From buses to bicycles, vehicles have to battle the elements. And the same goes for steel railings and any other metal-made product that spends its lifespan outdoors. To keep rust at bay, fabricators finish their metal products with a variety of corrosion-resistant coatings and paints. Whether that coating can properly form and stick inevitably makes or breaks a product in the face of harsh weather conditions.
In addition to their corrosion-resistant finishes, bikes, buses and balconies have another common bond: welded assemblies. When products require welds as well as coatings, fabricators are faced with the possibility of weld scale forming, which hinders a coating’s ability to adhere to a welded surface. And this isn’t exclusive to products that are made for the outdoors. Any metal product that requires a coating or paint can be compromised if weld scale is present.
Weld scale is a type of weld soil that stains or discolors the welded metal and the area around it. The degree of the weld scale depends on the type of welding method being utilized. To ensure the success of the subsequent pretreatment and coating operations, weld scale must be removed. To properly manage weld scale, fabricators must first understand how it is created. From there, they must choose the best way to remedy it, to keep their products’ coatings in tact – even through the deep of winter.
Identifying welding soils
The inorganic soils created during the welding process are localized in the area adjacent to the weld. These soils include weld spatter, weld slag, weld scale and various elements that can be found in the weld filler metal.
The area that displays the effects of the welding process is referred to as the heat-affected zone (HAZ). The HAZ is the area of base material that is not melted but has had its microstructure and mechanical properties altered by welding.
The heat from the welding process and subsequent recooling causes the changes in the HAZ. The extent of the property changes depends primarily on the base material, the weld filler metal, and the amount and concentration of heat input by the welding process.
For example, TIG welding creates a high-temperature electric arc that is focused on a small area. The intense heat coupled with the slower weld speed typically used for TIG may result in a wider HAZ. The arc in MIG welding may produce temperatures similar to TIG welding, but the weld speed is faster with MIG. When comparing the two welding methods on the same base material, MIG welding typically results in a smaller HAZ than TIG. The areas of the HAZ are shown in Figure 1.
Weld scale is an oxide layer created from the exposure of the heated base material to the surrounding oxygen-containing environment. The extent of the weld scale is dependent on the amount of heat generated during the welding process and is a visual indication of the extent of the HAZ.
The presence of weld scale inhibits the ability for a conversion coating to form and does not provide a receptive surface for the adherence of the coating. Even if initial adhesion of the coating is observed, the scaled areas have inferior corrosion resistance and must be removed to ensure the long-term performance of the coating. Examples of weld scale are shown in Figure 2.
If coating-adhesion failures on improperly prepared welds occur before the products ship to a customer, they are typically reworked. Reworking the part may include sanding and repainting the affected area, or it may include stripping or blasting the coating or paint from the entire part and reprocessing it through the surface treatment and paint operations. This results in increased process delays due to loss of production, increased labor costs and added material costs.
If the coating-adhesion failures occur at the customer site (field failure), the results can be damaging to a company’s reputation. Besides customer dissatisfaction, there may also be monetary penalties. A minimal amount of field failures typically results in warranty claims. Numerous failures may lead to large-scale returns of product with refunds expected or, ultimately, the loss of contracts.
Removal of weld scale
The formation of the proper pretreatment conversion coating and adhesion of subsequent coatings to welds is influenced by the cleanliness of the weld area. Therefore, weld soils – whether it’s weld scale, slag or spatter – must be removed to provide a receptive surface for subsequent conversion coating and coating applications.
There are two methods of removal: mechanical and chemical. Mechanical methods consist of manual or automatic abrasion. The typical mechanical methods used are grinding, sanding or blasting. Grinding and sanding can be done using abrasive wheels or belts as well as power abrasive brushes (wire brushes). Blasting is typically done using abrasive blast cleaning or abrasive media blasting.
The chemical method typically utilizes an acidic pickle solution, which is recommended, although neutral organic compounds have been used in specialized circumstances.
The acid pickle solution can be applied via spray or immersion and is typically integrated into the surface treatment operation. Thixotropic or gel products also are available that can be applied by non-atomizing, low-pressure spray or by brush in manual operations.
The types of acid pickle products commonly used for ferrous base materials contain primarily phosphoric acid or sulfuric acid with additional organic acids and additives. It is also common to see mixed acid products such as phosphoric/sulfuric acid or citric/sulfuric acid.
Whatever the method, it’s imperative for fabricators and manufacturers to manage weld scale in order to properly coat their products to give them the strength and longevity that customers expect.