The evaluation of the corrosion behaviour of Hercules™ alloy

Master Thesis


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University of Cape Town

Ni contributes about 60% of austenitic stainless steel manufacturing material price. This means that the price of austenitic stainless steel increases with an increase of Ni. Ni price fluctuation has led to major efforts to reduce its content in austenitic steels. Ni has been replaced with readily available, cheap elements such as Mn and N. Hercules™ is a low Ni austenitic stainless steel alloy that was developed at Mintek in the Advanced Materials Division as part of the new development of low Ni austenitic stainless steels. The typical content of Hercules™ comprises of 2 wt.% Ni, 9 wt.% Mn and 2.5 wt.% N When Hercules™ alloy was tested at Mintek for mechanical properties, it was found that it had higher tensile strength than Type 304 in the hot rolled and annealed condition. Therefore, it was concluded that it can be used for structural applications where high strength is required. The target applications for Hercules™ were reinforcement bars and fasteners. There has been work done in order to improve corrosion resistance of Hercules™ in order for it to be used in corrosive environments or for general purposes. A more corrosion resistant Hercules™ alloy with 0.5 wt.% Mo addition was developed. The current project focused on further characterisation of the corrosion resistance of Hercules™ B (with 0.5 wt.% Mo) and Hercules™ A (without Mo addition) against Type 304 and Type 202. Cyclic polarisation technique was used to test the susceptibility of Hercules™ to pitting, crevice and general corrosion in different solutions. Hercules™ B showed better resistance to pitting in 3.56 wt.% NaCl compared to Hercules™ A and Type 304 but, it showed poor resistance in the presence of an artificial crevice. When the concentration of NaCl was reduced to 1 wt.%, Hercules™ B showed better resistance to crevice corrosion compared to Type 304, while Type 202 consistently showed poor corrosion resistance during each test. Immersion tests in 6 wt.% FeCl3.6H2O were consistent with the results that were obtained from the cyclic polarisation technique. The critical pitting temperature (CPT) test was investigated using 6 wt. % FeCl3.6H2O immersion testing. All test alloys started pitting at 25℃, meaning that they all have a CPT value that is less than 25℃. The results obtained from ten-days immersion and cyclic polarisation test in 5 wt.% H2SO4 were also consistent with each other. All test alloys showed good performance in 5 wt.% H2SO4 by achieving a corrosion rate that is less than 0.1 mm/y. It was therefore, concluded that Hercules™ B has an overall corrosion resistance that is comparable to that of Type 304 in 5 wt.% H2SO4 and 1 wt.% NaCl.