Corrosion Behavior of Aluminide Intermetallic Coating on Stainless Steel in Aggressive HCl Solution
Authors: Liu Wenjuan , Wang Yu 
Published: February 2020

Metallic materials often go through corrosion when contacting with the aggressive agents present in the environment. Stainless steels which are widely used in various field owing to their low cost, sufficient corrosion resistance, workability and excellent mechanical performance also encounter great corrosion damage in the service environment, especially in aggressive chloride ions condition[1]. Chloride ions usually produce local breakdown of the protective layers, causing pitting corrosion which is the most dangerous form of localized corrosion. Therefore,stainless steels are often additionally protected(coating, corrosion inhibitors) when used in chloride environment. Currently, intermetallics coating is receiving more and more attentionowing to good high-temperature strength, high resistance to oxidation and corrosion, high melting point, high stiffness, good creep resistance and relatively low density[2].
In present work, the intermetallics coating on stainless steel is fabricated by pressure assisted diffusion bonding at the solid reaction condition. The fabrication process of coatings carried out in the open air along with the modest reaction condition, in terms of temperature and pressure, which makes it ideally suited for the production of commercially scalable materials[3].The pressure applied in the coating process will reduce the effects of kirkendall effect so that pore-free coating layer can be obtained on the stainless steel substrate. In this regard, the aluminide intermetallic coating of 430-SS/Al, 304-SS/Al was fabricated with Fe/Al produced as control.
The coating layer was fabricated by commercially 1100 aluminum bonding with substrate (430-SS, 304-SS and pure Fe) in a composite synthesis apparatus. Then, the intermetallic layers was exposed as the research objects and observed by SEM and EDS. It is indicated that the intermetallic surface layer is compact and dense on 304-SS substrate, compared with several small cracks presented on 430-SS substrate, and a tongue-like surface on pure Fe. Specifically, the cross-section of intermetallic layer was elucidated. As shown in Fig.1, the intermetallic layer grow intact on the substrate. Different aluminide intemetallic phases were formed due to incorporation of chrome, nickel elements by adjusting the composition of substrate. In total, all three intermetallic layers were compact. There was just one layer formed on pure Fe. While for 403-SS and 304-SS, one uniform layer and one eutectic layer were both presented[3].
The corrosion behavior of different intermetallic coatings were evaluated using liner sweep, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), besides weight-loss evaluation. The samples were cut into the rectangle with dimension of 20 mm×20 mm. For electrochemical test, three electrode-configuration was used, with Ag/AgCl as a reference electrode, Pt wire as a counter electrode, and intermetallic coating as the working electrode.Moreover, the corrosion morphology and corrosion product was analyzed with SEM, EDS, XRD and Ramen spectroscopy. The line sweep result indicated that the intermatillic layer on 304-SS was resistant in aggressive 0.5 M HCl solution, with RP(polarization resistance) 885 Ω·cm2, which is ten folds bigger than that on 430-SS (85.8Ω·cm2). For the intermetllic layer on pure Fe, the Rp was dramatically decreased to 6.48Ω·cm2. The EIS result as shown in Fig. 2 also confirmed the great corrosion resistance of intermetallic layer on stainless steel.
In summary, we fabricated intermetallic coating on stainless steel by incorporating different corrosion-resistant elements with an easily realized production process in the industry, with compatible good mechanical properties. These results provide an insight into the corrosion behavior of the intermetallic coating and establish a foundation for the choice of materials in the applied industries, taking into account of both mechanical and corrosion properties.
References
[1]G.T. Burstein, P.C. Pistorius, S.P. Mattin, Corrosion Science, 35 (1993) 57-62.
[2]T.Z. Li, F.C. Jiang, E.A. Olevsky, et. al, Materials Science and Engineering: A, 443 (2007) 1-15.
[3]Y. Wang, K. S. Vecchio, Materilas Science and Engineering:A, 649 (2016) 325-337.

Figure 1