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Ensuring a Successful Blasting Operation

MONDAY, SEPTEMBER 12, 2022

By Iann Bouchard, Eng, Winoa USA


Steel has many practical applications because of its strength and the ease with which it can be worked. However, a disadvantage of this metal is that it is subject to corrosion—an unwanted electrochemical or chemical surface process that damages metal. Corrosion can be fought in various ways. One of the most widely used methods is the application of suitable coating systems, normally in the form of paints.

Whether protecting steel with paint, rubber, metallization or another form of coating, its surface must first be prepared to ensure optimum resistance to corrosion over time.
Proper preparation depends on many factors:

  • Substrate (e.g., carbon steel, stainless steel, light alloys);
  • Type of coating, including its nature, planned thickness and use restrictions;
  • Blasting technique (e.g., wheelblast machine, compressed air);
  • Type and characteristics of the abrasive to be used, such as material, shape and hardness; and
  • Required specifications—including cleanliness and roughness—given by the user.

The adhesion and quality of coatings on metallic surfaces will depend on the cleanliness and roughness of the surface. Here, we focus on blasting operations, commonly referred to as shot blasting or sandblasting).

Fig. 1: Dust left on the surface following shot blasting adversely affecting the surface to be coated  figures: COURTESY OF winoa

How is cleanliness defined?

Cleanliness refers to the elimination of contaminating bodies (oxidation, mill scale, welding residues, coating residues) from the surface in question. It is defined by the initial state of the surface and the degree of residual contamination (such as mill scale, slag, oil, rust, soluble salts, dust) after cleaning (ISO 8501-1 standard).

Initial State (Rust Grade)

  • Rust Grade A : Practically no rust. The mill scale film is still attached.
  • Rust Grade B: Rust spots. The mill scale has started to come away.
  • Rust Grade C: Cavities. The mill scale film has started to disappear under the action of rust.
  • Rust Grade D: Uniform rust. Numerous cavities visible to the naked eye.

Table 1: Final State: Degree of Preparation
(Equivalent standardized level of cleanliness)

ISO 8501-1 SSPC SAE Corresponding Degree of Cleanliness
Sa 3 SP 5 White metal Approx. 99% (blast-cleaning to visually clean steel)
Sa 2½ SP 10 Near white Approx. 96% (very thorough blast-cleaning)
Sa 2 SP 6 Commercial Approx. 80% (thorough blast-cleaning)
SA 1 SP 7

Brush off to light blast

Free of non-ferrous components (oil, grease, dirt and lose paint).

Several degrees of preparation are described and illustrated by reference photos in the ISO 8501-1 standard. They correspond to different levels of residual contamination.

Fig. 2: Paint lifetime vs. cleanliness level  source: bahlmann, 1982

Expressed as a degree of cleanliness of the initial surface state. For example, the degree of cleanliness SA 2½ on a sheet of steel whose initial surface state was C is “C Sa 2½.

An environment of dry, decreased air becomes more important for the blasting operation as the target level of cleanliness is raised. Remember that bare metal is hungry for oxygen and, depending on the degree of humidity, it oxidizes quickly. The coating should therefore be applied quickly (i.e., within the hour of efficient metallization).

Rust grades and cleanliness levels are normally assessed by eye, comparing the surface with reference photographs from industry standards such as ISO 8501-1 and SSPC-VIS 1. Care should be taken to achieve consistency and reliability in the evaluation, keeping in mind that several factors can influence the surface cleanliness rating:

  • The observer and his subjective judgment of the situation;
  • Lighting, visibility and surrounding conditions; and
  • Level of training and the position of the evaluator, whose role might be a customer, operator or salesperson.

To eliminate these sources of reading errors, Winoa developed and patented the WA Clean, a tool that objectively measures the cleanliness level of a shot-blasted surface using optical analysis.

Fig. 3: Surface cleanliness being measured using WA Clean tool

HOW IS ROUGHNESS DEFINED?

Roughness is the shape of the surface profile (valleys and peaks). It has a significant impact on the adhesion of the coating and must be homogeneous and adapted to the coating applied.

Roughness can be characterized by several parameters
(Expressed in micrometres or mills)

Ra: Arithmetic average of all profile’s deviations (depth of the troughs and height of the peaks);

Rmax: Maximum height between a valley and a peak within an assessment length. It quantifies the maximum height to be covered by a coating. Paint consumption depends on this height.

Rz: Average of the five maximum heights of consecutive segments of the evaluation length. The comparison of Rz and Rmax makes it possible to evaluate the surface profile’s homogeneity.

Pc: Number of peaks per unit length (cm or inch). For surface profiles of equivalent height, it makes it possible to evaluate the average width and density of the peaks, which are elements of prime importance in coating addition.

Generally, a maximum number of homogeneously distributed peaks and valleys are sought in order to create as many anchoring points as possible. This ensures good coating resistance, provided the applied product is able to “wet” and coat 100% of the surface generated.

In order to obtain more accurate information about the surface morphology, the Rsk & Rku, skewness and kurtosis could be analyzed. Normally, we are looking for an angular surface profile, where Rsk indicates a peak predominance (Rsk > 0).

In summary, to define roughness, ranges from parameters Rmax, Rz and Pc must be defined. Achieving a Rsk with peak predominance will support on wettability and the anchorage of the coating. That, along with a clean and dust-free surface, will provide good coating adhesion strength properties.

Fig. 4: A closeup of a surface profile created from WA 3D

How to perform a successful blasting operation

Shot blasting has the advantage of being visually impactful, the results are fast and the effects on the surfaces are immediate. To be effective and obtain consistent and cost-effective results, however, a properly adjusted blast machine is essential:

  • The machine and operating mix must be stabilized.
  • The hot spot must be checked regularly.
  • The operating mix and abrasive additions must be continually monitored.
  • Coverage and intensity must be checked regularly.
  • Cleanliness and roughness must be monitored.

Fig. 5: Blast machine hot spot inspection and adjustment

We invite you to consult W Care (service division of Winoa) for training, advice, repair or optimization of your blasting processes and equipment.

CONCLUSION

Substrate and surface conditions, as outlined in ISO 8502, will irreparably affect the performance of a protective coating system. Selecting a high-quality abrasive media and working with a service partner will ensure minimal risk of coating failure. The benefits of this association will be 1) reduced surface contamination, 2) improved surface cleanliness and surface profile properties, and 3) lowered total operating costs.

For more information about Winoa, W Abrasives products or W Care services, or to request a visit from one of our specialists, go to winoagroup.com and wabrasives.com.

Claims or positions expressed by sponsoring authors do not necessarily reflect the views of TPC, PaintSquare or its editors.

 
ABOUT THE AUTHOR
Iann Bouchard, Eng, Winoa USA

Iann Bouchard is Group Marketing Manager at Winoa Group. His passion for helping his customers solve cleaning problems and improve blasting efficiency has led him to support sales, products, the communication team and service at Winoa since 2013. Bouchard’s expertise from Wheelabrator and his studies in engineering have allowed him to understand the challenges faced by the industry.

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