Before metal finishing or coating, metal surfaces have to be scrupulously cleaned of contaminants. Aerospace pickling serves this purpose. Prior to the application of a surface coating to steel, as for example zinc in the galvanizing process, it is necessary to remove all surface scale and oxide corrosion products. This is achieved by acid pickling.
If electrolytic cathodic hydrogen charging has occurred, during the dissolution of the scale the steel is anodic and the oxide cathodic, but on exposed steel surfaces free of scale, and once scale has been removed, there will be overall attack with ferrous ions being produced in anodic areas of the steel and hydrogen in the cathodic areas. The differing polarity of different surface regions is produced by the underlying microstructure. Although the majority of the hydrogen produced may escape as gas bubbles, some of it diffuses into the steel in atomic form. The current in the cell can be substantial and result in significant hydrogen ingress. With hardened or high carbon steels the effect may be so pronounced that internal voids and cracks may be produced, some evident as surface 'blistering'.
The electrode reactions are sensitive to the presence of impurities in the acids. Arsenic, for example, raises the overvoltage for the hydrogen evolution reaction and increases hydrogen absorption and diffusion into the steel. Sulphide in the acid promotes the anode reaction and the formation of ferrous ions.
Sulphuric acid anodising
Sulphuric acid anodising is also known as hard anodising and is one of the coating processes that follows on naturally from aerospace pickling. Although hydrochloric acid is more expensive than sulphuric acid, it is becoming increasingly used instead of sulphuric acid anodising, because the waste liquor can be recovered more economically. It is more active than sulphuric acid at an equivalent concentration and temperature, probably because the rates of diffusion of acid to, and ferrous ions from, the steel surface are greater. It is also more suitable for aerospace pickling prior to coating, since it gives less deposit on the steel and any residual iron chloride can be rinsed off more readily than residual iron sulphate deposits.
If aerospace pickling is employed then the pickling time should not be longer than is required for oxide removal. Protracted immersion will lead to hydrogen charging.
If the pickled steel is then immersed in a zinc bath and a coating produced, hydrogen is essentially 'locked in', since the diffusivity of hydrogen through the zinc is slow. Coatings of cadmium and copper have a similar effect and are used to retain hydrogen in steel after charging. Thus absorbed hydrogen in a galvanized steel will tend to concentrate below the zinc coating and may lead to hydrogen embrittlement in steel of high strength.
Aviation-database.com [http://www.aviation-database.com/] has lots of resources for the aircraft industry. The web is a vast source of information. Aviation-database collects the industry into one huge database of contacts. Aerospace pickling [http://www.aviation-database.com/ashton-moore-aerospace-metal-finishing.html] and sulphuric acid anodising are both aerospace metal finishing processes offered in the UK by Ashton & Moore in Birmingham.
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