Four Causes of Discoloration of Galvanized Steel Wire
The following shows four possible causes of discoloring springs made of our galvanized steel wire “Mac Wire”.
- Degraded material of wiredrawing lubrication film on the galvanized surface
- Heat diffusion caused by heat treatment at 250℃ or higher
- Corrosion of galvanizing
- Peeling caused by poor plating
Cause 1: Degraded material of wiredrawing lubrication film on the galvanized surface
Since “Mac Wire” is wiredrawn by a wet method, the plated surface of the wire is coated by wiredrawing lubricating liquid.
Using the wiredrawing lubricating liquid deteriorates the liquid over time and causes degraded material to be taken to the lubrication film.
If a large amount of degraded material is taken to the lubrication film, it may be carbonized by heat treatment after spring processing, causing changes in color tone and gloss of the galvanizing as well as color unevenness.
Therefore, we manage the concentration and temperature of the wiredrawing lubricating liquid to control the growth of degraded material and implement measures to prevent discoloration by advancing the replacement cycle of the lubricating liquid.
Cause 2: Heat diffusion caused by heat treatment at 250℃ or higher
Since zinc has a relatively low melting temperature, when heat treatment is performed at 250℃ or higher, the iron content in the surface steel starts to be thermally diffused (alloyed) into the galvanized layer.
When the diffusion layer reaches the plated surface, it changes color tone and gloss of the spring surface, resulting in color unevenness and discoloration.
Although the spring turns black, the corrosion resistance improves proportional to the heat treatment temperature.
Cause 3: Corrosion of galvanizing
Zinc is an amphoteric metal and reacts to both acid and alkali.
Therefore, if exposed to corrosion promoting substances, such as water, moisture, and perspiration, and acidic corrosive atmosphere, such as acidity and chlorine gas, the galvanized surface is corroded, causing white rust to be generated.
When white rust is generated, the rusted place turns to dark gray.
Furthermore, when spring processing is performed using corroded wire material, it causes uneven coiling and affects yield.
Cause 4: Peeling caused by poor plating
If the surface steel is exposed due to peeling caused by poor plating, performing heat treatment after spring processing will cause the peeled part to turn black due to the oxide coating (temper color) of the surface steel.
Those mentioned above are the four possible causes of discoloration of galvanized material.