Corrosion Resistance of Zn-Al Alloys

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Introduction to Zn-Al Alloy Corrosion Resistance

All zinc-based alloys demonstrate excellent resistance to corrosion across a variety of environments. The presence of aluminum in these alloys significantly enhances the well-established corrosion resistance of zinc, which constitutes the primary component of the alloys. It is important to note that for exposures to salt spray and solutions with high or low pH values, specific corrosion rates are not provided as they lack quantitative significance in relation to actual service conditions; only the comparative performance between alloys is meaningful for practical applications.

Chemical specifications for zinc alloys are standardized in ASTM B86 for No.2, No.3, No.5, and No.7 alloys, while specifications for ZA-8, ZA-12, and ZA-27 are detailed in ASTM B791.

Performance in Salt Spray Environment

Salt spray testing simulates the relative corrosion behavior of ZA-8, ZA-12, ZA-27, and No.3 alloys compared to pure zinc and aluminum or aluminum-rich alloys. This test provides a first-order estimate of zinc alloy performance in castings exposed to salt spray, whether in marine environments or on components exposed to salted winter roads.

For alloys containing aluminum levels up to 12%, the zinc-based alloys perform equally well or slightly better than pure zinc. ZA-27, with its higher aluminum content, behaves more similarly to an aluminum alloy and the galvanizing alloy Galvalume (Zn-5%Al-1.6%Si), demonstrating significantly improved resistance to salt spray corrosion. When corrosion does occur in zinc alloy plates, it typically proceeds through selective attack of the zinc-rich matrix.

Effect of pH on Corrosion Resistance

The pH level of the environment significantly affects the corrosion rates of ZA-8, ZA-12, and ZA-27 alloys relative to pure zinc and aluminum. In mildly acidic solutions (pH range of 4.0 to 7.0), the ZA alloys demonstrate superior corrosion resistance compared to zinc. ZA-27 performs better than both ZA-8 and ZA-12 in these conditions due to its higher aluminum content.

However, in alkaline solutions, particularly as the pH approaches 12.0, ZA-27 begins to corrode significantly. The performance of ZA-8 and ZA-12 alloys in alkaline environments more closely resembles that of pure zinc, and ZAMAK alloys would be expected to exhibit similar behavior.

Atmospheric Corrosion Performance

Zinc and its alloys demonstrate excellent atmospheric corrosion resistance, primarily due to the formation of a protective film known as a patina. This natural barrier is enhanced by the aluminum content present in the alloys. While exposure to acidic rainfall may cause partial dissolution of this protective film, it regenerates quickly during dry weather periods.

The corrosion rate of zinc alloys in atmospheric conditions is primarily controlled by three factors:

• The frequency and duration of moisture contact
• The amount of acidic atmospheric pollution
• Retention or removal of corrosive deposits

Sulfur dioxide represents the most detrimental atmospheric contaminant from a corrosion perspective. It acidifies moisture present on casting surfaces (forming acid rain), which subsequently dissolves portions of the protective film. While chlorides typically have a lesser effect on zinc alloys, the combination of chlorides and sulfur dioxide frequently produces more severe corrosion than either contaminant alone. Additionally, soot and dust retention increases condensation, enhancing corrosion activity. Other pollutants generally have less impact, and mildly alkaline conditions can occasionally be beneficial for long-term protection.

Performance in Mine Atmospheres

The exceptional corrosion resistance in humid atmospheres, combined with the inherent non-sparking properties when struck with iron tools, has led to the approval of zinc-aluminum alloys containing less than 15% aluminum for underground applications, whether in hazardous (explosion risk) environments or otherwise. Higher aluminum content alloys cannot be utilized in these environments due to the potential spark generation hazard upon impact.

Corrosion Resistance in Water Environments

The behavior of zinc-aluminum alloys in water can be directly related to zinc's performance in water and water-based solutions. This section explores their performance across several water categories:

Tap and Fresh Water (Cold)

The overall corrosion rate in cold water is minimal. In scale-forming water conditions, additional protection is provided by the natural scale formation.

Tap and Fresh Water (Hot)

In hard waters at "hot" water temperatures (50-70°C), the scale forms with a coarse-grained structure that adheres less effectively to zinc surfaces. Consequently, corrosion rates in this temperature range typically exceed those experienced at room temperature or at temperatures closer to boiling point. Appropriate inhibitors can effectively neutralize this corrosion tendency.

Industrial Process Waters

The compositional variations found in industrial process waters are too diverse for generalized guidance. For accurate assessment, in-situ testing for at least one year is recommended, though indicative results can often be obtained within a few months. Plant or development trials with zinc-aluminum castings are worthwhile for waters with pH values between 5 and 12. At approximately pH 12, low-aluminum content castings typically demonstrate superior corrosion resistance.

Sea Water Performance

In sea water environments, corrosion rates for zinc-aluminum alloys typically measure around 25µm/year in tropical seas and approximately half that rate in temperate oceans such as the North Atlantic. In tidal zones or areas regularly washed by waves, corrosion rates typically double compared to permanently immersed areas. Research in the North Sea has also revealed corrosion rates at depths of hundreds of meters below the surface can be up to twice as high as those near the surface. Higher corrosion rates should be anticipated in polluted sea water conditions.

In temperate sea water environments, zinc alloy castings often provide satisfactory service even in applications where zinc-coated steels (protected by relatively thin zinc coatings of approximately 0.1 mm) might be considered unsuitable.

Corrosion Resistance in Chemical Environments

Water-based Solutions

When exposed to rain, condensation, or water-based solutions, chemicals will react with the zinc component of the alloys. Generally, chemicals that dissolve in water to produce solutions with pH values below 5 or above 11.5 are corrosive to zinc-aluminum alloys.

Static and dynamic immersion trials involving various detergents, disinfectants, household cleaners, and soft tap water demonstrated that ZA-12 performs adequately for drainage fittings and is only slightly inferior to 63/37 brass in these applications.

Performance in Fuel Environments

Gasoline

Zinc-aluminum alloys completely immersed in water-free gasoline or gasohol do not experience significant corrosion. However, in hydrated fuel mixtures, corrosion products form on zinc alloys and may, in some cases, interfere with fuel system functionality. In water-containing fuels, voluminous white corrosion products typically form at the fuel/water interface.

Alcohol

Pure alcohols are considerably less corrosive to zinc-aluminum alloys than water. However, mixtures of water and alcohol demonstrate greater corrosivity than water alone. Consequently, zinc-aluminum alloys are not recommended for applications involving alcohol-water mixtures, such as those found in various beverages.

Diesel and Fuel Oils

ZA alloys do not experience corrosion from refined oils. However, sulfur or water present in ordinary fuel oils may form compounds with zinc that can potentially clog small orifices. Chroming treatments for castings may provide protection; zinc anodizing or chrome plating typically offers adequate protection in these environments.

Gear Oils

Research has shown that ZA-12 and ZA-27 alloys do not react with SAE 90 gear oil at temperatures up to 82°C (180°F). However, oil breakdown and associated corrosion will occur at elevated temperatures of 150°C (300°F).

Performance with Lubricants and Industrial Fluids

Lubricants and Hydrocarbons

The application of heavy oils or greases, particularly those of the extreme pressure type, results in smooth, light etching of surfaces and contributes to the formation of a natural protective coating. Lubricants used with zinc-aluminum alloys should be stable and free from acidity to maintain optimal protection.

Hydraulic Fluids

No significant corrosion is anticipated when zinc-aluminum alloys contact hydraulic fluids. For example, ZA-27 showed no apparent reaction after two weeks at 50°C (120°F) in static testing conditions.

Glycerol

Pure glycerol (glycerin) exhibits only limited smooth etching action on zinc, making it satisfactory as a hydraulic fluid for applications such as door checks. Similarly, glycerol/alcohol mixtures remain practically inert to zinc provided the mixture is pure and water-free. Water-containing or low-grade glycerol may cause some pitting of the alloy surface.

Specialized Applications and Chemical Exposures

Engine Coolants

Typically, a 33% solution of Prestone II in water (an engine coolant containing silicate inhibitors) does not attack No.3 and ZA-8 castings. The ZA-27 alloy displayed evidence of intergranular attack when tested in as-cast condition but showed no such attack when its surface was polished prior to testing.

Refrigerants

Freon 22 remains inert and stable in contact with zinc alloys, making these alloys suitable for refrigeration system components where this refrigerant is used.

Detergents and Cleaners

Solutions of ordinary bar soaps, which are mildly alkaline, are suitable for cleaning zinc alloys. These solutions develop a beneficial protective coating on the cleansed surface and perform satisfactorily in both warm and cold water applications.

Conclusion

Zinc-aluminum alloys demonstrate excellent corrosion resistance across a wide spectrum of environments and applications. The specific performance characteristics vary depending on aluminum content, with higher aluminum content (such as in ZA-27) generally providing superior protection in acidic environments, while lower aluminum content alloys perform better in alkaline conditions.

When selecting zinc-aluminum alloys for specific applications, consideration should be given to the exact service environment, including exposure to moisture, chemicals, temperature fluctuations, and atmospheric conditions. For critical applications or challenging environments, targeted testing is recommended to ensure optimal material selection.

The combination of corrosion resistance with other favorable properties—such as castability, dimensional stability, and non-sparking characteristics—makes zinc-aluminum alloys versatile materials for numerous industrial, marine, and specialized applications where longevity and reliability in corrosive environments are essential requirements.