Surface treatment: Sandblasting

In the heavy industry, whenever we speak about a complete part or a whole assembly of parts, regardless of weight and size, we refer not only to achieving the exact shapes and dimensions, but also to achieving the appropriate surface finishing.
And while technical equipment made from stainless steel or other non-ferrous metals need mostly pickling and passivation, carbon steel products require coating due to its corrosive nature.

Despite of the coating requirements, whether hot/cold galvanisation or RAL of paint, the preparation stage remains a key factor for the durability of the coverage. Correct preparation must be applied properly before starting any painting process in order to prevent prematurely failure of the coating.

Abrasive sandblasting is a widely used method for preparation of structural steel and carbon steel sheets.  It is applied in almost all steel-using industries, but mostly in marine and shipping and industrial steel structures. The process of sand blasting is performed mainly by manual labour (pic.1), since it allows better reach from different angles and sides.

Furthermore it allows work on sites, which makes it very suitable for the marine industry when replacing a ship’s coating (pic.2).

In production sandblasting is often replaced by shot blasting cameras with automated conveyers (pic.3)

Why do we need pre-coating sandblasting?

Carbon steel which is hot rolled leaves the last rolling phase with a temperature of about 1000C° and proceeds with cooling down. As the temperature declines the steel reacts with the oxygen in the atmosphere and forms a “mill scale” on its outer layer. The mill scale is a complex oxide which covers the surface of the steel (pic.4)

Mill scale is not a stable covering for the steel surface, so in time moisture from the air penetrates and leads to corrosion, or in other words rusting. The amount of rust depends mostly on how the materials are being handled (outdoors or indoors), the amount of moisture in the air and the time they are left without a coating.

Rust on structural steel after outdoor handling.

Abrasive blast cleaning or shot blasting very often offers the best solution for rust and mill scale cleaning as pre-painting preparation.  The method includes mechanical cleaning by continuing collisions of abrasive particles with very high speed with the steel surface. The blasting stream can be with either with air or water.

The surface treatment specification normally defines the method required, depending on the roughness.  After finishing the sandblasting, it is very important not to allow re-rusting and handling the materials properly in a dry-air environment, or simply proceed with either galvanisation or paint coating.

The necessity of carrying out a surface preparation before painting or galvanisation is actually more important than the actual process of painting and galvanisation as it impacts the life cycle of the coating itself. If rust and mill scale are not removed completely before coating, the remaining scales can later in time lead to falling apart of whole pieces of the coating (Pic.6 and pic.7)

How does abrasive sandblasting work?

Sandblasting or abrasive blasting a cleaning and smoothing process which works by forcing solid abrasive particles under very high pressure against steel or other surface.  It involves drawing the particles in to a mixing chamber of compressed air and sand which is pressured by turbines and then shot thru a nozzle against the steel surface.

Depending on the blasting equipment and the surface treatment requirements, wet blasting can be performed. With wet sandblasting, water is added in the mixing chamber, so that the blasting stream smoothens the finishing.

The mixture of water and particles and air can also be fused in the nozzles, right before shooting against the steel surface (pic.9)

When engineering your parts and technical equipment, it is very important to note the surface treatment requirements, in order to assure the lifecycle needed for the part.

At My Euro Steel we can provide our clients with both, smooth and rough sandblasting after assembly and even shot blasting before even cutting your parts from sheet metal or structural steel.

Just let us know what finish you need for your parts on our notes section, when requesting a quote.