When a product is hot dip galvanized it is submerged into molten zinc at 450°C. This temperature is at the lower end of the stress relieving range for heat treating of steels, which will relieve any inherent stresses in the steel. This could result in a dimensional change of a product, perceived as distortion if a change in shape of the product takes place.
Internal stresses to some degree are present in most steel structures and as a rule are not a problem when hot dip galvanized. Internal stresses are mainly in equilibrium with each other and would not result in distortion.
Some sources of internal stresses are:
- Residual stresses induced at the mill during rolling of structural section or plate
- Residual stresses created by bending or welding
- Lack of symmetry in sections such as channels or in built up sections
- Combination of thick and thin material in the same assembly
- Assemblies so large that they require double dipping to cover the entire surface of the product in the zinc kettle.
Explosions in the zinc kettle due to Insufficient venting holes can also contribute to distortion, especially when moisture has been trapped. (Refer to previous article)
Large overlapping surfaces which are seal welded can also contribute to distortion, should moisture be able to get in between the overlapped surfaces through a pin–hole or porosity in the weld.
Adequate venting and drainage are essential considerations when galvanizing and may mean the difference between life and death.
Vessels or hollow structures which incorporate enclosed sections must have provision for adequate venting during galvanizing. At galvanizing temperature any moisture present in closed sections is rapidly converted to superheated steam (a volume increase of approximately 1350 times), which may explode unless vented to the atmosphere.
For the safety of galvanizing personnel and equipment it is essential that venting be provided. Correct venting also ensure that when necessary the entire internal surface of work is properly galvanized and fully protected.
During the hot dip galvanizing process chemicals which are used to clean the steel is trapped between the overlapping surfaces, pin holes or porous areas. When the product is dipped into the molten zinc all moisture trapped will turn to super heated steam and evaporate at 450° C. The salts in the moisture will crystallize and stay behind inside the crevices. The salts that remain are normally zinc and or ammonium chloride salts from the flux solution.
Once the hot dipped galvanized products are exposed to moisture during inclement weather the crystallized salts trapped, will react with the moisture and leach out of the crevices leaving behind a stain that resembles rust.
Although not aesthetically pleasing, the staining of the galvanized surface will have absolutely no effect on the performance or life of the zinc coating. A single coat zinc rich paint can be used to camouflage the stains.
To ensure the safety of galvanizing personnel, large overlapping areas must be vented with a 6mm hole for every 100cm of overlapping area. The picture gives a good indication of the disastrous results when an overlapped surface is not properly seal welded and vented.
Narrow gaps between plates should be avoided. In particular, there should be no overlapping surfaces or back-to-back angles and channels. Trapped acid or flux chemicals may later weep from the joint causing surface discolouration of the hot dip galvanized coating. Where small overlapping areas are unavoidable, edges should be sealed by a continuous pore-free weld to prevent penetration of acid or flux chemicals. However, the galvanizer should be consulted before any area is sealed.