Effects of Sodium Phosphate and Sodium Nitrite on the Pitting Corrosion Process of X70 Carbon Steel in Sodium Chloride Solution

Yongyan Zhu,Jianli Zhang,Liang Li,Jiayi Ding

doi:10.3390/ma13235392

Abstract

In this paper, effects of sodium phosphate (Na3PO4) and sodium nitrite (NaNO2) on the pitting corrosion of X70 carbon steel in 0.10 mol/L NaCl solution were investigated by potentiodynamic polarization technique, electrochemical impedance spectroscopy (EIS) method, scanning electron microscope (SEM) and scanning electrochemical microscope (SECM). The SECM equipment was used to observe the dynamic processes of the pitting corrosion in situ. Na3PO4 or NaNO2 in the sodium chloride solution decreased the local anodic dissolution and increased the pitting resistance of the specimen. By analysis and comparison, it can be concluded that the inhibition effect of Na3PO4 is mainly due to the formation of a salt film, while the corrosion inhibition of NaNO2 is principally attributed to a protective oxide film on the electrode surface.

Highlights

When X70 carbon steel is in a corrosive environment, passive film on its surface breaks down at local points
When the concentration of Na3 PO4 increased to 0.080 mol/L, current peak did not emerge until at the 40th min (D4). These results clearly showed that as the concentration of Na3 PO4 increased, the number and the extent of the pits decreased obviously, and the induced time of the pitting corrosion increased distinctly
The results obtained by both the scanning electrochemical microscope (SECM) and scanning electron microscope (SEM) measurements show that Na3 PO4 and NaNO2 are both good inhibitors for X70 carbon steel in 0.10 mol/L NaCl solution

Summary

Introduction

When X70 carbon steel is in a corrosive environment, passive film on its surface breaks down at local points. It has been found that some inorganic anions can inhibit pitting corrosion of metals or alloys in corrosive environment. The inhibition effects and mechanisms of the anions have been studied by methods of weight loss and electrochemical measurements for more than 60 years [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]. Sodium phosphate (Na3 PO4 ) is usually considered as an effective corrosion inhibitor for metals in the corrosive environments containing

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Discussion

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