Anodising corrosion resistance

It is somewhat less corrosion resistant than tantalum, and like tantalum suffers from hydrogen embrittlement if it is made cathodic by a galvanic couple or an external e.m.f., or is exposed to hot hydrogen gas. The metal anodises in acid electrolytes to form an anodic oxide film which has a high dielectric constant, and a high anodic breakdown potential. This latter property coupled with good electrical conductivity has led to the use of niobium as a substrate for platinum-group metals in impressed-current cathodic-protection anodes. 

Properties of deposits These deposits invariably possess an appreciable number of discontinuities, which diminish their protective value. At such discontinuities there is an inherent susceptibility to corrosion because of the galvanic relationship that necessarily exists between A/, and. Corrosion resistance may, however, sometimes be improved by sealing (cf. anodising) or by chromate passivation. 

Since the natural passivity of aluminium is due to the thin film of oxide formed by the action of the atmosphere, it is not unexpected that the thicker films formed by anodic oxidation afford considerable protection against corrosive influences, provided the oxide layer is continuous, and free from macropores. The protective action of the film is considerably enhanced by effective sealing, which plugs the mouths of the micropores formed in the normal course of anodising with hydrated oxide, and still further improvement may be afforded by the incorporation of corrosion inhibitors, such as dichromates, in the sealing solution. Chromic acid films, in spite of their thinness, show good corrosion resistance. 

One of the most important characteristics of aluminium and its alloys is the thin oxide film which forms on their surfaces when exposed to the atmosphere. If the oxide film is broken it will reform quickly, and this gives these materials excellent corrosion-resistance. This oxide film can be artificially thickened to give added protection - a process known as anodising - and can be easily coloured to provide a highly decorative appearance. 

Hard anodising is a process where the anodising parameters are changed, leading to a much thicker, harder, abrasion- and corrosion-resistant film. The film formed is dark in colour, so black is the only sensible film colouring. 

Haynes G. In Corrosion Tests and Standards Apphcation and Interpretation, Baboian R., ed. ASTM Manual Series MNL 20, Philadelphia, PA, 19103, 1995, pp. 91-97 Hirai S., Shimakagen K., Aizawa S. Alkaline corrosion resistance of anodised coated with zirconium oxide by a sol-gel process. J. Am. Ceram. Soc. 1998 81(12) 3087-3092 Hirayama R., Harayama S. Electrochemical impedance for degrades coated steel having pores. Corrosion 1991 47(12) 952-963... 

Shi Z., Song G.-L., StJohn D. (2001), A method for evaluating the corrosion resistance of anodised magnesium alloys . Corrosion and Prevention 2001, ACA, Newcastle,... 

Song G.-L (2004a), Environment-friendly. Non-toxic and Corrosion Resistant Magnesium Anodisation 2004904949 (Australia). 

Z Shi, GL Song, A Atrens, Influence of anodising current on the corrosion resistance of anodised AZ91D magnesium alloy. Corrosion Science, 2006, 48, 1939-1959. 

Sulphuric acid anodising Alloys easy to anodise. Protection, decoration, good corrosion resistance, resistance to scoring Architecture, building, kitchen utensils... 

Within the composition limits of contemporary alloys, the iron and silicon content does not have a major influence on the corrosion resistance of semi-products of this family. Resistance to atmospheric corrosion is good. Pure alloys 1199 and 1190 are anodised when used outdoors. 

It has long been known that even when delivered in appropriate tempers, their susceptibility to corrosion is, in general, too high to allow their use without protection in most environments atmosphere, water, etc. [24]. Protections commonly used are paint, cladding with 1050A, and anodising. They are, however, not sufficient to compensate these alloys intrinsically low corrosion resistance due to their high copper content. 

Tests have shown that the corrosion resistance of aluminium in water is not influenced by the injection of 10 ppm chlorine, which is well above the usual level in biological water treatment plants (Figure D. 1.8) [16]. Aluminium resists water and the chlorine environment of swimming pools very well. When unprotected, it will develop the usual water staining, and when anodised at 20 pum, neither pitting corrosion nor water staining will develop for several years [17]. 

These are excellent corrosion inhibitors for aluminium (see Section B.5.8) and are widely used as such zinc chromate is a pigment for primer coatings, potassium chromate is used (as solutions at 3-7 g l at 100 °C for 30 min) as a sealing bath after anodising that confers to aluminium components an outstanding corrosion resistance. However, their use is more and more restricted due to their toxicity. 

Sheet aluminium can be given a colour by a similar process. The aluminium is first made the anode in a bath of chromic acid (p. 377) when, instead of oxygen being evolved, the aluminium becomes coated with a very adherent film of aluminium oxide which is very adsorbent. If a dye is added to the bath the oxide film is coloured, this colour being incorporated in a film which also makes the remaining aluminium resistant to corrosion. This process is called anodising aluminium. 

BS 1490 Major alloying conslituenis (nominal "It) Condition strength (N/mm ) Sand Chill Fluidity Resistance to hot tearing Pressure tightness Machin- ability Resistance to corrosion Protective anodising Comments... 

Zinc alloys have good resistance to corrosion, and die-castings can be left as-cast However, in situations which are particulariy corrosive, chro-mating or anodising is used for added protection. Where functional and decorative finishes are required, the most widely used is chromium plating, and the best results are obtained when an initial layer of copper is followed by layers of nickel and chromium. Other finishes such as painting and plastics coating can also be applied. 

Maintenance is minimal, even when no extra protection (painting, anodising) is provided. When aluminium is painted, replacement of the paint is less frequent and less urgent because the underlying metal generally has good resistance to corrosion. However, the use of aluminium does not make maintenance unnecessary, especially... 

---