Scope
1.1 This specification covers the requirements for a coating of zinc mechanically deposited on iron and steel basis metals. The mechanical zinc coating is provided in several thicknesses up to and including 107 μm. The coating as deposited shall have a uniform silvery appearance, and a matte to medium-bright luster. Zinc coatings are usually applied to provide corrosion resistance. The performance of a zinc coating depends largely on its thickness, the supplementary treatment if any, and the kind of environment to which it is exposed. The seven heaviest classes of coatings offer suitable alternatives to hot-dip galvanizing.
NOTE 1—The performance of this coating complies with the requirements of Specification A153/A153M and MIL-C-81562.
Process
The Process for applying a zinc coating mechanically involves: prepping the surface of the parts to be coated, by dipping them in a chemical bath, then applying a thin metal coating by immersion in a chemical solution. Next, the prepped parts go into a tumbler, which includes the parts to be coated, a zinc powder and impact media. The parts are run through the tumbler which uses the impact media to drive the powdered zinc into the part to be coated. From there the coated parts are rinsed and dried and a secondary treatment may or may not be added, ie for Type II Zinc.
Classes – Zinc Coatings are classified on the basis of thickness as shown below:
Mechanical Zinc Coatings Classification
| Class | Minimum Thickness, μm | ||||
|---|---|---|---|---|---|
| 110 | 107 | ||||
| 80 | 80 | ||||
| 70 | 69 | ||||
| 65 | 66 | ||||
| 55 | 53 | ||||
| 50 | 50 | ||||
| 40 | 40 | ||||
| 25 | 25 | ||||
| 12 | 12 | ||||
| 8 | 8 | ||||
| 5 | 5 | ||||
| Source: ASTM B695-04 | |||||
Types
Type I – plain zinc as coated, without supplementary treatment. Type 1 zinc is useful for lowest cost protection where early formation of white corrosion products is not detrimental. It is also used for higher temperature applications up to approximately 120°C where the effectiveness of chromates is greatly reduced. (Appendix X2.1)
Type II – colored chromate conversion treatment. Chromates that have a color (yellow, olive drab, bronze, etc.) are used to delay the appearance of white or red corrosion products on the plated article, or to provide a color desired by a customer for a specific purpose. (Appendix X2.2)
Table 1: Minimum Hours to Failure
| Type | White Corrosion | ||||||
|---|---|---|---|---|---|---|---|
| Class | 55-110 | 50 | 40 | 25 | 12 | 8 | 5 |
| I | ᴬ | ᴬ | ᴬ | ᴬ | ᴬ | ᴬ | ᴬ |
| II | 72 | 72 | 72 | 72 | 72 | 72 | 72 |
| Type | Red Rust | ||||||
| Class | 55-110 | 50 | 40 | 25 | 12 | 8 | 5 |
| I | None | 300 | 250 | 192 | 96 | 56 | 36 |
| II | None | 300 | 250 | 192 | 96 | 72 | 72 |
| Source: ASTM B695-04 | |||||||
The following data, based on widespread testing, may be used as a guide to compare the behavior of zinc in various atmospheres. The values are only indicative, because individual studies in various parts of the world have resulted in figures that vary widely from these averages.
Table Appendix X1.2: Corrosion Rates
| Atmosphere | Mean Corrosion Rate |
|---|---|
| Industiral | 5.6 μm (0.22 mil/year) |
| Urban Nonindustrial or Marine | 1.5 μm (0.06 mil/year) |
| Suburban | 1.3 μm (0.05 mil/year) |
| Rural | 0.8 μm (0.03 mil/year) |
| Indoors | considerably less than 0.5 μm (0.01 mil/year) |
| Source: ASTM B695-04 | |
Hydrogen Embrittlement
X3.1 A major advantage of mechanical deposition is that it does not produce hydrogen embrittlement in hardened steel during the coating process. However, pronounced embrittlement can be produced in certain cleaning processes. The mild degree of embrittlement that might result from following proper procedures with cleaning methods permitted in this specification normally is self-relieving within a day’s time at room temperature.