance of the sample decreased.As the concentration of HCl solution increased from 1%to 5%,I_corr increased by 12.8%.However,when the concentration of HCl solution increased from 5%to 10%,E_corr increased by 78.8%.Hence,the electrochemical corrosion can be divided into two ranges of 1%-5%and 10%-20%,their reaction mechanism was different.The corrosion reaction rate of the sample was relatively slow when the HCl concentration was 1%-5%.The rust layer had a double-layer rust layer structure,whose inner layer was a dense thin layer and outer layer was a porous and loose thick layer.R_r and R_ct were large and the sample had well corrosion resistance,manifested by the combined effect of charge transfer and diffusion.The corrosion reaction rate of the sample was relatively fast when the HCl concentration was 10%-20%.The rust layer showed a single-layer structure,which was dense and thin.R_r and R_ct were relatively small and the reactions are controlled by charge transfer.The electrochemical reaction of 17-4PH stainless steel in different concentrations of HCl solution was similar to that of 15-5PH stainless steel.As the solution concentration increased,I_corr gradually increased,E_corr,R_r and R_ct decreased.When HCl increased from 5%to 10%,I_corr increased by 27.6%,the rust resistance decreased by24.5%,and R_ct decreased by 24.2%.When the concentration of HCl solution increased from 10%to 15%,R_r increased by 111.1%.R_r decreased by 55.7%,and R_ct decreased by 78.2%.The reaction mechanism of 17-4PH stainless steel in different concentrations of HCl solution was also divided into two ranges of 1%-10%and 15%-20%.Compare17-4PH stainless steel to 15-5PH stainless steel,in the same concentration of HCl solution,I_corr of 17-4PH stainless steel was smaller than that of 15-5PH stainless steel,E_corr,R_r and R_ct were same greater,and 17-4PH stainless steel had better corrosion resistance.The higher content of Cu and Cr in 17-4PH stainless steel was an important factor that resulted in better corrosion resistance of 17-4PH stainless steel.At the situation of 120 MPa stress and 20%concentration of HCl,15-5PH and17-4PH stainless steels still had no stress cracks after 432 h of stress corrosion.In the case of prefabricated cracks,the crack gaps didn't expand.In the same stress corrosion conditions,the C-ring specimen prefabricated crack notch corrosion of 17-4PH stainless steel was milder than 15-5PH stainless steel and 17-4PH stainless steel showed better stress corrosion resistance.Both of the two types of stainless steels stress corrosion were comprehensive corrosion,the surface of material exhibited a concave and convex shape after stress corrosion.Compared with 15-5PH stainless steel,17-4PH stainless steel had higher Cr and Cu content on the substrate surface after stress corrosion.