Abstract
Titanium has been added to ferritic stainless steels to combat the detrimental effects of intergranular corrosion. While this has proven to be a successful strategy, we have found that the resulting Ti-rich inclusions present on the surface play a significant role in the initiation of other forms of localized corrosion. Herein, we report the effect of these inclusions on the localized corrosion of a stainless steel using macro and micro electrochemical techniques. Through the use of scanning electrochemical microscopy, we observe the microgalvanic couple formed between the conductive inclusions and passivated metal matrix. The difference in local reactivity across the material’s surface was quantified using a 3D finite element model specifically built to respect the geometry of the corrosion-initiating features. Combined with electron microscopy and micro elemental analysis, localization of other alloying elements has been reported to provide new insight on their significance in localized corrosion resistance.
Highlights
Owing to their cost effectiveness,[1] austenitic stainless steels (SS)are often replaced with ferritic SS containing low amounts of Ni
A series of EDX line scans revealed the composition of these inclusions to be mainly titanium, niobium and nitrogen (Fig. 1b/d, c/e), with most inclusions being TiN, in agreement with previous studies on SS 444.13,15,19 The mean ratio of Ti to N was calculated as a mole percentage to be 1:0.96 using 5 EDX point analysis from five different inclusions
scanning electrochemical microscopy (SECM) measurements over the heterogeneous SS 444 substrate revealed the enhanced reactivity of the inclusions in comparison to the passivated Fe–Cr metal matrix, and suggest a lack of passive film over the inclusions entirely as proven to be true for previously described manganese sulfide (MnS) inclusions.[34]
Summary
Are often replaced with ferritic SS containing low amounts of Ni. ferritic Fe–Cr alloys are susceptible to intergranular corrosion (IGC), which greatly decreases the material’s lifespan. In SS, this tends to occur along grain boundaries at the Cr-depleted zones that form adjacent to chromium carbide precipitates.[2] IGC can be minimized by limiting the amount of C and N within the alloy, specific heat treatments, and by alloying the metal with stabilizing agents such as Ti and Nb. In SS, this tends to occur along grain boundaries at the Cr-depleted zones that form adjacent to chromium carbide precipitates.[2] IGC can be minimized by limiting the amount of C and N within the alloy, specific heat treatments, and by alloying the metal with stabilizing agents such as Ti and Nb These elements have higher chemical affinities to C and N than Cr does and so will preferentially precipitate, leaving the Cr in solid solution. This allows for the formation of protective Cr oxides, increasing the material’s resistance to IGC.[3,4,5] Despite the successful reduction of IGC in alloyed ferritic SS, its mechanical strength can still be greatly reduced by pitting corrosion, usually associated with some discontinuity over the metal surface, such as a grain boundary, a defect/scratch, or an inclusion within the metal’s microstructure.[6]
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