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

The author, reviewing 45 years of experience in the UK, considers that the increase during that time in specialised research and teaching in corrosion science has not produced its maximum possible benefit to corrosion prevention in practice, and that application of economic cures for corrosion troubles is both more necessary and more difficult than explanation of their cause. He suggests that teaching, research, and the activities of scientific and technical societies should now be aimed more at diffusion of directly useful information to users of metal, at the development of improved inspection and test methods and the provision of useful specifications and codes of practice.

This paper is divided into three parts. The first deals with the general aspects of failure investigations discussing the kind of staff and facilities required and emphasising the need for speedy, accurate reporting: The second part is concerned with the nature of engineering failures and analyses the results of almost 1000 investigations with regard to failure modes, field of application and components that fail. The third and major part describes a variety of typical failure investigations that illustrate the concepts outlined in Part 1. These mclude problems of bad fitting, corrosion and abrasive wear on plain bearings; the corrosion of exhaust valves and spark plug electrodes by deposited lead compounds and methods for measuring operating temperatures on inaccessible components; exhaust valve sinkage (valve seat recession) on engines running on ash-free fuels such as naturalgas and unleaded gasoline; the size factor in mechanical testing with special reference to anchors; fretting fatigue; cavitation damage in fuel injection systems and diesel engine cylinder liners, and stress corrosion cracking of Admiralty brass condenser tubes cooled by Thames river water.

A draft of this paper prepared by the authors during the Spring of 1973 has been submitted to a detailed discussion at the Annual Meeting of Working Party NO. 2 (Delft, 14 June, 1973) whose participants are listed below.†

All modifications proposed by the members of the Working Party received the agreement of the authors who then prepared the present version. Final approval was then required and obtained from all the members of the Working Party and from the Scientific Advisory Committee of the E.F.C.

R. Scimar—President of W.P.2

Full scale trials of ten paint systems, applied on the plate girders of the Loudwater Viaduct, are described. The best performance during four years of exposure was given by a solvent less epoxy system, but its application requirements were stringent and may be difficult to meet in practice. For high corrosivity sites, a chlorinated rubber paint system should give a good performance with easy application, although transportation damage between fabrication shop and site may be a problem. A general conclusion is that aluminium pigment should be excluded from conventional micaceous iron oxide paints. In several instances aluminium has been leached out as aluminium salts leading to deterioration. of the paint system and rusting of the substrate.

The importance of the micro-climate to which sheltered bridge steelwork is exposed is discussed. Steel girders are not washed by rain and the combination of corrosive contaminants and long periods of wetness can be a particularly severe condition for paint coatings under a bridge deck. Further research on the characterisation of bridge sites and evaluation of priming paints has been initiated.

Climatological and air pollution factors greatly affect the corrosion damage to steel highway bridges. This paper gives first results of a programme to define quantitative criteria by which the potential corrosivity of a prospective bridge location can be assessed.

At seven sites throughout the country, the corrosion rates of a special mild steel, a low alloy steel and zinc are being measured under both open and sheltered exposure conditions. Meteorological and air pollution data are also being collected.

To eliminate the considerable delays which can occur in negotiating the building of permanent test compounds, a mobile test rig has been developed, which can be easily transported to any prospective bridge location.From the first year's results, it is apparent that sheltered test specimens give a much closer indication of the corrosivity under a bridge deck than do open exposure specimens. Of the environmental factors, atmospheric chlorides and sulphur compounds are the major factors influencing corrosion rate. It was not possible, at this stage of the work, to produce a correlation between corrosion rates and meteorological data.

The objective of this research contract, for the Department of the Environment, was to assess the behaviour of the various primer types, commonly used on structural steel in the special environment under road bridges. In the first instance unrusted steel was used in order to reduce the number of variables. The work has shown that even in this sheltered situation the protection afforded by primers is dependent on the water resisting qualities of the binder as well as on the inhibitive pigment used. The results also demonstrated that, by choice of a particularly aggressive site (Norfolk Bridge, Shoreham), assessments of relative performance of different primers could be made almost as rapidly as, and more reliably than, by artificial laboratory tests. Further work is in hand to explore the effects of paint binder more thoroughly and to assess the effects of rust contamination before blast cleaning and priming.

Corrosion rates of mild steel when coupled to each of 31 other metals and alloys have been determined in 5 natural environments using disc-type specimens. These environments were industrial, urban/rural and marine atmospheres, natural water and sea water. The relative order of effect on the corrosion rate of mild steel in each environment has been tabulated. The results show that the performance of mild steel in a given couple can differ appreciably in different environments.

Chemical and microprobe analyses have been used to study the composition of the oxide films formed on iron by 0·05 M potassium chromate, pH 4–8.

Chemical analyses have shown that the air-formed oxide film was thinned and evenly reinforced with a normal iron chromium spinel having a composition in the range Fe²⁺ (Fe³⁺ _0·5 Cr³⁺ _1·5)O₄ — Fe³⁺ Cr³⁺ O₃

Microprobe analyses indicated enrichment of chromium at scratch lines, but the effect was small compared with the overall thickening of the films.

The corrosion of 3S aluminium (Al-Mn alloy) and its inhibition in solutions of phosphoric acid has been studied. The extent of corrosion increases with increase in acid concentration and with the length of the period of immersion. At 0·5% inhibitor concentration the efficiency of the inhibitors increases in the order: triethylamine < dicyclohexylamine < diethylamine < di-n-propylamine < cyclohexylamine < n-butylamine < ethylamine < trimethylamine < ethylenediamine < hexamine < isopropylamine < dimethylamine < methylamine < n-propylamine. At constant acid concentration the inhibitor efficiency increases with increase in the concentration of the inhibitor, whereas at constant inhibitor concentration the efficiency decreases with increase in the concentration of the acid. Very little correlation appears to exist between the pK_b values or the molecular weight of the inhibitor and the extent of inhibition. Maximum inhibition is achieved in those cases where the pH of the inhibited solution lies in the range 4·5 to 8·5, but where the solution is required for use at low pH values, then hexamine appears to be the most suitable inhibitor.

Three deteriorated structures exposed to different environmental conditions in Egypt and a non-deteriorated structure have been inspected. Visual and microscopic examination of reinforcing steel samples from the different constructions revealed the occurrence of pitting corrosion in the reinforcement of a building exposed to a marine atmosphere, local and general attack in the reinforcement of a Filtration Building for the Domestic Water Project and stress corrosion cracking in the horizontal reinforcement of some concrete columns of a building made of Portland blast furnace slag cement. It has been found that the type of corrosion is mainly determined by the type of aggressive ions present in the concrete and ta a lesser extent by the microstructure of steel. Examination of the non-deteriorated structure led to the conclusion that low concentrations of aggressive ions in, and high alkalinity of, the concrete, relatively low C and S contents of the reinforcement and a non-aggressive surrounding atmosphere establish the ideal conditions for the durability of the structure.