The mechanism of thermal oxide film formation on low Cr martensitic stainless steel and its behavior in fluoride-based pickling solution in conversion treatment

Abstract

In this research, first a multi-analytical approach was carried out to propose a mechanistic model for the formation of the thermal oxide film on low Cr stainless steel. It describes the effects of kinetically and thermodynamically driven phenomena on the growth of a Fe-Cr complex oxide film at high temperatures. Second, the behavior of the thermal oxide film in the presence of fluoride ions, as a relevant pickling agent in conversion treatment, was investigated. It was shown that fluoride ions enter the oxide film and form a complex with Fe and preferentially dissolve hematite islands through a complexation mechanism.

Unraveling the mechanism of the conversion treatment on Advanced High Strength Stainless Steels (AHSSS)

“…Fluoride ions in the conversion solution dissolve the less stable component in the oxide film. Therefore, Fe-O bonds are broken and dissolved as the less stable components of the oxide film during immersion in the fluoride-containing conversion solution [4]. The fluoride ions can diffuse through the oxide film by selective dissolution of the oxide film.…”

“…1(c,d)) indicate local attack of the oxide surface (see red arrow). This etching can be the result of the pickling step by fluoride ions which intends to release some metal ions from the surface [4]. EDX analysis was performed together with SEM imaging to identify the elemental composition of the treated sample with a depth resolution of around 1 µm.…”

“…However, the application of lighter material in the car body can result in the deterioration of safety as the mechanical strength has a direct relation with the weight of the car. In this regard, Advanced High Strength Stainless Steels (AHSSS) have been developed for body-in-white applications as ultra-high strength stainless [1][2][3][4]. The optimal composition and structure in these alloys provide ideal corrosion resistance together with excellent mechanical properties [5,6].…”

“…Despite the fact that the application of conventional grade stainless steel (such as S.S. 304 or S.S. 316) in the automotive car body can have several advantages, they don't perform effectively during the pretreatment step [44,45]. It was mentioned earlier that the dissolution of the surface in the pickling step is a crucial starting point for the formation of conversion coating [4,31,46]. Therefore, the presence of a protective Cr oxide film on the surface of these substrates prevents the dissolution of the metal surface during the pickling step and initiation of the conversion process.…”

“…In this regard, first the ability of this material to dissolve in the industrial pickling process was investigated [4]. It was indicated that for this stainless steel the thermal oxide film mainly consists of a Fe-Cr oxide solid solution with Fe3-xCrxO4 spinel structure instead of an intact Cr oxide layer.…”