Preformed pits as initiation sites for stress corrosion of austenitic stainless steels

A study has been made of the effect of preformed pits on the corrosion behaviour of a heat-treated AISI304 austenitic stainless steel under constant load equal to 80% sp(0.2), at polarization potentials in the imperfect passivity region and under open circuit conditions, exposed to aqueous solutions with diverse SO4/Cl ratios.With polarization in the passive range and a ratio of 1/0.2, as in the first stage of stress corrosion, intergranular attack on the bottom of pits with critical dimensions has been observed. On the walls of the cracks there are characteristic signs of transgranular attack. An increase in temperature causes an increase in the rate of the phenomenon, without leading to the appearance of different forms of corrosion. With polarization in the active range the trend of the crack is always transgranular. Sometimes the crack walls appear to be marked with fine parallel pleats, parallel grooves and micropits. In some conditions - low temperature and very small pits - the formation of martensite, attacked in a highly selective manner, prevents stress corrosion

Composition and structural modifications to the AI/AISI 316L sintered steel interface induced by heat treatments

After heat treatment, the aluminium coating, deposited on sintered AISI 316L austenitic stainless steel forms an interdiffusion layer. Aluminium diffuses beyond the layer/steel interface, mostly along the interconnected porosities, penetrating into the sintered steel for about 80 gm, and causing a nitrogen exchange reaction with the Cr2N phase. This leads to the formation of AlN particles. At the beginning these are tiny, then they subsequently coalesce and migrate outwards through the interdiffusion layer. The vapour phase aluminium diffusion is in accordance with thermodynamical data.At the layer/coated sintered steel interface, on the steel side, the cavity surfaces are covered by aluminium and chromium mixed films. In uncoated heat treated sintered steels, the chromium oxide films are present in all the volume. The anodic behaviour of aluminium coated and heat treated (700 -900°C) sintered steels is much better than that of the uncoated steels heat treated at the same temperatures.The improvement can be attributed to the Cr2N and Al reaction which makes passivation easier. With 1000°C heat treatment, a worsening is observed compared to the lower temperatures

Diffusion Processes of Aluminium Coatings for the Protection of Austenitic Stainless Steels

An aluminium-coated austenitic stainless steel has been subjected to heat treatments of various durations. The process has led to the formation of inter diffusion zones with characteristic non-directional and columnar structures.The formation of in termetallic compounds such as NiAl or Ni3Al prevails in the non-directional type, while substitutional solid solutions occur in the "columnar " type.One or both these structures can be obtained by balancing and regulating process activity through adequate heat treatments.The development of one or both structures can heighten the characteristics of the coating as regards coating protection or oxidation resistance at high temperatures

Aluminium based protective coatings produced on AISI 304 stainless steel

The diffusion of Aluminium is one of the most promising methods to build superficial coatings for stainless steel protection. Heat treatments at 800 °C performed on rods of AISI 304 steel, Aluminium coated by means of electrodeposition, displayed the possibility of forming intermetallic compounds. Depending on the duration of the heat treatment and on the cooling kinetics, these compounds can be continuous. At high cooling kinetics (water cooling) a two-phase structure composed of [MATH] and [MATH] (substitutional solid solution) is obtained. At slow cooling rates (in furnace), a substitutional solid solution and some precipitates of [MATH] and [MATH] can be observed. At intermediate cooling rates (oil cooling and air cooling), only the substitutional solution and the [MATH] phase are present. Using furnace cooling from 800 °C until 500 °C, permanence at this temperature for 192 h and cooling furnace, the two-phase structure obtained is composed of the substitutional solid solution and the [MATH] phase. The existence of these phases and their composition have been reported by X-ray diffraction patterns and microanalysis. The possibility of forming a natural composite, constituted by a hard phase of aluminides diffused in a substitutional solid solution, has an important consequence on the mechanical and protection properties of these coatings. Moreover, the diffusion of Al improves the adhesion of coatings

Mechanical and electrochemical alteration of sorbitic and austenitic steels charged with hydrogen

Experiments made with the method initiated by Hyspecka and Mazanek have revealed phenomena not encountered with other methods of investigation, such as the recovery of mechanical properties while still discharging, and the simultaneous effect on electrochemical voltage, which can only be due to a trasformation of the hydrogen state inside the metal. It was also observed experimentally that those processes are generalized, may be iterative, and occur for times correlated with the density of discharge.An attempt was also made, with very significant results, to correlate the variation in these mechanical properties, depending on the hydrogen charging time, with the much more easily achieved variation in electrochemical characteristics. Study of these phenomena was combined with examination of the fracture surfaces and numerous longitudinal diametral sections under the fractures, using a Cambridge 600 scanning electron microscope (SEM). The results for sorbitic steels were published as they became available (2-8) and an interpretative model was proposed based on them; the new results for austenitic steels, and for electrochemical tests, also seem capable of being referred to that model

ALUMINUM PROTECTIVE COATINGS PRODUCED ON TERNARY FE-CR-NI ALLOYS

Protective coatings can defend a material from degradation when it is exposed to severely aggressive environments such as those encountered in the chemical, petroleum and aerospace sectors. Chromium-, aluminium- and silicon-based coatings produced with these elements alone or in combination have been used for quite some time to increase resistance both to hot corrosion and high temperature oxidation, owing to the fact that most structural materials with adequate mechanical properties do not stand up well in difficult environmental conditions If, however, they possess both these positive characteristics their cost is prohibitive. The coating ofNi-based superalloys by diffusing aluminium has been exhaustively studied, but this is not the case where austenitic stainless steels are concerned. The purpose of the work reported here was to characterize the microstructure and microchemistry of aluminium diffusion coatings produced on stainless steels by means of preparation of synthetic alloys