Corrosion Protection & Prevention
All coatings whether metallic or non-metallic, when applied to steel and other metals, provide a protective barrier envelope for the prevention of corrosion of the underlying metal or alloy. Non-metallic coatings (paints, lacquers, oils and waxes, etc.) are electrically non-conductive and relatively inert. When uniformly applied, protection is provided so long as the surface film is not damaged. In contrast, metallic coatings are conductive and have a more active role in the prevention and control of corrosion.
The Galvanic Series of metals and alloys indicates the behaviour to be expected when dissimilar metals are in contact, including where metallic coatings are applied to other metal surfaces and where they are to be used in applications where the surface is wetted. For example, where steel is protected by coatings which are inherently more corrosion resistant (more noble) the protection is maintained so long as the coating envelope is complete. At points of surface damage, pin-holes and porosity accelerated local attack on the underlying metal can be expected, as with nickel/chromium coatings on steel components.
Conversely, surface coatings of the less noble metals such as Al, Cd,Zn, Zn-Al (Zn Flake) are not only protective as an envelope but in addition actively prevent base metal corrosion wherever local damage to the coatings occurs. This action is known as “sacrificial protection” because the surface film corrodes preferentially, albeit slowly, to the underlying steel. This phenomenon occurs in ‘normal’ environments but under some circumstances, the sacrificial protection does not occur, e.g. zinc on steel at temperatures above 85°C.Metallic coatings on metals, other than steel, have different effects depending upon the position of the metals in the galvanic series. This is why, for example, some coatings perform badly in contact with aluminium.
The Galvanic Series can also be an important consideration in the design of assemblies to be used wholly or intermittently in wet service conditions in which dissimilar metal contacts occur. The degree to which this factor may affect a particular assembly can depend upon other aspects also and expert advice is often justifiable. There are beneficial influences of using specific coatings to prevent or diminish contact corrosion, e.g. zinc and zinc flake are suitable coatings which, when applied to steel and stainless steel, can protect connecting surfaces. In such circumstances, brass, nickel and copper coatings would accelerate aluminium corrosion. Excellent advice on bimetallic corrosion is given in BSPD 6484 “Commentary on corrosion at bimetallic contacts and its alleviation”.
Illustration of Zinc Sacrifical Protection
Conversion / Passivate Coatings
To further enhance the corrosion resistance of zinc, the surface can be given a conversion coating with, usually, a chromium bearing solution. These coatings often contain hexavalent chromium which is a material that is controlled under the EU End of Life Vehicles Legislation effective July 2007. In most cases the hexavalent chromium can be replaced with trivalent chromium (which is considered harmless) but to the detriment of the corrosion resistance. This can be reinstated by the Leach-Seal process (JS 500 lacquers). The colours obtained are clear, yellow to iridescent, opaque, olive drab or black.
A zinc coating can be modified by using a special solution to deposit an alloy. In some instances these alloys slow the rate of corrosion as they alter the magnitude of the current that flows due to the reduced potential.
Galvanic Series of Metals and Alloys in Sea Water
Ref. La Que and Cox, Proc. Amer. Soc. Test Mat 40, 670 (1940)
Published “standard electrode potential” tables are obtained from thermodynamic data. They are seldom used to predict the degree or nature of the corrosion reaction occurring.
If you would like further information on corrosion protection and prevention from an industry expert or to find out how Anochrome can assist you, please email us via the Contact page, or alternatively you can call us on 01902 397333.