Using macro and micro electrochemical methods to understand the corrosion behavior of stainless steel thermal spray coatings

Abstract

High-velocity oxygen fuel thermal spray stainless steel coatings are desirable for their excellent erosion resistance. However, the fabrication process can lead to a decrease in corrosion resistance in comparison to the original bulk material. Here we produced stainless steel coatings on stainless steel substrates using varying deposition parameters to investigate the corrosion properties of the resulting composite steels and elucidate the corrosion behavior both on the macro and micro scale. Macro potentiodynamic polarization measurements carried out in corroding environments demonstrated the rate of degradation of the Fe-Cr alloy coating. After short immersion periods, the coatings showed iron-like active corroding behavior and no passivation regions on the anodic branch. Over time, the coating's corrosion behavior began to change to signify similar results to that of pure chromium. Ex-situ electron microscopy and elemental composition revealed a Cr oxide rich layer left on the coating's surface. Micro electrochemical techniques including scanning electrochemical microscopy and scanning micropipette contact method were employed over the coatings and powdered material, respectively, to show that the lack of protective passivity the thermal spray coatings possess is mostly inherited from the atomized powdered stainless steel material.

Electrochemical Behavior, Microstructure, and Surface Chemistry of Thermal-Sprayed Stainless-Steel Coatings

“…Evidence of such films was shown in Figure 6, where the anodic current of coating HVAF#1 nearly stabilizes at 35 μA.cm −2 . Similar behavior was reported for SS444 HVOF coatings in [18]. At the HVAF #1 coating cross-section, the formation of corrosion products rich in Cl highlights the ability of the electrolyte to penetrate through the voids between splats.…”

“…On the other hand, the results also mean that Cr distribution on the coating surfaces is very heterogeneous. For example, the EDX analysis performed on precipitates present in both the feedstock and coatings revealed an enrichment in chromium and silicon, which was also reported in [18]. These factors may play an important role in the coating's electrochemical behavior.…”

“…When comparing polished coatings (not exposed to the tests) to those exposed to the test solution for 3-weeks, one notices an increase in the Cr/Fe ratio. This phenomenon may be attributed to a preferential dissolution of Fe at the electrode surface which was also observed in [18] and on early studies regarding the passivity of Fe-Cr alloys [27]. If one considers the standard electrode potentials of Cr(III) and Fe(II), respectively −0.74 V and −0.41 V versus a standard hydrogen electrode, one could say that the shift observed in the E corr of the coatings during the first week of immersion is caused by the surface Cr-enrichment.…”

“…The aim of the present work is to further explore the potentials and limitations of ferritic SS444 thermal-sprayed coatings. The results reported in [18] are extended, now including a thorough in-flight particle analysis as well as stainless steel coatings produced by HVAF. The coatings' electrochemical behavior is monitored for longer immersion times and is compared to a bulk disc produced by arc melting the spray feedstock.…”

“…When compared to austenitic steels, SS444 combines higher resistance to stress-corrosion cracking and lower nickel concentrations, reducing costs and making it a suitable material for applications including food processing [15], drinking water transport [16], and protection against hydrogen, chlorides, sulfides, and nitrides in oil distillation towers [17]. A recent report [18] on the corrosion behavior of SS444 coatings produced by HVOF revealed their inability to develop a protective passive layer. The effect was attributed to the powder atomization process that led to important modifications of the steel's original chemical composition.…”