The Role of Mo Addition in the Improvement of Corrosion Resistance of Weathering Steel Under a Wet-Dry Cyclic Condition
Authors: Sugawara Yu , Okamura Yu , Murase Masatsugu 
Published: December 2020

It is known that Mo improves corrosion resistance of weathering steels in atmospheric corrosion environments.1 Mo dissolves as MoO4
2- in neutral solutions. If MoO4
2- is adsorbed to the surface of rust particles, it is expected that Mo-containing rusts are selectively permeable to cations and inhibit the permeation of chloride ions into the steel matrix.2 However, there is little report on the role of Mo addition in the improvement of the corrosion resistance of weathering steels. In this study, solution chemistry of electrolytes permeating through rust layers of a Mo-added weathering steel was investigated. In addition, a model interface between Mo-containing rusts and the steel matrix was fabricated, and its corrosion behavior under a wet-dry cyclic corrosion test was observed. From the results obtained in this study, the beneficial effect of Mo on the corrosion resistance of weathering steels was discussed.
Commercial-type weathering steel (WS: Fe-0.5Cr-0.3Cu-0.2Ni) and 0.5 mass% Mo-added weathering steel (Mo-WS: Fe-0.3Cu-0.5Mo) sheets were used in this study. All the steel sheets were polished up to 100–150 µm in thickness with SiC papers. We conducted a wet-dry cyclic corrosion test based on ISO 16539 Method A. The dew point was kept at 301 K. An electrolyte layer of 1.0 mm in thickness was formed every cycles using 0.18 M NaCl. In order to examine the solution chemistry of the electrolyte permeating through rust layers, a model specimen simulating a rust/steel interface was fabricated. The steel sheets were placed on a silica glass. A pH test paper or a filter paper was also placed between the steel sheet and the silica glass. Those were fixed by a masking tape and an epoxy resin. Using the specimen, we conducted the cyclic corrosion test to form rust layers. When a penetration hole through the steel sheet was formed by rust growth, the permeating electrolytes were trapped by the pH test paper or the filter paper. The pH of the electrolytes was evaluated by the color change of the pH test paper. The electrolytes absorbed in the filter paper was extracted to pure water, and Cl- concentration was measured by ion chromatography. In addition, we observed the corrosion behavior of the steel sheets in the model specimen simulating the rust/steel interface without pH test and filter papers. When a penetration hole through the steel sheet was formed under the wet-dry cyclic corrosion test, the steel sheet in the back side corroded by the permeating electrolytes. The corrosion behavior of the steel sheet in the back side can be regarded as that in the rust/steel interface.
After 10–15cycles of the corrosion test, the penetration holes were observed from the back side of the model specimens with both WS and Mo-WS. In both cases, the pH of the electrolytes was about 1. The Cl- concentration for Mo-WS was approximately 1.0 M, one-third of that for WS. From the EDS analysis for the cross section of the rust/steel interface of Mo-WS, the signal of Mo was distributed in the whole rust layers. It is confirmed that Mo-containing rust layers suppressed the permeation of Cl- into the rust/steel interface. Black and brown rusts were observed from the WS sheet in the model specimen immediately after the penetration by rust growth. Raman spectroscopic analysis showed that the black rust was Fe3O4, and the brown rusts were β-FeOOH, which is often detected from rust layers formed in high chloride environments. In the case of Mo-WS, only black rusts (Fe3O4) were observed. This result also supported the inhibition of the Cl- permeation through Mo-containing rust layers. It is thought that the Mo addition suppressed the enrichment of Cl- and the formation of highly reducible β-FeOOH at the rust/steel interface, leading to the decrease in the corrosion rate.

Reference:

1) H. E. Townsend, Corrosion, 57, 497(2001).
2) N. Sato, in Passivity of Metals and Semiconductors, Proceedings of the Eighth International Symposium, M. B. Ives, J. L. Luo, and J. R. Rodda, Editors, P. 283, The Electrochemical Society, Pennington, NJ (2001).