How can you avoid flash rust during wet blasting?

Flash rust is defined as the corrosion that occurs on the surface of a metal shortly after any wet method of surface preparation. This type of rusting typically occurs within minutes after cleaning operations are completed.

During wet abrasive blasting, the passive oxide layer that normally protects the metal can become damaged or removed completely, leaving the underlying metal substrate prone to corrosion as it is exposed to air and moisture. (Learn more about wet abrasive blasting in the article Understanding the Industry Shift to Wet Abrasive Blasting.) The rate of flash corrosion depends on the amount of dissolved oxygen in the water, impurities present in the water, the amount of ionic species on the metal's surface and the total drying time.

Figure 1. Web abrasive blasting in action.

Small amounts of flash rusting can fall within the tolerance of some coatings. However, applying coatings to a surface with heavy flash rusting can negatively affect the adhesion of the coating to the metal.

This type of corrosion can be avoided by employing the following measures:

• Use water with minimal impurities.

Water quality is one of the biggest contributing factors in flash rusting. Chlorine ions (Cl^-) found in some water sources are extremely electronegative and therefore highly reactive (more so than dissolved oxygen). Therefore, using water filtered by distillation or reverse osmosis can help minimize the possibility of flash corrosion.

• If possible, avoid acid washes because these can increase the corrosion rate.

• Ensure the metal substrate is free from contaminants.

Contaminants left on the surface after cleaning can draw moisture out of the air and onto the exposed substrate; this can promote flash rusting. Contaminants may be deposited on the metal by rain (acid rain), airborne contaminants or by preexisting rust that was spread across the surface during wet blasting. A clean surface after blasting is therefore essential to prevent flash rusting.

• Add a corrosion inhibitor to the blast water.

Inhibitors can prevent flash rust in a couple of ways. Some inhibitors are designed to purge contaminants from the surface. Other inhibitors contain chemicals that protect the exposed surface by passivating the processes that cause oxidation. Ultimately, inhibitors work to slow the rusting process, thus providing a short amount of “protection time” before the final coating is applied.

This piece originally appeared on Graco.com. It has been reposted here with permission.