This corrosion occurs when the surface is exposed to reducing mediums. The entire stainless steel surface can be attacked uniformly when the passive film disappears from the surface without being able to reform. different types of corrosion that affect stainless steels general corrosion The corrosion resistance of precipitation hardened steel is close to that of austenitic steels but not as good as that of standard duplex steels. Like martensitic steels, precipitation hardened steels have excellent mechanical resistance and hardness properties. Most precipitation hardened steels have a duplex structure although some have a martensitic structure. These alloys also contain copper (1.25-2.5%) or aluminium (0.9-1.35%) that form intermetallic precipitates after thermal treatment. These steels contain from 12.25-18% chromium and 3-8.5% of nickel. On the other hand, these steels have excellent resistance to stress corrosion cracking caused by chlorides. The low chromium and high carbon contents of these alloys provide them with limited corrosion resistance, the lowest of any stainless steel type. Consequently, the alloy can be used in steam turbines, shafts, hard coatings.

Heat treatment will produce a martensitic microstructure that imparts great hardness and mechanical resistance. These steels normally contain 11.5-14% of chromium together with a higher concentration (0.15%) of carbon than other types of stainless steel. High chromium and molybdenum concentrations provide this alloy with excellent corrosion resistance: better than the corrosion resistance of all other types of stainless steel (pitting and stress cracking resistance), which explains the widespread use of these alloys in marine environments: pumps, valves, pressurised pipes, etc … martensitic steels The austenite phase produces greater ductility and its mechanical strength is greater than that of each of its phases. Each of the alloy’s two phases has interesting properties. These steels contain 18-29% chromium and 1-4% molybdenum together with a regulated concentration of nickel (3.5-7.5%) and/or austenite stabilisers (nitrogen, manganese) that encourage the formation of a balanced ferrite and austenitic structure at ambient temperatures. duplex (or austenoferritic) steels (table 2) On the other hand, ferritic steels are extremely resistant, if not immune, to stress corrosion cracking in the presence of chlorides. Ferritic steels are also less corrosion-resistant than other meterial. Ferritic steels have less valuable mechanical properties, specifically low ductility. These steels contain from 10.5 to 27% chromium. Main austenitic and duplex steels ferritic steels However, we should note that austenitic steels are sensitive to stress corrosion cracks in the presence of chloride.Ĭlick here to create your account in order to view the illustrations Table 2. When molybdenum is added (up to 6%), this further increases austenitic steel’s corrosion and pitting resistance, especially in a water-based medium that contains chlorine. Austenitic steels is also the one who, after 'duplex' steels, has the best corrosion resistance. Nickel encourages the formation of an austenite microstructure (crystalline structure, face-centred cubic) that endows it with valuable mechanical properties, high ductility and which make it easy to machine, properties that cannot be found in other stainless steels. These steels (the most often used in water treatment applications) contain more than 16% of chromium and 6% of nickel by weight, very often less than 18% of chromium and 8% of nickel. Depending on its crystalline structure, stainless steel falls into one of five categories: austenitic, ferritic, duplex, martensitic and precipitation hardened. In effect, beyond this content, under oxidising conditions (oxygen in the air or oxygen dissolved in water will suffice), a thin but stable layer forms and strengthens (see below for increasing Cr, Ni, Mo contents). linear polarisation resistance measurementīy convention, stainless steel is an iron-chromium alloy with a chromium content of more than 10.5%.effect of surface condition and water cleanliness.

corrosion in cast iron = graphitisation.

high temperature passivating of iron using magnetite.

natural water used in carbon steel passivation.

formation of protective layers and passivation.

cathodic reactions and corrosivity in water treatment.

anodic reactions and the tendency to corrosion.

industrial processes and effluent treatment.

treatment and conditioning of industrial water.

degasification, odour control, evaporation.

flocculators – settling tanks – flotation units.

fundamental biological engineering processes applicable to water treatment.

fundamental physical-chemical engineering processes applicable to water treatment.

Does 316 stainless steel rust