The Effect Of Nitric Passivation On Corrosion Resistance Of ZG06Cr13Ni4Mo Low-carbon Martensitic Stainless Steel

Compared with traditional martensitic stainless steels,low-carbon martensitic stainless steels achieve excellent strength,good corrosion resistance,and weldability by reducing the content of carbon and impurity elements.Also,the matched toughness is obtained due to the complex microstructure consisting of lath martensite and diffused austenite.Owing to its cost-effectiveness and good mechanical properties,low-carbon martensitic stainless steel has huge application prospects in oil exploitation and offshore facilities.However,it exhibits corrosion resistance intermediate to those of carbon steel and duplex stainless steel since the main component lath martensite phase in martensitic stainless steel has a lower corrosion potential than ferrite and austenite,and the carbide particles generated at the grain boundary cause the surrounding area to be Cr-depleted.For severely corrosive environments such as the marine atmosphere,the corrosion resistance needs to be further improved.Nitric acid passivation,as a widely used stainless steel protection method,has the advantages of low cost,simple process and wide application range.Based on this,this paper studied the effect of nitric acid passivation on the short-term and long-term corrosion behavior of ZG06Cr13Ni4Mo stainless steel.Furthermore,an attempt was made to explain the phenomena discovered in the passivation process through the improved Point Defect Model.Results indicated that the general corrosion resistance of ZG06Cr13Ni4Mo stainless steel after acid passivation was much higher than that of the untreated sample.Also,the pitting potential increased after acid passivation.The passive films formed on stainless steel consist a hydroxide cover layer and an oxide inner layer.The thickness of the passive film on acid-passivated sample was 3-4 times that of the untreated.The dramatic increase in the corrosion resistance of the passivated sample was due to the formation of a thicker passive film fewer defects,rather than by Cr enrichment in the passive film.The corrosion resistance of acid-passivated stainless steel was still better than that of the untreated sample after 50 days of salt spray/drying cycle accelerated test.The rust layer formed on the surface of the samples cannot provide significant protection for the metal.The repairs of the passive films of the untreated and passivated samples occurred during the exposure.However,owing to the large thickness of the "native passive film",the passivated sample's passive film was partially dissolved and then repaired during the test.The existence of Cr2O3 in concentrated nitric acid is due to the kinetic equilibrium of growth and dissolution of passive film.Using the theoretical model,the variation rule of the thickness of the passive film was calculated.The steady-state thickness of the passive film depends on the oxidizability and pH of the nitric acid solution,as well as the applied potential.Normally,the thickness increases with the concentration of nitric acid,but excessively high concentration will increase the destruction rate of the passive film and reduce the thickness.The difference in thickness and composition of the passive film on different steels with the same passivation treatment is related to the intrinsic corrosion resistance of the metal.The metal with better corrosion resistance tends to form a thinner and denser passive film.