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

This paper is the second in a series looking at understanding the factors controlling and predicting marine aerosol concentration on land. It looks at results from three transects across the Australian continent. In each transect, the airborne salinity was measured, using the wet candle method at distances from 10 m to 40-300 km from the coast. The positions of the transects were selected to give a significant variation in the factors controlling salt production and transport. For example, one transect in South Australia was established where both high whitecap activity is likely to promote salt production and flat terrain and prevailing winds are likely to favour transport. Another, in Queensland, was established where calm seas will limit salt production and very seasonal winds and high relative humidity and rainfall will limit transport. On the basis of this experimental study, the general validity of the fundamental concepts put forward in Part 1 is assessed. Further, the feasibility of building a mathematical model to predict salinity is determined and the main factors causing variations in salinity on land are outlined. The results are then used to assist in the interpretation of previous work in the literature.

This paper present results from a large number of airborne salinity measurements in Australia and south-east Asia (SEA). Salinity measurements were made in the open on clear ground, in the open in urban regions, and on the facades of buildings. The data were used to assess the validity of the concepts regarding saline sources and transport presented in previous papers. In particular, the salinity measurements in SEA are used to assess the validity of the models for ocean produced salt aerosol. A classification scheme for defining sheltering by man made and natural landforms and types of water bodies was developed. The data were then used to assess the sheltering effect of urban landforms and of building facades. Lastly, the data were used to assess the possible contribution of narrow and wide bays to airborne salinity.

The author has studied several failures in the field of bent and branched copper-nickel pipelines exposed to marine environments contaminated by ammoniacal byproducts. The cause of the failures was investigated by laboratory tests on the Cu-5.37 wt-%Ni alloy used for the failed pipes. The tests were conducted under slow strain rate testing (SSRT) conditions, in aqueous ammonia and ammoniacal sea water environments. These studies revealed that the presence of ammonia in sea water impairs the load bearing capacity of the alloy, and the aqueous ammonia environments tend to cause brittle stress corrosion cracking failures, which are often premature and/or catastrophic. However, in addition to induced residual stresses from manufacturing/processing, the operating conditions in marine environments subject these pipes to external forces and widely varying pressures and fluid flow rates, which can cause both static and cyclic stresses. Experiments conducted under SSRT conditions, however, could not predict the operative failure modes and mechanisms for the alloy when exposed to ammoniacal environments under the combined action of static and cyclic stresses. In the present investigation, the stress corrosion failure characteristics of the alloy studied previously in SSRT, were subject to testing under static load with superimposed fixed interval, low amplitude loading cycles. These tests were conducted in air and in 5 and 10 wt-% aqueous solutions of ammonia. The results and observations help to elucidate the role of superimposed cyclic stresses on the stress corrosion failure behaviour of the alloy in the ammoniacal environment.

The corrosion behaviour of carbon steel under combined water treatment (CWT) conditions was investigated for the purpose of determining the minimum adequate oxygen concentration. Corrosion tests were carried out in flowing water (pH 9.0) under simulated CWT conditions at 250° C for 500 h, and electrochemical measurements were also carried out during these test periods. The corrosion potential of carbon steel shifted in the noble direction and the polarisation resistance increased with increasing oxygen concentration in the range of 5 to 15 μg L⁻¹. The corrosion rate calculated from weight loss measurements in oxygenated water containing 15 μg L⁻¹ of dissolved oxygen was less than that in deaerated water. In the case of the water containing 25 μg L⁻¹ of dissolved oxygen, the corrosion loss was much smaller than that for deaerated water. In addition, no significant change of corrosion behaviour was observed for oxygen concentrations in the range 25 to 100 μg L⁻¹. The present work has shown that the oxygen dosing concentration for CWT could be decreased, the minimum oxygen concentration required to maintain protective oxide films having been estimated to be in the range 15-25 μg L⁻¹.

Unalloyed and low alloyed steels undergo predictable uniform corrosion and are potential candidates for high level nuclear waste disposal containers. In this study, low alloyed silicon steel (< 5 wt-%Si) has been tested in a silicate containing corrosive solution at 90 ° C in order to establish whether the addition of silicon to iron would improve the resistance of the surface film to corrosion in an argillaceous (i.e. clayey) soil. Electrochemical tests were conducted for short term immersion of the steel coupons and showed that the addition of silicate to the corrosive solution decreased the corrosion rate, whereas the addition of silicon to the steel increased the critical passivation current peak. A comparison with an unalloyed carbon steel showed that the latter material was more easily passivated than the Si alloyed steel. Weight loss tests were used to study the long term behaviour of the steels in the corrosive medium at 90° C. The inhibiting effect of sodium silicate was significantly increased after some hours of immersion, and the Si alloyed steel was more easily protected than the carbon steel. The surface film consisted of sheet silicate. The influence of the silicate concentration was discussed and correlated to the nature of the dissolved species: silicic acid and/or colloidal particles.

The effects of two methods for the surface addition of yttrium on the oxidation behaviour of 304 stainless steel have been investigated. Isothermal oxidation tests were performed at 1000° C in air for 100 h. The results show that ion implantation and sol-gel coating have similar effects, reducing significantly the scale growth rate. In situ X-ray diffraction analyses clearly demonstrate that surface additions of yttrium allow the oxidation rate of type 304 stainless steel to be reduced by limiting the growth of non-adherent iron rich oxide. In situ analyses were also carried out during cooling in order to observe the structural evolution of oxides formed at high temperature.

The addition of nitrogen to duplex stainless steel weld metal has been achieved and the resultant weld metals have been assessed for their microstructural features and corrosion behaviour. It was found that, although autogenous welding of duplex stainless steels is not recommended, the addition of nitrogen through the shielding gas mixture can help to obtain quality welds with the desired phase balance and corrosion resistance.

AISI type 304L stainless steel (SS) was assessed for its corrosion resistance in nitric acid solution. The effect of variation in nitric acid concentration (1 to 10N) and temperature (298 K, 313 K, 333 K, 348 K) on the corrosion resistance of 304L SS was investigated using potentiodynamic polarisation studies. The corrosion resistance of the alloy was also studied in 1N nitric acid containing different concentrations of chloride ion (1000, 5000, and 10 000 ppm) in order to understand the tendency to pitting corrosion. The results show that the corrosion resistance of the material deteriorated for acid concentrations greater than 4N. The temperature of the acid was found to have a profound effect on the corrosion resistance, with corrosion becoming more prominent at temperatures of 333 K and above. Chloride ions up to a concentration of 10 000 ppm were found to have no effect on the pitting resistance of the material studied.

Various protective methods may be used to improve the corrosion resistance of steel, and the application of coatings of zinc and its alloys is one of them. The efficiency of zinc in corrosion protection is due to its behaviour as a sacrificial anode. To enhance the corrosion protection, zinc has been alloyed with more noble metals such as cobalt, nickel and iron. In this work zinc-cobalt alloys were electrodeposited onto steel from an alkaline electrolyte. The investigation was carried out on electrodeposits with low and high cobalt contents. An in situ study was performed using a TEM equipped with a hot stage to relate the heat treatment with changes in structure and, consequently, corrosion behaviour. The images of the specimens as deposited and after heat treatment showed the presence of a phase that was identified as Co₅Zn₂₁. However only the diffraction pattern of the heat treated specimen fitted the strongest lines for this compound. The corrosion tests showed differences in the corrosion resistance before and after heat treatment, for alloys with low and high cobalt. The heat treated specimens displayed more active behaviour.