Wet abrasive blasting, also known as dustless blasting or vapour blasting, is a method that uses high-pressure water to propel abrasive particles at a surface in order to remove contaminants such as coatings, corrosion, or dirt. Wet abrasive blasting can be an effective method for cleaning and surface preparation, but it is possible that it could cause flash rusting in some circumstances.
Flash rusting is a form of corrosion that can occur when ferrous (iron-based) metal surfaces are exposed to moisture and oxygen. Flash rusting can occur when metal surfaces are cleaned using methods that leave behind a film of moisture, such as wet abrasive blasting. If the metal is not dried properly or is left in a humid environment, the moisture on the surface can react with the oxygen in the air to form rust.
Flash rusting can present a significant problem when preparing metal structures for coating and painting, as it forms very quickly on steel that has been newly blasted, especially when sandblasting or other conventional dry blasting are methods are used. If not checked and controlled, flash rusting can fatally undermine the integrity of steel structures, boats, ships and machinery and cause protective coatings to fail.
Blasting mechanically damages the passivating layer, which protects the metal from corrosion. In the case of vapour abrasive blasting, water is also present on the surface and in direct contact with the metal through the ruptured passivation layer.
If there are salts on the surface, they will dissolve in the water to form an electrolyte solution and a corrosion cell will form. This fast-acting attack can cause visible rusting in as little as 30 minutes.
Flash rust is caused by salt deposits on the surface of newly-blasted metal (a constant hazard in marine environments) which, when mixed with water, cause corrosion cells to form. The problem is even more exacerbated and accelerated in humid environments. Sodium chloride can pull moisture out of the atmosphere at 75% relative humidity. Other, less abundant salts draw moisture at as low as 25–35% relative humidity. But as relative humidity increases, the rate of corrosion increases exponentially.
In wet abrasive blasting, a slurry of water and sandblasting media is propelled at high pressure onto the surface. A combination of the blast media cushioned by water removes the coating/corrosion but soluble salts are not left on the blasted surface as the water dissolves them and then washes the salt away.
The use of a rust inhibitor solution such as RapidHold™ SuperStrength provides additional protection against flash rusting for upto 72 hours. The rust inhibitor can be added to the slurry mix of water and sandblasting media while blasted. For instance, the RapidBlast™ wet blasting machine also has an option for wash down whereby the surface can be rinsed off using the rust inhibitor solution mixed with water.
While light levels of flash rust can fall within tolerances for some coatings, in all cases they will degrade adhesion. Applying a coating over heavy flash rust will cause the coating to fail, as well as jump-start further corrosive reactions.
RapidHold™ SuperStrength is an environmentally friendly rust inhibitor solution that prevents flash rust while wet blasting. It is also a great additive for rinsing after soda blasting, dustless blasting, slurry blasting and vapor blasting. It effectively removes salts and other contaminates.
In summary, wet abrasive blasting has the potential to cause flash rusting if a good quality rust inhibitor is not used as instructed or if the is left exposed to moisture after the blasting process. To prevent flash rusting, it is important to follow proper drying and protection procedures after wet abrasive blasting.