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Surface Treatment Process - Blast Cleaning Quality

Surface treatment process is a complex and crucial part of manufacturing. It consists of several separate processes, such as blast cleaning, painting and chemical washing. These processes differ from each other and the requirements for each process are different in different manufacturing plants. Blastman Robotics provides automatic solutions for blast cleaning and our aim is to provide our customers with the best possible blast cleaned surface. We need know the requirements of other processes in the production line, so that we can provide the best possible surface. Furthermore, we need to understand the characteristics of these processes. This blog will discuss general requirements for different types of surface treatment processes.

The surface treatment process typically starts with heat treatment, washing or chemical treatment, when producing new products. Repair facilities are different, because they typically skip heat and chemical treatments, since it’s already been done in manufacturing. Washing is typically part of the repair process. The process parameters for these processes affect the surface condition. This in turn changes the initial surface condition for the next process, which is typically blast cleaning. Therefore, the supplier for blast cleaning needs to know what happens to the surface, so that they can take it into account, when designing their process. For example, if washing is completely discarded and the workpiece is machined, there will be oil on the workpiece. Oil has an enormous impact on blasting and it needs to be considered. Another example would be the duration and temperature of heat treatment. Mill scale is produced during heat treatment and the thickness of the layer affects blasting.

After the first part of the process the workpiece is typically blast cleaned. For this part, we need to know the initial condition of the workpiece and the requirements of the next process. There are two main parameters that need to be optimized: preparation grade and roughness. These two parameters define the adhesion and lifetime of the coating.

The preparation grade basically means how clean the workpiece is after blasting. It’s defined by standards that are based on visual inspection. There are many standards used to evaluate a blast cleaned surface, but two are most commonly used. The Sa standard is used in Europe and the SSPC standard is used in North America. Preparation grade affects the lifetime and adhesion of the coating. It also affects the duration of the blasting process. Therefore, it is crucial that the desired preparation grade is well defined and monitored. This way the process can be designed to be as efficient and cost-friendly as possible.

Table 1. Brief overview of blast cleaning standards.

The roughness of the workpiece influences the adhesion of coating on the workpiece. Roughness can be easily and accurately measured with modern devices. There is no direct correlation between greater roughness and adhesion. This means that all coatings have specific limits for roughness and these limits depend on the properties of the coating and the thickness of the coating film. Incorrect roughness can drastically reduce the adhesion of the coating and reduce coating lifetime. The roughness limits are usually provided by the coating supplier.

After blast cleaning, a coating, usually paint, is applied on the workpiece. This is often the last part of the surface treatment process and the first thing the customer sees. The coating is also a protective layer against corrosion for the workpiece. The surface created in the blast cleaning process needs to fulfill all the requirements of the coating. Otherwise adhesion and lifetime will not be optimal. A surface with a low preparation grade has a lot of impurities, such as sulfites and chlorides, which cause blistering on the coating film. A surface with roughness below the specified limits doesn’t provide enough area for the coating to adhere properly. A surface with roughness above the specified limits is not fully covered by the coating and the protective layer of the coating doesn’t cover the entire surface. This can be fixed by applying a thicker film, but it usually reduces adhesion.

Sakari Veijola

Blastman Robotics Ltd