Corrosion Resistance of Zn-Al Alloys

All zinc-based alloys have excellent resistance to corrosion in a variety of environments. In general terms, the presence of aluminum in the alloys enhances the well-known corrosion resistance of zinc, which is the main constituent of the alloys. Note that for exposure to salt spray and high and low pH solutions, the actual corrosion rates are not given because they have no quantitative significance in relation to service conditions; it is only the comparative performance which is significant.

Chemical specifications for zinc alloys are given in ASTM B86 for No2, No3, No5, and No7 alloys, and ASTM B791 for ZA-8, ZA-12 and ZA-27.

Effect of Salt Spray Environment

A salt spray test was used to simulate the relative corrosion behavior of ZA-8, ZA-12, ZA-27 and No.3 alloy as compared to that of pure zinc and aluminum or aluminum-rich alloys. This test can also give a first order estimate of the performance or zinc alloys in castings exposed to salt spray, either in a marine environment or for castings to salted winter roads.

For aluminum levels of up to 12% the zinc-based alloys perform as well as or slightly better than pure zinc. Because of its higher aluminum level, ZA-27 behaves more like an aluminum alloy and the galvanizing alloy Galvalume (Zn-5%Al-1.6%Si) and is much less affected. Corrosion of zinc alloy plates proceeds through selective attack of the zinc-rich matrix.

Effect of pH

The effect of pH on the corrosion rate of ZA-8, ZA-12 and ZA-27 in relation to zinc and aluminum. In mild acidic solutions (pH range of 4.0 to 7.0), the ZA alloys are more corrosion resistant than zinc, with the ZA-27 alloy performing better than ZA-8 and ZA-12 due to its higher aluminum content.

In alkaline solutions the ZA-27 alloy begins to corrode significantly as the pH of the solution approaches 12.0. The performance of the ZA-8 and ZA-12 alloys approximate that of zinc; the performance of ZAMAK alloys would be expected to be similar.

Atmospheric Corrosion

Zinc and its alloys have good atmospheric corrosion resistance. Resistance to atmospheric corrosion is due to the formation of a protective film, often referred to as a patina, which is enhanced by the aluminum content of the alloys, Exposure to acidic rainfall will cause partial dissolution of the protective film, but the film will reform quickly during dry weather.

Zinc corrosion rate is controlled mainly by:

the frequency and duration of moisture contact,

the amount of acidic atmospheric pollution, and

retention or removal of corrosive deposits.

Sulfur dioxide is the worst atmospheric contaminant from the point of view of corrosion. It acidifies any moisture present on the casting surface (e.g. acid rain), which in turn partially dissolves the protective film as described above. Chlorides normally have a lesser effect on zinc alloys. However, when both chlorides and sulphur dioxide are present the effect is frequently more corrosive than either alone. Soot and dust retain condensation and thus enhance corrosion. Other pollutants usually have less effect -mild alkaline conditions can even be beneficial.

Mine Atmospheres

The excellent corrosion resistance in humid atmospheres, combined with the inherent non-sparking nature of zinc-aluminum alloys when struck with iron tools, has resulted in zinc-aluminum alloys with less than 15% Al contents being approved for underground locations, hazardous (explosion risk) or otherwise. Higher aluminum alloys cannot be used in this environment because of the danger of spark generation when struck.

Corrosion Resistance in Water The behavior of the zinc-aluminum alloys in water can be related to the behavior of zinc in water and water-based solutions. The major categories are tap and fresh water, industrial process waters and sea water.

Tap and fresh water, cold. The overall corrosion rate in cold water is very low. Where the water is scale-forming some additional protection is afforded.

Tap and fresh water, hot. In hard waters the scale which forms at "hot" water temperatures in the range of 50 to 70°C has a coarse-grained structure with less adhesion to the zinc surface. In this temperature range, corrosion rates can be expected to be greater than that experienced at room temperature, or at temperatures closer to the boiling point. Suitable inhibitors nullify the corrosion tendency.

Industrial Process Waters. The compositional variations in industrial process waters are too large for any general guidance to be given. An in-situ test, preferably for at least a year, is desirable although indicative results can often be obtained in only a few months. Plant or development trials with zinc-aluminum castings are worthwhile for waters of pH 5 to12. At around a pH of 12, low-aluminum castings are likely to be the more corrosion resistant.

Sea Water. Corrosion rates are typically 25µm/year in tropical seas and half that in temperate oceans, such as the North Atlantic. In tidal areas or areas washed by waves, the corrosion rate is typically double that for permanently immersed areas. Trials in the North Sea have also shown up to twice the corrosion rate at depths of hundreds of meters below the surface as compared with corrosion rates near the surface. Higher corrosion rates should be expected in polluted sea water.

In temperate sea water, zinc alloy castings will often give satisfactory service even where zinc-coated steels, which are protected only by a relatively thin zinc coating of approximately 0.1 mm, might have been reported as unsuitable.

Corrosion Resistance in Chemical Environments

Water-based solutions. In the presence of rain or condensation, or else in water-based solutions, chemicals will react with zinc. Chemicals that dissolve in water to give a pH of less than 5, of greater than 11.5, are generally corrosive to zinc-aluminum alloys.

Static and dynamic immersion trials with various detergents, disinfectants and household cleaners as well as soft tap water showed that ZA-12 is adequate for drainage fittings and only slightly inferior to 63/37 brass.

Gasoline. Zinc-aluminum alloys totally immersed in gasoline and gasohol do not corrode significantly when water-free. In hydrated fuel mixtures corrosion products do form on zinc alloys and may, in some cases, interfere with the functioning of fuel systems. In water containing fuels voluminous white corrosion products will form at the fuel/water interface.

Alcohol. Pure alcohols are considerably less corrosive than water, but mixtures of water and alcohol are more corrosive than water alone. As a result, zinc-aluminum alloys are not recommended for alcohol-water mixtures, such as found in various beverages.

Diesel and fuel oils. ZA alloys are not corroded by refined oils, but sulfur or water present in ordinary fuel oils may form compounds with zinc which can clog small orifices. Chroming of castings may be helpful: zinc anodizing or chrome plating should provide adequate protection.

Gear oils. The ZA-12 and ZA-27 alloys were found not to react with SAE 90 gear oil at temperatures up to 82°C (180°F). Oil breakdown and associated corrosion will occur at 150°C (300°F).

Lubricants and hydrocarbons. The use of heavy oil or greases, particularly the extreme pressure type, result in smooth, light etching of the surfaces and the formation of a natural protective coating. Lubricants used should be stable and free from acidity.

Hydraulic fluids. No corrosion is to be expected. For example no apparent reaction occurred with ZA-27 after two weeks at 50°C (120°F) in a static test.

Glycerol. Pure glycerol (glycerin) has a limited smooth etching action on zinc, and is satisfactory as hydraulic fluid used in, for example, door checks. Similarly, glycerol/alcohol mixtures are practically inert to zinc provided the mixture is pure and free of water. Water or low-grade glycerol may cause some pitting.

Engine coolants. Typically a 33% solution of Prestone II in water (an engine coolant solution which contains silicate inhibitors) will not attack No.3 and ZA-8 castings. The ZA-27 alloy displayed intergranular attack when tested as-cast, but not when its surface was polished before testing.

Refrigerants. Freon 22 is inert and stable in contact with zinc alloys.

Detergents and cleaners. Solutions of ordinary bar soaps are mildly alkaline and are suitable for cleaning zinc alloys. They develop a beneficial protective coating on the cleansed surface and are satisfactory for both warm and cold water applications.