Formation Process and Stability of Co-W Oxide from an Electroplated Co-W Alloy Coating at 1000 °C for Cr-Based Steels
Authors: Gan LU , Murakami Hideyuki , Saeki Isao 
Published: February 2020

1. Introduction.
Stainless steels are known as corrosion-resistant iron-based alloys which have wide range of applications in the modern society because of their good durability, relatively high electronic conductivity, good mechanical properties and low costing. Ferritic stainless steel is one kind of stainless steel which consists primarily of iron and chromium. The so-called passive film formed on the surface of ferritic stainless steel which is a very thin and self-healing film formed by Cr oxide provides the corrosion prevent property in high temperature environment.
However, there is a critical issue related to the use of Cr based-steels which is the evaporation of the Cr, the volatile species of Cr are CrO3 and CrO2(OH)2 at high temperature. Some researchers reported that heating process would reduce the Chromium level which leads to the loss of corrosion resistance and transformation of microstructure. The evaporation of Cr results to the limitation for ferritic application. The evaporated Cr would poison the cathode materials in SOFC, and the acute oral toxicity for Cr (VI) ranges between only 50 and 150 µg/kg while the chromium(III) compounds were known to be carcinogenic.
For the extensive application of ferritic stainless steels in high temperature environment such as the tubes in automobile exhaust system or boiler and stable the performance of these stainless steels, many efforts have been made. The use of Mn containing materials to form a Mn-Cr spinel oxide which can reduce the chromium evaporation to 30-40%. But the evaporation still occurred. Recently, researchers showed a tendency to prepare a coating on the stainless steels using various coating methods such as PVD, thermal spraying or electroplating.
In our previous work, we have found the Co-W alloy coating formed by electroplating spontaneously formed a Co-W oxide layer at 800 °C, and this layer performed as a Cr barrier to stop the Cr outside penetration completely. In this research, we focus on the stability and barrier property of Co-W oxide layer at 1000 °C. Co-W alloy with 5 at% W was prepared by electroplating method on type 430 stainless steels. And the products after 1000 °C oxidation test in air were characterized in detail in order to elucidate the Cr barrier property offered by this coating.
2. Experimental.
A commercially used Type 430 was set as the substrate whose composition was 16.2Cr–1.0Mn–0.7Si–0.12C–0.02S–0.04P (mass%). The steel was cut into small coupons. Thin Co-W alloy coating was deposited on the coupons to obtain a 10-µm-thick Co–W layer (85 g m−2). All solutions were prepared with analytical grade reagents and deionized water. The composition of Co-W electroplating was measured with an energy dispersive X-ray fluorescent analyzer (XRF) to confirm the composition to be Co-5 at%W. In the following paragraph, we called this alloy coating as Co-5W.
After plating, the samples were oxidized in air with 2 kinds of heating processes. Process 1 (P1) is to heat the sample to 1000 °C directly and keep the temperature until 25 hours. Process 2 (P2) is to do the pre-treatment in air at 800 °C for 3 hours to form a Co-W oxide layer and then raise the temperature to 1000 °C.
After the oxidation test, the characterization was made with 3D-digital scanner, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDXS).
3. Results and Conclusion.
The oxidation behaviors at 1000 °C of type 430 stainless steels, with electroplating Co-W alloy coatings, was studied. The results are summarized as follows:

After heated directly at 1000 °C, the coating layer showed reticulate cracks. While with a pretreatment at 800 °C, the surface showed no cracks.

The formation of the cracks may lead to discontinuous Co-W protective layer and could provide a route for Cr and Fe outwards penetration.

The crystal structure has been observed with TEM analysis. The Co-W oxide would grow to large cells while the Cr oxide remained small size approximately amorphous.

The initial oxidation behaviors have been checked to confirm the order of the compounds to be oxidized. Higher oxygen potential resulted to the oxidation of Cr in early stage of oxidation.

As shown in figure(A)(B)(C)(D) below, Co-W oxide continuous layer was not formed when oxidation directly at 1000 °C. However, with a pretreatment at 800 °C, the formed Co-W oxide can be stable at 1000 °C and it can block Cr outwards penetration at 1000 °C, as figure(E)(F)(G)(H). And the Co-W coating provided an oxidation resistance at 1000 °C as well.

Figure 1