S. Torchio

Electronic properties of oxide films and pitting susceptibility of type 304 stainless steel

Abstract Semiconducting surface films formed by thermal treatment on type 304 stainless steel at temperatures ranging 150–300°C can markedly influence the pitting susceptibility of the underlying metal. Oxygen defective films ( n -type conductivity) can create conditions which favour the nucleation and development of pitting corrosion. On the contrary, oxide films with an excess of oxygen with respect to complete stoichiometry ( p -type conductivity) are more protective against pitting attack. Thermal treatments which can modify the original structure of such oxide films can substantially modify the susceptibility to the pitting corrosion. Pitting susceptibility can be evaluated through the pitting density obtained in chemical tests using glycerol-ethanolic AlCl 3 + FeCl 3 + LiCl solutions. In particular, thermal treatments at 150°C in dry air can transform an oxygen-defective nonstoichiometricfilm, previously obtained by oxidation at 300°C displaying n -type conductivity and high pitting susceptibility, into a metal defective non-stoichiometric oxide film displaying p -type conductivity and low pitting susceptibility.

Stress—corrosion cracking of austenitic stainless steel in hydrochloric acid media at room temperature

Abstract Stress—corrosion cracking of solution quenched, type 304, stainless steel can occur at room temperature in HCl solutions ranging between 5·10 −1 M and 1M HCl. The cracking observed in HCl solutions is similar to that previously observed in H 2 SO 4 + NaCl and HClO 4 + NaCl solutions. Cracking occurs at − 0·200 V (NHE), in the active potential region, it is under cathodic control, and it develops in conditions under which the corrosion rate of the external surface area is more or less constant and independent of the HCl concentration, in the range 10 −1 M−1 M HCl. At higher HCl concentrations, corrosion rates increase and uneven, general corrosion occurs instead of cracking. The development of pitting and stress—corrosion cracking under active conditions precludes the conclusion that active—passive cells always play a role in localized corrosion and, in particular, in stress—corrosion cracking. Under these conditions, it has been shown that sensitized and non-sensitized specimens behave similarly (giving rise in both cases to transgranular cracking); active—passive cells, due to chromium depletion at the grain boundaries, are not involved. Active—passive corrosion mechanisms can however arise at more noble potentials (0·100−0·200 V NHE), as in the case of HClH 2 O 2 solutions of specific concentration, producing intergranular corrosion of the stainless steel in the sensitized condition.

Chemical etching and pitting ofstainless steel

The use of potentiostatic tests for the evaluation of the pitting corrosion susceptibility of stainless steel may be questionable since they relate only to the anodic characteristics of the metal surface. On the other hand chemical tests provide evidence of both the anodic and cathodic characteristics of the metal and thus reproduce more closely the real corrosion conditions. A series of chemical etchants useful for the determination of the pitting corrosion susceptibility of various types of stainless steels is described. Etchants were glycerol-ethanolic solutions of FeCl2 + AlCl8 + LiCl. The influence of each simple etchant component, the steel type and the metal surface preparation has been evaluated in regard to the intensity and morphology of the attack. Cl− and Al3+ increase the etchants aggressivity; high Fe3+ concentration is very efficient for revealing the pit growth process; low Fe3+ concentrations stimulates the pits nucleation; water produces an inhibiting action.

Stress corrosion cracking of type aisi 304 stainless steel at room temperature; influence of chloride content and acidity

Abstract The influence of chloride concentration and acidity on the occurrence of stress corrosion cracking (s.c.c.) of type AISI 304 austenitic stainless steel at room temperature has been studied. The results obtained show that s.c.c. in acidic chloride solutions can occur in a wide range of acidity as a function of the total chloride content. Cracking occurs in the active potential range.

The behaviour of iron and low alloy steels in anhydrous organic solvents—methanolic solutions

Abstract The data supplied by electrochemical methods such as potentiodynamic polarization curves and potentiostatic anodic transients show that in methanolic solution a region of unstable passivity for Armco iron and Ni-Cr-Mo low alloy steels results from the overlapping of certain ranges of solution composition (e.g. acidity up to 10 −3 M as sulphuric acid, chlorides 10 −4 to 10 −3 M, water 0.01–0.5%). Slow strain rate tests indicated that the instability of the protective oxide films can induce susceptibility to stress corrosion cracking in the presence of applied stresses.

Factors influencing the susceptibility to intergranular attack, stress corrosion cracking and de-alloying attack of aluminium brass

The occurrence and the severity of intergranular attack (IGA), stress corrosion cracking (SCC) and de-alloying attack in Al brass alloys have been investigated in an acidic chloride-citrate solution. Tests performed on laboratory prepared alloys, in which As and P additions were made in order to cover the whole composition range admitted by various national specifications indicated that the susceptibility to the different forms of localized corrosion, is mainly related to the As and P contents and it can be markedly influenced by tempering treatments and by cold-working. The optimum corrosion performances of Al brass are displayed by alloys containing As in the 0.01–0.04% range and P ⩽ 0.0025%.

The effects of quenching conditions on the electrochemical behaviour of Fe34Ni36Cr10P14B6 amorphous alloys

Abstract The influence of quenching conditions on the anodic behaviour of amorphous Fe 34 Ni 36 Cr 10 P 14 B 6 alloy in 1 N H 2 SO 4 and 1 N HCl solutions has been investigated by means of standard electrochemical techniques accompanied by Auger electron spectroscopy and scanning electron microscopy. Both sides of 20 and 60 μm thick ribbons prepared by the planar flow casting technique at two different quenching rates were tested. Strong similarities have been found between the anodic behaviour of the surfaces which underwent the fastest quenching rates (both sides of the 20 μm thick ribbons and the dull sides of the 60 μm thick ribbons) while more noble corrosion potentials and lower currents of passivity were displayed by the shiny sides of the 60 μm thick ribbons, which underwent the lowest quenching rate. These results are interpreted as the result of concurrent factors related both to the chemical composition of the passivity films and to the characteristics of the amorphous structure, i.e. the amount of quenched-in defects.

The stress corrosion cracking of admiralty brass in sulphate solutions

Abstract The field of occurrence, the severity and the morphology of the stress corrosion cracking (s.c.c.) of Admiralty brass exposed, under open circuit conditions, to sulphate solutions have been determined, as a function of acidity and of Cu 2+ content, by means of ‘U’-bend and of constant load tests. Depending upon the solution pH, cracking exhibits two different modes: intergranular in strongly acidic solutions and transgranular in a wide range of moderately acidic and in alkaline solutions. The transition in the cracking mode occurs near the borderline conditions of Cu 2 O stability; in the same conditions the maximum susceptibility to s.c.c. has been observed. Different roles played by Cu 2 O in the nucleation and in the propagation stages of transgranular cracking are discussed.

The influence of arsenic and phosphorus on the stress corrosion cracking of aluminium brass in chloride-citrate solution

Abstract The presence of As and P, in the range usually found in commercial Al brass, strongly influences the stress corrosion cracking (s.c.c.) behaviour of the alloy tested in acidified (pH 4.0) 1 M sodium chloride + 0.25 M tri-sodium citrate + 0.015 M cuprous chloride solutions. Time to failure, recorded during constant load tests, and maximum crack depth measurements on sections of U-bend specimens both confirmed the deleterious effect of the presence of As and P, in relatively high amounts, to the s.c.c. resistance of the alloy. General corrosion, dezincification, intergranular and/or transgranular cracking were found during metallographic examination of the fracture zones.

Stress corrosion cracking of aluminium brass in acidic chloride solutions

Abstract Stress corrosion cracking of aluminium brass in non-ammoniacal acidic chloride solutions has been studied. The most intense transgranular penetration was found in the range of active potentials in synthetic sea water containing 1.5 g/l. of CuCl at pH 1.5, in 1 M sodium chloride solution containing 1.5 g/l. of CuCl at pH 2.0 and in 1 M sodium chloride + 0.25M tri-sodium citrate solution containing 1.5 g/l. of CuCl at pH 4.0. Experimental evidence that high As and high P contents in the alloy can play an important role on the morphology and on the intensity of the corrosion process is presented.