Elsevier

Corrosion Science

Volume 161, December 2019, 108189
Corrosion Science

Short Communication
On the unusual intergranular corrosion resistance of 316L stainless steel additively manufactured by selective laser melting

https://doi.org/10.1016/j.corsci.2019.108189Get rights and content

Highlights

  • SLM-produced 316L SS has shown markedly high intergranular corrosion (IGC) resistance.

  • This unusual IGC behaviour was explained by the avoidance of localised Cr depletion.

  • The IGC was affected by grain boundary character of the SLM-produced 316L SS.

Abstract

The intergranular corrosion (IGC) resistance of 316L stainless steel (316L SS) produced by selective laser melting (SLM) was investigated using microscopy analysis and electrochemical measurements. The IGC resistance of SLM-produced 316L SS, determined using a double-loop electrochemical potentiokinetic reactivation test, was found to be substantially higher than that of conventional 316L SS. This unusual behaviour was explained by the fact that no Cr-rich precipitates were detected for SLM-produced specimens after long-term sensitisation heat-treatment and those SLM-produced specimens exhibited a high frequency of twin boundaries and low-angle grain boundaries along with fine grains, leading to the avoidance of localised Cr depletion.

Introduction

Austenitic stainless steels are widely used in engineering structures operating at elevated temperatures such as in steam generating plants as piping and superheating tube materials. Under such harsh environmental conditions, intergranular corrosion (IGC) of conventional stainless steels is frequently observed, especially in sections joined by welding at temperatures between 500 and 850 °C. Additive manufacturing (AM), an emerging net-shape layer-wise fabrication process, is being adopted for producing structural components used at elevated temperatures because of its ability to produce integrated complex parts in a single step without the need for joining like welding. Selective laser melting (SLM) is a powder-bed AM technique, in which a component is produced by selectively melting consecutive layers of powder on top of each other using a high-energy laser beam [[1], [2], [3]]. Upon irradiation, the powder is melted and forms a tiny melt pool. This generates extremely high temperatures up to 105 °C and rapid cooling up to 106-108 °C/s within the melt pool [4,5]. The thermal history that materials experience during SLM processing is very different from that established by conventional manufacturing techniques [6,7]. The rapid melting and solidification in combination with cyclic heating and cooling upon the deposition of subsequent layers result in a microstructure that differs from traditionally-produced parts [[8], [9], [10]]. This has resulted in unusual properties such as weaker erosion-corrosion resistance of SLM-produced 316L stainless steel (hereafter 316L SS) [11].

It is well-known that an important microstructural characteristic of a crystalline material that could be highly influenced by the thermal history during the production process is the grain boundary (GB) character. Such change in GB character could lead to changes in the IGC behaviour of SLM-produced stainless steels. Research on IGC resistance over the years has shown that the GB character has a crucial influence on precipitation behaviour and IGC susceptibility [[12], [13], [14], [15], [16]]. Measurements of GB character, with special emphasis on coincidence site lattice (CSL) boundaries, have been the focus of GB engineering in improving corrosion resistance. In spite of extensive studies on the microstructural evolution and properties of SLM-produced 316L SS [[17], [18], [19], [20], [21], [22]], the IGC behaviour of SLM-produced stainless steels has yet to be studied in any detail. A recent report has found that the SLM-produced 316L SS exhibited a more rapid sensitization of GBs upon exposure to elevated temperature compared to the wrought counterpart, however, the precipitation behaviour and its subsequent influence on the IGC resistance are not well understood [23].

In the present study, an attempt was made to provide insight into how the nonconventional thermal history associated with SLM processing can influence the GB character, precipitation behaviour and IGC susceptibility with the support of microscopy analysis and electrochemical measurements.

Section snippets

Materials and heat-treatment

Commercially available gas-atomized spherical 316L SS powder, with a particle size range between 5 and 40 μm, was used to produce specimens. Gas-atomisation was performed under argon gas atmosphere. An SLM®-125HL machine was used to produce cubic specimens with dimensions of 1.0 × 1.0 × 1.0 cm3. Prior to the SLM fabrication, the build plate was pre-heated to a temperature of 200 °C and the build chamber was purged with purified argon until the oxygen level was reduced to below 100 ppm. The main

Results and discussion

In general, it is believed that the loss of intergranular corrosion resistance at the GBs in stainless steels is due to the formation of GB chromium carbides that cause localised Cr depletion in the region adjacent to these precipitates [[25], [26], [27]]. The intergranular attack is believed to be accelerated by the increasing potential difference between grain interiors (cathode) and GBs (anode). The DL-EPR test is a common method for assessing such behaviour based on the assumption that only

Summary

The IGC resistance of an additively manufactured 316L SS was studied using a combination of microscopy analysis and electrochemical measurements. The relationship between the IGC and GB character of the SLM-produced 316L SS was established for the first time. No Cr-rich precipitates were detected for SLM-produced specimens after a long-term sensitisation heat-treatment. Subsequently, DL-EPR tests showed substantially lower DOS values for the SLM produced 316L SS compared to its commercial

Data availability statement

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Acknowledgement

Financial support from Deakin University Postgraduate Research Scholarship (DUPRS) is greatly appreciated. Deakin University’s Advanced Characterisation Facility is acknowledged for use of the microscopy instruments and assistance from Dr Adam Taylor and Dr Mark Nave.

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