Superheated air and moisture can be an explosive force when hot dip galvanized articles are not properly vented or drained. A new report and GAA’s updated Design Manual offers guidance in line with Australian and New Zealand standards.
It’s not hard to imagine how flying shards of steel and molten zinc at 450 degrees celsius can cause some serious damage, but there’s always YouTube if you need a stark reminder.
The Galvanizers Association of Australia (GAA) has released a new report on drainage and venting to help professionals keep the hot dip galvanizing process effective and safe (it’s part of their technical publications – so free signup required). The updates also include a range of 3D interactive models to demonstrate the industry-recommended placement for drainage and venting holes for various structural connections.
When hot dip galvanizing ferrous materials, there are three main considerations to keep in mind: safety, quality and aesthetics. Correct draining and venting are critical to ensure all three.
“Galvanizers, fabricators and engineers can work together to achieve a good-looking finish for their galvanized products, without compromising on safety or effectiveness,” says Ann Sheehan, Corrosion and Sustainability Officer at GAA and author of the updated guide.
“By better understanding the drainage and venting requirements, we can all plan and implement designs that lead to a better outcome from the hot dip galvanizing process.”
If you galvanize a hollow section without the right-sized and located holes in it, the article can float and fail to create a consistent coating across the entire material. If the hollow section is sealed before galvanizing, the molten zinc will superheat any moisture inside and cause parts to explode.
Trapped fluids from pre-treatment cleaning processes can also expand rapidly when heated by molten zinc. For a safe, efficient and effective coating, both the zinc and pre-treatment solutions need to make contact with all surfaces and displace all air before flowing out of and running off the article.
When it comes to drainage holes, size and location matter. Larger holes in the right locations drain all liquids quickly and leave less chance of flaking finishes or blowouts. The faster the zinc drains from the article, the better looking the finish. Having at least two holes helps with zinc flow and lowers the risk of trapped liquids. Ideally, holes should be placed as close to corners as possible.
“Holes shouldn’t be located in the centre of end plates and connections. Even though it might look good, it’s a poor drainage location and increases the risk of trapped liquids. We like to see diagonally opposed holes close to the edges or connections where possible,” says Sheehan.
Larger pieces, such as poles, are often suspended from chains when going through the hot dip galvanizing process. Unfortunately, the chains can cause touch marks that spoil an article’s aesthetics. For a better looking finish and more effective coating, engineers and fabricators should plan ahead and create designs that already include hang points (even if they are temporary).
The guide from GAA also explains how to apply correct venting to overlapping surfaces, which can be particularly dangerous if not adequately prepared.
“We’ve captured a lot of detail and industry knowledge in this guide that will help everyone stay across the technical specifications and principles that lead to better outcomes for hot dip galvanizing. Our advice is based on the Australian and New Zealand standards, and we’re happy to discuss any further specifics that the industry might have,” says Sheehan.
The new information is available in the Design section of the GAA Design Manual online with 3D interactive models and a downloadable guide available throughout the section. The models are also a useful reference tool to teach new employees.