Measuring Critical Pitting Temperatures Of Stainless Steels By Electrochemical Noise Technology

Pitting corrosion can cause serious harms to the use of stainless steels as a major formof the stainless steels’ corrosion. So pitting corrosion possesses wide concern in the field ofstainless steels’ corrosion researches. Critical pitting temperature (CPT) is widely applied as amajor parameter of stainless steels’ pitting resistance ability for its fast and quantitativemeasurement. Currently, DC electrochemical measurement and electrochemical noisetechnique are used to test CPT. DC electrochemical measurement includes Potentiodynamicmeasurement and Potentiostatic polarisation measurement. The advantages of the DCmeasurement is fast, high accuracy and widely accepted, which has brought into ASTMstandard. whereas, these methods must apply potential to the material, which is inapplicablefor corrosion monitoring under actual environmental conditions. On the contrary, theelectrochemical noise technique is a nondestructive and non-interference electrochemicalmeasurement. So people try to use electrochemical noise technology for the measurement ofcritical pitting temperature of materials. However, some electrochemical noise measurements’accuracy is low because of the lack of clear physical meaning in criterion. This paper, fromthe physical meaning of pitting occurrence, has proposed a new experimental criterion in howto define the CPT under electrochemical noise test conditions, which is based on theelectrochemical noise tests of304stainless steels. That is, Using Arrhenius formula toanalyze noise data, and then defining the transition point of activation energy from negativeto positive as the critical pitting temperature of the metal. Prior to this transition point, theactivation energy is positive and the reaction is non-spontaneous. During this period, themetastable pitting and the material’s repassivation take place at the same time, whenrepassivation process is predominant. As a result, pitting development is inhibited. After thecritial point, the activation energy is negative and the reaction is spontaneous, during whichthe stable pittings’ autocatalytic reactions dominate and the stable pittings generate andcontinue to grow up. In order to validate the applicability and accuracy of this criterion,several tests of critical pitting temperature under three typical corrosion conditions wereperformed respectively, they are:(1)CPT of2506duplex stainless steel in0.5%NaClsolutions.(2)CPT of2506duplex stainless steel in0.1M NaCl at various molybdateconcentrations.(3)CPT of2506duplex stainless steel with different surface roughness.(4) CPT of2506duplex stainless steels which sensitive at650℃for different time. Resultssuggest that, the results of using new criterion on analyzing data are similar to the results ofexternal potentiostatic method which describes the changes of the corrosion current densitywith temperature scans. Thus, A conclusion can be obtained that this criterion can be appliedinto the evaluation of CPT of materials effectively.In the final chapter, we preliminary explored the influence to the CPT of stainless steelby temperature cycling state, the test results showed that with the increasing of thetemperature cycling unmber, the passive flim’properties increased, the CPT rised, but thepitting number and depth increased gradually.