P. Bala Srinivasan

Effect of weld metal chemistry and heat input on the structure and properties of duplex stainless steel welds

Abstract The excellent combination of strength and corrosion resistance in duplex stainless steels (DSS) is due to their strict composition control and microstructural balance. The ferrite–austenite ratio is often upset in DSS weld metals owing to the rapid cooling rates associated with welding. To achieve the desired ferrite–austenite balance and hence properties, either the weld metal composition and/or the heat input is controlled. In the current work, a low heat input process viz., EBW and another commonly employed process, gas tungsten-arc welding have been employed for welding of DSS with and without nickel enhancement. Results show that (i) chemical composition has got a greater influence on the ferrite–austenite ratio than the cooling rate, (ii) and even EBW which is considered an immature process in welding of DSS, can be employed provided means of filler addition could be devised.

Microstructure and corrosion behavior of shielded metal arc-welded dissimilar joints comprising duplex stainless steel and low alloy steel

This work describes the results of an investigation on a dissimilar weld joint comprising a boiler-grade low alloy steel and duplex stainless steel (DSS). Welds produced by shielded metal arc-welding with two different electrodes (an austenitic and a duplex grade) were examined for their microstructural features and properties. The welds were found to have overmatching mechanical properties. Although the general corrosion resistance of the weld metals was good, their pitting resistance was found to be inferior when compared with the DSS base material.

Characterisation of tribological and corrosion behaviour of plasma electrolytic oxidation coated AM50 magnesium alloy

Abstract A cast AM50 magnesium alloy was plasma electrolytic oxidation treated in a silicate based electrolyte using a direct current power source. Plasma electrolytic oxidation coatings produced at four different visible discharge conditions were characterised for their microstructural features, composition, tribological behaviour and corrosion resistance. The chemical composition, thickness and the roughness of the coating were found to be influenced by the processing voltage and thick coatings produced at relatively higher voltage levels provided a better wear and corrosion resistance.

Development of decorative and corrosion resistant plasma electrolytic oxidation coatings on AM50 magnesium alloy

Abstract Plasma electrolytic oxidation of AM50 magnesium alloy was performed in alkaline phosphate electrolyte with and without the addition of titania sol. The coatings produced in the phosphate electrolyte were constituted with MgO and Mg3(PO4)2. The coatings obtained in the phosphate electrolyte with the addition of titania sol were blue in colour and contained additionally the TiO2 and Mg2TiO4 phases. The phosphate PEO coating provided a two order of magnitude improvement in corrosion resistance to the AM50 magnesium alloy as was shown by the potentiodynamic polarisation measurements. With differences in the physical appearance, microstructural morphology and phase composition, the coatings produced in titania sol containing electrolytes offered an improved corrosion resistance compared to the mere phosphate PEO coating.

Effects of inhibitors on corrosion behaviour of dissimilar aluminium alloy friction stir weldment

Abstract Dissimilar weldments comprising age hardenable AA7075 and AA6056 aluminium alloys were produced by friction stir welding. Corrosion behaviour of the weld nugget/thermomechanically affected zone and parent materials was characterised in chloride solutions. Chromate, molybdate and cerium nitrate were added to the chloride solution and their effect on the corrosion behaviour of the weldment was assessed. Electrochemical studies on different regions of the dissimilar friction stir weldment, performed in inhibited 3·5%NaCl solutions, suggest that chromates are better in terms of inhibition efficiency and inhibit all regions of the weld area to a similar extent. From the viewpoint of environmental issues, both molybdate and cerate may also be thought of as effective inhibitors; however, their efficiencies are definitely below that of chromates in neutral chloride environments.

Corrosion behaviour of duplex stainless steel weld metals with nitrogen additions

Abstract The addition of nitrogen to duplex stainless steel weld metal has been achieved and the resultant weld metals have been assessed for their microstructural features and corrosion behaviour. It was found that, although autogenous welding of duplex stainless steels is not recommended, the addition of nitrogen through the shielding gas mixture can help to obtain quality welds with the desired phase balance and corrosion resistance.

Evaluation of Chromate Films by Electrochemical Impedance Spectroscopy

Conventional chromate-fluoride baths have been employed for producing chromate films on aluminium and its alloys. In the present investigation, the effect of addition of ferricyanide to the conventional bath on the coating formation rate has been studied. The corrosion resistance of the resultant films have been evaluated by Electrochemical Impedance Spectroscopy. An attempt has also been made to reduce the level of toxic chromates in the conventional bath. The corrosion performance of the chromate films produced from the newly formulated baths has been compared with the coatings resulting from conventional and accelerated baths

Acceleration of ambient temperature phosphating by an electrochemical pulse technique

Abstract An attempt has been made to accelerate the phosphating process by an electrochemical pulse technique. It has been observed that by the application of square wave pulses of various amplitudes and widths it is possible to achieve coatings with better properties than coatings produced by the immersion process. Properties such as the coating weight, porosity and corrosion resistance of the resultant phosphate coatings have been evaluated by conventional stripping, potentiostatic cathodic polarization and electrochemical impedance techniques respectively to assess the performance. X-ray diffraction studies showed that the samples phosphated by all the methods contain the Zn3(PO4)2·4H2O (hopeite) phase only. From the studies it has been observed that the coatings produced by the various electrochemical pulse methods are thicker, less porous and more corrosion resistant than coatings produced by the simple immersion method.

Environmental cracking behavior of submerged arc-welded supermartensitic stainless steel weldments

Supermartensitic stainless steel welds produced by submerged are welding were assessed for their microstructure and properties. Slow strain rate tests conducted on these specimens revealed that both the parent material and the weld metals are susceptible to cracking under conditions of hydrogen (H) charging.

Sliding wear behavior of salt bath nitrocarburized medium carbon steel

Salt bath nitrocarburizing is a well-known thermochemical diffusion process for enhancing the tribological and corrosion properties of ferrous components. The current work describes the role of a compound layer developed during nitrocarburizing, both in the ferritic and austenitic regimes of Fe-N-C system, on the sliding wear behavior of a medium carbon steel. The wear behavior of the nitrocarburized steel discs was assessed by the pin-on-disc tests (ASTM G 99-99) under different normal loads running against a hardened SAE52100 pin. It was observed that the compound layer on the surface not only controlled the wear rate but also resisted the adhesive wear/transfer of material from pin to disc, aside from providing low-friction coefficients.