Why must material with hollow sections and other welded sections be vented prior to galvanizing?
Improper venting can cause several concerns:
Firstly, any hollow section must be vented to allow complete flooding of the cavity with zinc and the expulsion of any trapped air. Failure to allow this will result in black spots or non-coated areas inside the section ultimately resulting in a non-conformance. Similarly, many open types of material must be properly vented to ensure a proper galvanizing job. Examples of proper venting and draining are shown below.
Secondly, if a sealed, hollow section is submerged, the trapped air will heat up and expand. This expansion can cause enough pressure in the product to split the steel and cause an explosion putting all plant personnel at serious risk.
What is Reactive Steel?
Reactive steel is a term used to describe the reaction between the steel and the zinc. If the presence of silicon and / or phosphorus is above certain levels the zinc coating becomes abnormally thick and grayed out. If the reactivity is severe enough actual flaking of the zinc coating may also occur.
Many elements have been experimented with over the years to determine if their presence in the galvanizing bath would influence the reactivity of steel. The most influential of these is nickel. At Red River Galvanizing a nickel bath is used and maintained at the proper levels through monthly testing and analysis.
In general, a silicon equivalent value can be looked at and plotted onto a graph detailing the effects of silicon and phosphorus on galvanizing. The silicon equivalent value can be calculated according to: Sieq = (Si (wt%) + 2.5P (wt%)) where Si is silicone and P is phosphorus. This is called the “Sandelin Effect” and is represented graphically below.
My material is presently painted or has a lacquer finish. Can it be galvanized?
In short, the answer is yes. However, prior to galvanizing, the paint or lacquer layer must be removed. Although soaking the material in a caustic solution may be enough to remove most of the material, it does take time. Often 10 or more hours of emersion are required to completely remove the coating. Alternatively, and preferably, sandblasting the material prior to galvanizing is far more cost effective and less time consuming.
Why does some material appear shiny while others appear dull?
Differing surface appearances are almost always explained with either steel chemistry or steel thickness. Levels of silicon and phosphorus high enough to be termed “reactive” will produce a coating that will be significantly thicker, and as a result, much duller in appearance. Secondly, thicker steels will also develop thicker alloying layers of zinc. This thicker coating will also exhibit a duller appearance, however, unlike reactive steels, the thicker coating resulting from thicker steel (view thickness chart for more info) will not chip and flake off as is the case with reactivity.
What is your typical turn around? As a customer, how can I help improve this?
At Red River Galvanizing our goal is a typical 2 to 3 day turn around. Although this may be influenced by the workload at the specific time, we encourage all customers to call ahead to book a time to drop the material off. This will help ensure the material will be completed on time.
How long does a galvanized coating last?
The length of time a galvanized coating will last is directly proportional to the amount of zinc that has bonded to the steel surface. As zinc corrodes preferentially to steel, the more zinc present to corrode, the longer it will take to expose the base metal it has bonded to. Another major contributor to coating lifespan is the environment in which it is placed. Dry, arid environments will far outlast humid, more corrosive environments. In general, service life can be depicted as follows:
Can warping of material occur during galvanizing? How can this be controlled?
Due to the extreme heat of the galvanizing process, warping can occur in some instances. Once the material being galvanized reaches the correct temperature stresses in the material, either from the rolling or the fabrication process, may be partially released causing undesirable distortion. Although the frequency of this is rare, the fabricator can take certain steps to further help minimize this occurrence. Examples of these are:
Use symmetrically rolled sections where ever possible to better allow a more uniform expansion and contraction of the material;
Use parts that are as close to similar thickness as possible to once again allow a more uniformed expansion and contraction of the material;
Bend material to as large a radii as possible during the fabrication process;
Always try and use balanced welding techniques to help reduce uneven stresses being introduced into the material during fabrication;
Brace material temporarily to improve the stiffness of an assembly prior to galvanizing.
Why does the original shiny surface dull or mat out over time?
The top layer of the zinc coating is almost 100% zinc. Zinc, although more noble than steel, will corrode. This is the basis of cathodic protection. As this layer begins to corrode, it oxidizes and is unable to maintain it’s luster. This is a normal part of the lifespan of a galvanized coating and should therefore be expected.
What is ‘Cathodic Protection’?
Cathodic protection, sometimes referred to as galvanic protection, is an electro-chemical process by which zinc, metalurgically bonded to steel, will corrode preferentially relative to the steel it is bonded to. By allowing zinc to bond to steel a galvanic or corrosion cell is created. The steel becomes the cathode and the zinc the anode. When a galvanic or corrosion cell is created a difference in electrical potential between the two dissimilar metals is realized. Electrons will flow from the more active metal (zinc) to the less active (steel). The zinc actually ionizes while at the same time producing electrons, which the steel will receive through the metallic connection with the zinc. The result is the steel will become negatively polarized and hence protected from corrosion.
Why should material be ‘seal welded’?
When steel is welded in such a way as to create an overlap the potential exists to trap air between the two points. This area, if not completely sealed by weld, can allow liquids from the preparation steps of galvanizing to enter and remain until galvanized. Once the steel is heated to galvanizing temperature, this liquid will vaporize and blow out of the non-welded portions causing unsightly black spots. This area, which, if significant enough, can cause a non-conformance or at least an area, which requires post galvanizing repair.
How large of a part may be galvanized?
Red River Galvanizing Inc has processing tanks and a galvanizing kettle, which are 35 feet in length, 4 feet in width and 9 feet in depth. Any material within these dimensions can be galvanized in a single emersion (single dip). Material larger than this can be galvanized in two emersions (double dip). For specific details on our capabilities of double dips please contact us for quotation.
Can castings be galvanized?
In short, castings can be galvanized. However, unlike regular steel material they should be shot blasted prior to galvanizing. This removes the non-metallic material such as sand that is part of the casting process. Once this material is removed acid pickling can take place and the part can be successfully galvanized.
What are the typical guidelines for galvanizing threaded parts?
Once threaded material is galvanized, the treaded portions are larger and will not work with standard nuts or bolts. Therefore, allowances need to be made to allow for the buildup. Please refer to the following table for typical guidelines for threaded material.
Are salt spray tests reliable in determining the lifespan of a hot dip galvanized coating?
For years salt spray tests have been used to approximate environments where galvanized material will be utilized. For equally as long, they have been criticized for their inability to accurately duplicate specific environments. A salt spray test bombards the immature zinc coating with a very aggressive element (salt). It attacks the most outer layer, referred to as the patina. Patina develops on galvanized material several months after galvanizing. Therefore, subjecting zinc to an aggressive environment prior to it maturing and forming its patina completely can often lead to results revealing a much shorter lifespan than would naturally be found.
What is white rust?
White rust is a white, chalky deposit sometimes found on galvanized material. This chalky substance is actually zinc oxide but is commonly referred to as white rust or wet storage stain. White rust represents the accelerated corrosion of the zinc surface. Storing galvanized materials too tightly together in humid, warm locations generally causes it. In order to combat the formation of white rust, both proper storage and chromating should be practiced. Chromating is the emersion of newly galvanized material into a chromate solution. This places a coating on the material, which will degrade over time while the maturing zinc forms its protective patina. Red River Galvanizing does utilize a chromating step to minimize the frequency of white rust.
The material I had galvanized appears to be rusting around the welds. Why is this?
Typically, rust appearing around welds of freshly galvanized material can be removed easily with a cloth or wire brush. This indicates either the presence of an imperfection in the weld or a non seal-welded fabrication process. Liquid from the cleaning steps prior to galvanizing, enters through this imperfection and leaches out after galvanizing. As the steel surfaces behind the welds have not been galvanized, rusting can occur and the liquid runs out over the galvanized surface leaving a rust colored stain.
What is the typical cost of galvanizing compared to other coating techniques?