Evaluation of the Corrosion Resistance of Phosphate Coatings Deposited on the Surface of the Carbon Steel Used for Carabiners Manufacturing

Abstract

This study aims to evaluate the corrosion resistance of carbon steel, used for carabiners manufacturing, coated with three different types of phosphate layer. The phosphate layers have been obtained by phosphate conversion coating with three different types of phosphate solutions: zinc-based solution, zinc-iron-based phosphate solution, and manganese-based phosphate solution. Additionally, the test was performed on zinc phosphate samples impregnated with molybdenum bisulfate-based oil and zinc phosphate samples further coated with a layer of elastomer-based paint. Considering the areas where the carabiners are used (civil engineering, navigation, oil industry, rescue operations, etc.), the corrosive environments studied are rainwater, Black Sea water, and fire extinguishing solution. The structure of the deposited layers was studied by scanning electron microscopy, while the interface structure between the alloy and corrosive environment was analyzed by electrochemical impedance spectroscopy. According to this study, the corrosion resistance of zinc-based phosphate coated samples and zinc/iron-based phosphate coated samples is higher than that of the studied carbon steel samples, despite the corrosion environment. Also, the most aggressive corrosion environment was the fire extinguishing solution.

Phosphate Surface Treatment for Improving the Corrosion Resistance of the C45 Carbon Steel Used in Carabiners Manufacturing

“…The appearance of galvanic corrosion can be correlated with the high porosity of the manganese phosphate layer as well as with the high content of iron and nickel ions. Therefore, micro-piles form on the surface of the alloy between two different materials joined by a strong electrolytic medium [ 17 ]…”

“…In the voltammogram recorded in the fire extinguishing solution, the anodic branch and the cathodic branch are curved and distant from each other, where the anodic branch (direct curve) is above the cathodic curve (reverse curve), indicating that during cyclic polarisation, an accentuated metal passivation occurs. This behaviour is because some of the FES organic substances are preferentially adsorbed on the alloy surface, influencing the repassivation potential [17][18][19]. Therefore, if in the case of seawater and rainwater the repassivation potential has lower values compared to the values of the corrosion potential, in the case of the fire extinguishing solution, the repassivation potential has a value close to the corrosion potential value.…”

“…In the fire extinguishing solution, the cathodic branch is located below the anodic branch, this indicating a rather accentuated passivation. In this case, passivation occurs by clogging the pores with corrosion products, as well as by adsorption of organic components from the solution [17].…”

Immersion Behavior of Carbon Steel, Phosphate Carbon Steel and Phosphate and Painted Carbon Steel in Saltwater

“…After 45 or 36 days, and 85 or 75 days, respectively, the samples were removed from the solution and analyzed using the method of electrochemical impedance spectroscopy (EIS) to study the structure of the surface layer and its evolution with the immersion time. The initial studies were performed one hour after immersion in solution (the time required for sample mounting operations in the electrochemical cell and thermostat) [ 26 ]. Only samples kept in saltwater for 85 days were used to study the surface morphology.…”

“…However, to our knowledge, there is no study which approaches the corrosion behavior evaluation of the carbon steel coated with Zn/Fe phosphate layer and phosphate-painted carbon steel immersed in a natural saltwater environment. Based on previous preliminary studies [ 17 , 26 ], the goal of this study is to analyze the influence of immersion time in saltwater of three types of samples: carbon steel, phosphate carbon steel, and phosphate and painted carbon steel, while a new phosphating solution suitable for carbon steel coating was introduced. Moreover, the corrosion mechanism and parameters for different immersion times were analyzed using EIS, SEM, and EDS.…”