F. Mazza

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.

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%.

Electrochemical behaviour and stress-corrosion cracking of titanium in alcoholic solutions

Abstract The electrochemical behaviour of titanium in neutral methanolic and ethanolic solutions containing chlorides, and corrosion tests performed with U-bend specimens in similar solutions, indicate that stress-corrosion phenomena occur on titanium when particular anodic and cathodic conditions are settled. In neutral aerated ethanolic solutions the oxide film is stable and its electronic properties (ionic and electronic currents) are similar to the ones observed in aqueous solutions. Exception is made only for the breakdown potential. In ethanolic solutions, stress-corrosion occurs only in the presence of depolarizing species which can set mixed potentials more noble than the one characteristic of oxygen (i.e. FeCl 3 ). Breakdown potential weakly depends on the chloride concentration, but the adsorption of chlorides on the oxide surface, in ethanolic solutions shifts the steady state potential of the metal to more active potentials so that, from the electrochemical point of view the more concentrated NaCl solutions, appear to be less effective in promoting stress-corrosion, the corrosion potentials settled at the higher Cl − concentrations, being less noble than the potentials settled at the lowest Cl − concentrations. In this case the role of Cl − in the dissolution of titanium in ethanolic solution seems to be restricted to the formation of complex ions. The stability of titanium oxide films in methanolic solutions is very weak and, in the anodic sense, is restricted to a very narrow range of potentials. Presence of oxygen can create mixed potentials more noble than the breakdown potentials and thus the stress-corrosion occurrence. The beneficial effect of cathodic polarization in neutral alcoholic environment can be due to the establishment of corrosion potential less noble than breakdown potential of the oxide film.

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.

A study of the corrosion products of aluminum brass formed in sodium sulfate solution in the presence of chlorides

Abstract The surface film forming on Al brass specimens immersed in stagnant Na2SO4 solutions containing chlorides at pH values 3.0–7.25 was examined by using chemical, electrochemical and X-ray techniques. In the absence of chlorides the surface film consists of oxides (CuO, Cu2O) and cupric basic sulfates Cu3(SO4)2.4H2O, stable in the whole range of pH; the surface film is quite homogeneous and no dezincification or localized corrosion occurs. In the presence of chlorides, the surface film consists also of cuprous chloride (CuCl) and of cupric [Cu(OH,Cl)2.2H2O] and aluminum oxychlorides [Al45O45(OH)45Cl] and aluminum oxides (Al(OH)3]. The weight of the corrosion products is a maximum in solutions containing 5 × 10−3 M NaCl at each pH value. In the most acidic solutions the surface film is physically and chemically extremely unhomogeneous, thus favouring the occurrence of dezincification or localized corrosion phenomena.

Galvanic platinizing and passivity of stainless steel in H2SO4

The corrosion resistance of stainless steel in H2SO4 can be increased by the presence of electrodeposited Pt on the metal surface. Such an effect is only partly attributable to a greater efficiency of platinum as a cathode for O2 reduction, which is the sole mechanism in the anodic protection of stainless steel by galvanic coupling with Pt. The greater resistance of platinized stainless steel to corrosion is due to selective deposition of platinum in form of small round particles (~ 0·01 μ dia.), under appropriate conditions of platinizing; these particles can block the most active sites of the stainless steel surface. Electron-microscope observations on thin film samples provided evidence for a certain selectivity of the Pt electrodeposition toward emerging dislocations and grain boundaries. This selectivity is linked with the degree of stability of the passivity state and can be improved by a preliminary passivating treatment.

The influence of chloride ions on the stress corrosion cracking of Al-brass in acidic sulphate solutions

Abstract Chloride ions can play a stimulating or an inhibitive role on the occurrence of the stress corrosion cracking (SCC) of Al-brass in acidic sulphate solutions depending upon their concentration, the pH and the Cu 2+ content of the electrolyte. The stimulating effect is observed either in environmental conditions in which competition between the precipitation of Cu 2 O and CuCl can occur or in conditions in which Cu 2 O is thermodynamically stable but the presence of a critical amount of Cl − induces an alteration of its defect structure. The inhibiting effect is observed at high Cl − concentrations where CuCl is the prevailing stable phase.