MATERIALS SCIENCE AND ENGINEERING

CORROSION BEHAVIOR OF DUPLEX AND MARTENSITIC STRUCTURAL ALLOYS CARBURIZED UNDER PARAEQUILIBRIUM CONDITIONS

This investigation concerns the corrosion behavior of duplex stainless steels and martensitic stainless steels carburized under paraequilibrium conditions, i.e. processing conditions under which carbon diffuses over distances of several μm, whereas all other atomic species are effectively immobile [1]. The alloys being studied in this part of the project are SS-410 and Ferralium 255.

SS-410 is a martensitic stainless steel and is known for high corrosion resistance and high strength and toughness [2]. Ferralium 255 is a duplex stainless steel that contains approximately equal volume fractions of austenite and ferrite. Loaded in tension, it has a yield stress of approximateloy 860 MPa and a high corrosion resistance [3]. In natural seawater tests, Ferralium 255 has a corrosion rate of 3 mm/yr. For comparison, exposing austenitic 316L to these conditions results in a conversion rate of 38 mm/yr [3].

Low-temperature carburization was found to dramatically enhance the corrosion resistance of austenitic stainless steels [4,5]. For example, the carburized case of austenitic SS-316 was not attacked during etching, whereas the bulk material was attacked (Fig. 1). Corrosion resistance can be measured experimentally by applying an electric potential difference to a specimen versus a counter electrode and measuring a current density response. When comparing specimens, a more corrosion-resistant metal will exhibit a lower current density response, and therefore lower corrosion reaction rates. Figure 2 shows that compared to non-treated material, low-temperature carburized austenitic stainless steel (SS 316L) exhibits a lower current, and thus better corrosion properties at steady state [6]. TEM (transmission electron microscopy), XPS (X-ray photoelectron spectroscopy or ESCA – electron spectroscopy for chemical analysis), XRD (X-ray diffraction), and light-optical microscopy will be used to characterize the surface conditions of the carburized alloys before they are corrosion tested. After marine corrosion testing, scanning electron microscopy (SEM) and XPS will be utilized to evaluate corrosion modes and contributing micromechanisms. The passivity of the carburized surfaces of these two alloys will determine their corrosivity in seawater.

Cross-sectional optical micrograph showing the carburized layer in an austenitc stainless steel (SS-316) after low-temperature carburization at 470 °C for 246 h [2].

Crevice corrosion testing of carburized SS 316L (austenitic, low carbon) versus wrought C-22 alloy and Ti–6Al–4V [6].