Degradation in pitting resistance of 316L stainless steel under hydrostatic pressure

Unexpected iron corrosion by excess sodium in two-dimensional Na-Cl crystals of abnormal stoichiometries at ambient conditions

At ambient conditions, we found salt crystals formed from unsaturated solutions on an iron surface; these salt crystals had abnormal stoichiometries (i.e. Na₂Cl and Na₃Cl), and these abnormal crystals with Cl:Na of 1/2-1/3 could enhance iron corrosion. Interestingly, we found that the ratio of abnormal crystals, Na₂Cl or Na₃Cl, with ordinary NaCl was relative to the initial NaCl concentration of the solution. Theoretical calculations suggest that this abnormal crystallisation behaviour is attributed to the different adsorption energy curves between Cl^--iron and Na⁺-iron, which not only promotes Na⁺ and Cl^- adsorbing on the metallic surface to crystallise at unsaturated concentration but also induces the formation of abnormal stoichiometries of Na-Cl crystals for different kinetic adsorptionprocess. These abnormal crystals could also be observed on other metallic surfaces, such as copper. Our findings will help elucidate some fundamental physical and chemical views, including metal corrosion, crystallisation and electrochemical reactions.

Effects of Temperature and Pressure on Corrosion Behavior of HVOF-Sprayed Fe-Based Amorphous Coating on the Mg-RE Alloy for Dissolvable Plugging Tools

To retard the degradation of the magnesium alloys for dissolvable ball seats, Fe-based amorphous coatings were deposited on dissolvable Mg-RE alloy substrates using high velocity oxygen-fuel spraying technology. The results show that the Fe-based amorphous coatings possess low porosity (0.82%) and high amorphous contents (91.4%) and their corrosion resistance decreases with the increase of temperature or pressure. However, with the help of Fe-based amorphous coatings, the degradation time of dissolvable Mg-RE alloy has been significantly prolonged. In particular, the service life of coated Mg-RE alloy exceeds 360 h at temperatures below 50 °C and reaches 87 h at 120 °C and 80 atm. Under high temperature and high pressure, the compactness of passive films decreases and the chemical activities of ions and metal elements increase, leading to the degradation of corrosion resistance of Fe-based amorphous coatings. In long-term corrosion, the crystallized splats are prone to corrosion because of the multiphase structures. The corroded crystallized splats are connected to the inevitable pores by the corroded intersplat regions, resulting in the formation of corrosion channels and the corrosion failure of coatings. This study provides a useful guidance for the corrosion protection of dissolvable plugging tools made of magnesium alloys.

Investigation on the Relationship between the Surface Chemistry and the Corrosion Resistance of Electrochemically Nitrided AISI 304 Stainless Steel

The relationship between the surface chemistry and the corrosion resistance of electrochemically nitrided AISI 304 stainless steel samples has been investigated. The nitriding treatment was carried out in HNO3 0.1 M and HNO3 0.1 M + KNO3 0.5 M at room temperature. Samples were subjected to the nitriding procedure for 30 minutes under a cathodic potential of -0.7 VAg/AgCl. The chemical composition of the nitrided layers was assessed by X-ray photoelectron spectroscopy (XPS). Depth profiles of the main elements present in the nitrided layers were also obtained by XPS by etching them with argon ions. The corrosion behavior of the nitrided samples was evaluated by potentiodynamic polarization. The results showed that the nitrided layers consisted of a mixture of chromium nitrides, chromium oxides, iron oxides/oxyhydroxides, and nickel oxide. The best corrosion resistance was obtained by electrochemical nitriding in the HNO3 0.1 M + KNO3 0.5 M solution. This result could be correlated with the composition and thickness of the nitrided layer.