Influence of Phosphorus on Initial and Long-Term Atmospheric Corrosion Behavior of Steel Materials
Authors: Katayama Hideki , Hayama Chihiro , Kadowaki Mariko , Murase Yoshiharu , Hara Toru , Hara Yuka , Tsutsumi Yusuke , Watanabe Hikari , Shitanda Isao , Itagaki Masayuki 
Published: December 2024

Phosphorus is contained as an impurity in the manufacturing process of steel materials, and it is known to be difficult to remove it completely, and it is also thought to influence the corrosion resistance of steel materials. However, the effect of phosphorus on corrosion resistance has not been clarified, because the steel materials used in previous research reports also contain copper and chromium. The aim of the present study was to systematically examine the influence of phosphorus on the corrosion behavior from early to long-term.

Fe-P binary alloys containing 0.5, 1.0 and 1.5 mass% P were used as the samples. The sample grinded with SiC and diamond paste were prepared for electrochemical measurements. Anodic polarization measurements were performed in 25℃ of boric acid-borate buffer solution (pH 8.0) containing 10 mM NaCl in a non-deaerated environment. Platinum was used as the counter electrode and an Ag/AgCl electrode (3.33 M KCl) as the reference electrode. Before the polarization measurements, the immersion potential was measured for 10 min. and then cathodically treated at -1.0 V for 10 min. The sweep rate was 20 mV/min. In addition, the surface was observed by SEM and EDS after polarization at 0.2 V for 80 s to investigate the starting point of localized corrosion of the Fe-1.5 mass% P.

EDS analysis was performed on all sample surfaces before electrochemical measurement. The distribution of phosphorus concentration was not observed on Fe-0.5 mass% P, whereas horizontal banded macro-segregation of phosphorus was observed on Fe-1.0 mass% P and Fe-1.5 mass% P. Furthermore, as a result of EDS analysis with a higher magnification, intergranular segregation of phosphorus was observed only on Fe-1.5 mass% P.

In the anodic polarization curves of all samples, a peak was observed in the potential range of about -0.66 to -0.39 V, which could be attributed to the active dissolution of the sample. No increase of the current density was observed in the lower potential range than about 0.05 V, suggesting that the sample surface was passivated in this potential range. On the other hand, the current value increased significantly in the potential range higher than 0.05 V, due to the occurrence of localized corrosion. The current density of Fe-1.5 mass% P increased at lower potentials than for the other samples.

Polarization measurement of Fe-1.5 mass% P was conducted at a constant potential to investigate the starting point of localized corrosion. A rapid increase in anode current was observed after approximately 10 seconds. SEM observations and EDS analysis were conducted on the sample surface after the test. A number of localized corrosion of about 5 µm in size was observed on the sample surface after the test. EDS analysis showed that higher concentrations of Si and Mn were detected in the corroded areas. In addition, localized corrosion with an elongated shape was also observed on the Fe-1.5 mass% P surface. Since no alloying elements (Mn and Si) derived from inclusions were detected in the corroded areas, this localized corrosion is considered to be due to intergranular segregation of P.

The exposure test of all samples was conducted at Miyakojima Island, Japan for 5 years. The corrosion loss of Fe-1.5 mass% P was the smallest compared to the other samples, suggesting that the higher the P content, the lower the corrosion loss. EPMA analysis was conducted for the cross-section of the rust layer of samples after long-term exposure. P was partly concentrated in the rust layer of Fe-0.5 mass% P, but Cl penetrated into the rust layer and reached the steel substrate. On the other hand, P was fully enriched in the rust layer of Fe-1.5 mass% P and Cl was present only on the surface of the rust layer. The protective function of the rust layer due to phosphorus enrichment may have contributed to the long-term improvement of corrosion resistance.