Galvanizing aluminum addition

FIGURE 10.23 Effect of temperature and aluminum addition on inhibitive effect of aluminum alloy layer formation during hot-dip galvanizing. (From D. Horstamnn, Proc. 7th Int. Conf. on Hot Dip Galvanizing, Paris, France, 1964 Zinc Development Association, London, 1967. With permission.)... 

Aluminum coatings on steel will perform in a manner similar to zinc coatings. Aluminum has good resistance to many atmospheres in addition, being anodic to steel, it will galvanically protect exposed areas. Aluminum-coated steel products are quite serviceable under high-temperature conditions, for which gooa oxidation resistance is required. 

Pure aluminum cannot be used as an anode material on account of its easy passivatability. For galvanic anodes, aluminum alloys are employed that contain activating alloying elements that hinder or prevent the formation of surface films. These are usually up to 8% Zn and/or 5% Mg. In addition, metals such as Cd, Ga, In, Hg and T1 are added as so-called lattice expanders, these maintain the longterm activity of the anode. Activation naturally also encourages self-corrosion of the anode. In order to optimize the current yield, so-called lattice contractors are added that include Mn, Si and Ti. 

The quality control of galvanic anodes is reduced mainly to the analytical control of the chemical composition of the alloy, to the quality and coating of the support, to an adequate joint between support and anode material, as well as to restricting the weight and size of the anode. The standards in Refs. 6, 7, 22, 27, 31 refer to magnesium and zinc anodes. Corresponding specifications for aluminum anodes do not exist. In addition, the lowest values of the rest potentials are also given [16]. The analytical data represent the minimum requirements, which are usually exceeded. 

Paints are complex formulations of polymeric binders with additives including anti-corrosion pigments, colors, plasticizers, ultraviolet absorbers, flame-retardant chemicals, etc. Almost all binders are organic materials such as resins based on epoxy, polyurethanes, alkyds, esters, chlorinated rubber and acrylics. The common inorganic binder is the silicate used in inorganic zinc silicate primer for steel. Specific formulations are available for application to aluminum and for galvanized steel substrates. 

Sulfamic acid (HOS02NH2). Clean at only 120 °F (49 °C) for cast iron and up to 140 °F for marstenitic SS (60 °C). It is not suitable for galvanizing or aluminum over 150 °F, but it is suitable for copper, brass, and SS. Sulfamic acid tends to be used only in small systems because of its relatively high cost. It is a ciystalline solid and so is easily transported. Additionally, in the diy form it is relatively safe and has a negligible effect on skin. Typically, it is used at 10% strength, and when 5% sodium chloride is added, it is reasonably successful at dissolving ferric oxide. 

The covers are constructed from aluminum or some type of steel. This should provide adequate protection from being pierced or cut through. The metals are corrosion resistant so that they will not corrode or become fragile from extreme weather conditions in outdoor applications. The bolts used to install each of these systems are galvanized steel. In addition, the bolts for each cover are installed on the inside of the unit so they cannot be removed from the outside. 

Adhesives of HP type have been shown to bond bronze, lead, nickel, magnesium, copper, aluminum, steel, and stainless steel, in addition to most of the other substrates that earher offerings were capable of bonding. They did, however, continue to show weaknesses when zinc surfaces were bonded. So these adhesives may not be well suited for certain appKcations in the automobile area where galvanized steel is being bonded (Note that later generations covered in this chapter do not necessarily have this shortcoming). 

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