Corrosion Engineering,Science and Technology: The International Journal of Corrosion Processes and Corrosion Control

Published data on the anodic dissolution, reduction of oxygen on, and free corrosion of silver alloys in aqueous media are reviewed. With the possible exception of Ag–Sn, addition of less noble metals enhances the dissolution rate. Fundamental studies suggest that, at less anodic potentials, dissolution rate is controlled by solid state diffusion of Ag atoms in the lattice.

A n environmental test cell and associated equipment are described, in which direct visual and photomicroscopical information on the initiation of simulated atmospheric corrosion as it occurs in situ have been obtained.

The environmental flow rate and angle of presentation to the metal surface, temperature, relative humidity, and sulphur dioxide concentration are controlled and monitored at values typical of natural conditions. Special emphasis has been placed on corrosion initiation associated with deposited particulate matter related to atmospheric pollution. Inspection of the particulate matter and induced corrosion has been undertaken by SEM/EDAX and attempted by Auger spectroscopy. Examination using ammonium sulphate or ammonium chloride particles separately showed that, whilst the critical humidity for corrosion initiation on iron relates to the humidity over the saturated solution of the salt in each case, thereafter the humidity permitting continued corrosion is most probably affected by the equilibrium humidity over the appropriate soluble corrosion product. Other work demonstrated that sulphur dioxide promotes both the development of aerial corrosion protuberances in ammonium chloride initiated corrosion and the appearance of localised corrosion around sand particles. More generally, the amount of sulphur dioxide presented to iron specimens was found to be a more important corrosion parameter than its atmospheric concentration at relative humidities over 90%. Such considerations are relevant to the corrosion protection of exposed plant and buildings, where design aspects should take account of consequential air flow patterns (flow rate and direction), so minimising both turbulence and deposition of particulate matter.

This research has involved investigation of the simulated atmospheric corrosion of zinc, aluminium and especially iron in situations where particulate pollutants initiate attack at various relative humidities, in the presence and absence of sulphur dioxide. Metal surfaces holding small population densities of particulate material. collected at sites near well-defined sources in rural. industrial and marine locations. have been examined directly and after exposure in simple humidity cabinets. or in a specially designed cellfor in situ observation. Additionally. SEM/EDAX examination has assisted in identifiying the nature and corrosivity of the individual particles. The four most aggressive species were salt and salt/sand from marine or de-iced locations. the emitted ash from iron smelters. plume ashfrom municipal incinerators and coal mine dusts. In each case the corrosiveness was associated with the presence of chloride ions and/or sulphur compounds. Other particulate types. such as fly ashes. had more limited corrosivity. Thus. corrosion was promoted in the vicinity of coal ash in the presence of sulphur dioxide. whilst oil fly ash was relatively non-corrosive even in the presence of sulphur dioxide. Considering the much higher inorganic content of coal fly ash, and the greater porosity of oil fly ash. this suggests that the corrosion mechanism depends upon the catalytic promotion of sulphate formation from sulphur dioxide by inorganic species, such as Fe²⁺ and Mn²⁺ ions, which then promotes the anodic reaction, and not simply the absorbed presence of sulphur dioxide per see Chalk particles were non-corrosive even in the presence of sulphur dioxide, due perhaps to the removal of sulphur dioxide as insoluble sulphite/sulphate. The desirability of controlling the emission or distribution of particulates from certain plants is indicated.

Laboratory studies have been carried out to study the corrosion-erosion behaviour of constructional materials in the manufacture of wet process phosphoric acid. Experiments were conducted in the static and simulated premixer condition of the Nissan process based on the hemihydrate–dihydrate route. Static corrosion tests were carried out with stainless steels AISI 316L, HV–9A and Sanicro–28 in a sulphuric–p, osphoric acid mixture, and the effect of additions of hydrofluoric acid, hydrofluosilicic acid and chloride were also studied. In the second series, tests were conducted in the simulated premixer condition with HV–9A. The behaviour of imported and indigenous rock has been compared and the effect of kieselguhr was studied. The effect of the coarseness of the rock and temperature on the corrosivity of the system was also investigated.

Stress corrosion tests were carried out on friction welds in prestressing wire. Testing was undertaken in 3% sodium chloride solution, using both plain and pre-cracked samples. The results indicated that there was no significant difference between the see sensitivity of parent wire and that of friction welds. However, enhanced pitting attack at friction welds was found. associated with the microstructural reorientation in this area.

This paper is a continuation of work published as Parts I–III (BCJ, 1975, 10, 17; 1978, 13, 28; 1980, 15, 41) where a method was suggestedfor protecting Al–2·5 wt% Mg alloys against sec in saline water, using suitably oriented (normal orientation of the cells to the future stress direction) electrolytically prepared (in 15% H₂SO₄ at 25°c) γ₁-Al₂O₃(I). The results were optimised using a c.d. of 6 A/dm² to prepare γ₁-Al₂O₃ (I and II) and a thickness of 3·4 μm (III). In III a correlation between sorptive and/or catalytic properties of the γ₁ ⁻,γ_1,2 ⁻ and γ₂-Al₂O₃ and their protective properties was found. In the present work the anodising bath temperature was varied and maximum protective properties were found at 25°c for 3·4 μm and 13·5 μm thick γ₁-Al₂O₃ and at 40°C for 23 μm thick γ₁-Al₂O₃. Thus the optimum mean increase of the time to failure for 13·5 μm thick γ₁-Al₂O₃ is 72%, for 23 μm 100%, andfor 3·4 μm 220% compared with that for the unanodised surface. Explanations for this are suggested, based on the differing secondary structure of the oxide at different temperatures, as revealed by SEM.

New aluminium–thiourea inhibitors for mild steel in water are reported. These inhibitors, a combination of aluminium sulphate, thiourea and cationics, are tested using the weight loss method according to NACE Standard TM-01–69. It was found that the corrosion rate of mild steel in distilled water can be decreased markedly (up to 79·0%) if aluminium ions are used in conjunction with thiourea and cationics. The concentration of the aluminium ions in the mixture and the nature of the anion which accompanies these ions are critical for the inhibitive efficiency of the combination tested; thiourea and cationics appear to augment this efficiency if their dosage (and their composition–where cationics are concerned) are chosen properly.

The mechanism of the corrosion of lead in sea water has been investigated in stationary and laminar flow conditions (from 0·3 to 3 m/min). Some experiments were carried out in NaCl and Na₂SO₄ solutions in order to discriminate more efficiently between the action of certain ions (Cl⁻, SO₄ ²⁻) that might affect the corrosion kinetics, while using the same environmental parameters (temperature, dissolved oxygen, fluid flow rate).

These solutions all had the same anionic concentration and the same pH value as sea water. The primary lead passivation film consists of basic salts resulting from the simultaneous precipitation of various anions (OH⁻, Cl⁻, sO₄ ²⁻) together with Pb²⁺ ions. In particular, the non-conductive corrosion products forming the primary film leading to ‘mechanical’ passivation in sea water are Pb(OH)Cl and Pb₃(OH)₂(CO₃)₂ and, after 360 h immersion, they were found to be: Pb(OH)Cl, Pb₃(OH)₂(CO₃)₂, PbCO₃, PbCl₂, PbO, Pb₂O₃, and PbCl₂. SOO₄ ²⁻ ions do not inteifere with the lead passivation mechanism in sea water. In stagnant solutions, CO₃ ²⁻ ions do, however, contribute to the formation of a compact and adherent film of basic carbonates and double salts inhibiting lead corrosion. But in moving solutions, lead corrosion is solely linked to the activity of Cl⁻ ions and the anodic dissolution mechanism is the same as for NaCl-containing solutions.

Electrochemical measurements, visual appearance and estimation of dissolved iron were applied in order to assess the performance of lacquered tinplate cans filled with melon cubes and stored at 15,25,35 and 45°c. Visual appearance rating was found to relate well with polarisation resistance, indicating for a heavily attacked can a low visual score and a low resistance value. Corrosion potential values of the lacquered cans changed little with change in storage time and temperature, apparently due to the small dissolved metal concentration in this product. Corrosion currents, as obtained from potentiodynamic measurements, were found to increase linearly with time, and Q₁₀ values of about 1·8 were calculated for the metal exposure rate. An equation relating current density with time and temperature, which enables the prediction of shelflife of the food-lacquered can system, was found. Corrosion currents obtained from linear polarisation and potentiodynamic measurements were found to be related, indicating that linear polarisation, which is a fast and simple method, may be used for estimating corrosion currents. A relationship between current density and dissolved iron was also found and either or both of these values can be used as a criterion for the shelflife of the product itself.