Salt Spray Hour Data Sheet on Electroplated Zinc
This white paper outlines the ASTM requirements as stated in ASTM B633 for electrodeposited coatings of zinc on iron and steel. In accordance to this standard, there are six finish types and four thickness classes. each has their own corrosion requirement for minimum salt spray hours. Below, you will also see a bar graph of salt spray hours met for different plating types as tested.
Table 1 specifies the different types of coatings and the minimum salt spray hours to white corrosion as stated in ASTM B633.
| Table 1 - Finish type and Corrosion Resistance Requirements | ||
| Type | Description | Minimum Salt Spray hr. (White Rust) |
| I | As plated without supplementary treatments | - |
| II | With colored chromate coatings | 96 |
| III | With colorless chromate conversion coating | 12 |
| IV | With phosphate conversion coating | - |
| V | With colorless passivate | 72 |
| VI | With colored passivate | 120 |
Table 2 specifies the different thickness of the coating and the service condition that relates to each as stated in ASTM B633.
| Table 2 - Thickness Classes for Coatings | |||
| Classification Number and Conversion | Service condition | Thickness Minimum | |
| Coating Suffix | (um) | (in) | |
| Fe/Zn 25 | SC 4 (Very Severe) | 25 | 0.001 |
| Fe/Zn 12 | SC 3 (Severe) | 12 | 0.0005 |
| Fe/Zn 8 | SC 2 (Moderate) | 8 | 0.0003 |
| Fe/Zn 5 | SC 1 (Mild) | 5 | 0.0002 |
SERVICE CONDITION AS STATED IN ASTM B633:
SC 4 - Very Severe
Exposure to harsh conditions, or subject to frequent exposure to moisture, cleaners and saline solutions. Likely damage of denting, scratching or abrasive wear.
SC 3 - Severe
Exposure to condensation, perspiration, infrequent wetting by rain and cleaners.
SC 2 - Moderate
Exposure to dry indoor atmospheres but subject to occasional condensation, wear or abrasion.
SC 1 - Mild
Exposure to indoor atmospheres with rare condensation and subject to minimum wear or abrasion.
The bar graph shows the salt spray hour results to first white corrosion, 5% white, and hours to red rust for all standard plating types along with variations of these coatings. All platings tested were in the thickness classes of SC 2 and SC 1. Hours to white rust will not vary between different coating thicknesses of the same type of plating assuming another process has not been done. As seen on the chart there are some proprietary coatings that show hours with a sealer or after being baked. These hours are of a higher capability than as minimum stated in ASTM B633. All hours are as shown from testing by platers on their plating's capabilities.
ZINC PLATING - HOURS TO WHITE AND RED CORROSION
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