Metallic abrasives are popular in stationary blasting chambers or halls because of their high recyclability. However, this does come at the cost of flexibility in application and potential production inefficiencies.

It is important that users understand the benefits and deficiencies of using metallic abrasives and ensure they are not making their decisions based on false economies. A review of the facts provides a better understanding:

Flexibility

Metallic stainless-steel abrasives (stainless steel grit) are predominantly suitable for the surface preparation of stainless steels. Their use on aluminum surfaces is chemically possible but is not often practiced due to the performance outcome. The bulk density and hardness of stainless-steel abrasives have excessive influence on the aluminum substrate and can cause deformation. The use of metallic stainless-steel grit is only economically efficient when you can recycle the product numerous times.

Metallic steel abrasives (steel grit) are typically used for the surface preparation of steel surfaces. A large grain size is usually used to achieve increased recyclability. Steel grit cannot be used to blast stainless steels, due to the resulting galvanic corrosion, as detailed below.

Galvanic Corrosion (Bimetal Corrosion)

Why are stainless-steel shot and steel-shot not suitable for steel and stainless-steel substrates, respectively?

ASTM defines galvanic corrosion as “accelerated corrosion of a metal because of an electrical contact with a more noble metal or nonmetallic conductor in a corrosive environment.1” Consequently, galvanic corrosion can result from abrasive embedment or metallic abrasive dust on the surface of dissimilar metals. It is therefore important to avoid contact between different metals in fabrication and blasting processes, where metallic abrasives need to be matched to the metal substate it is being used on.

GMA Garnet™ abrasive is a non-conductive inert natural mineral abrasive which does not react with metallic substrates. It is versatile enough to be used on various types of ferrous and non-ferrous metal surfaces. Moreover, its flexibility allows for usage in both indoor controlled environments and outdoor situations such as site blasting works.

Recycling

Does increased recyclability provide an advantage and how does the abrasive change during the blasting process? Stainless steel grit and steel grit round off with each impact on the metal substrate. Round abrasive particles have a peening effect on the surface and produce a more rounded surface profile. This in turn means less adhesion for heavy duty Industrial coating systems which require substantially angular and coarser blasted surfaces. It is therefore necessary to regularly top up with new grit in order to maintain surface profile requirements.

Many industrial and government bodies, for example the Illinois Department of Transportation (USA)2, only accept surfaces that have been blasted with angular abrasives for metallizing or secondary (after fabrication) /maintenance works. This requires substrates blasted by shot blasting systems to undergo a secondary blasting process, adding additional costs. With each use the abrasive loses its coarseness and consequently blasting speed, this then impacts production efficiencies.

Depending on the garnet product type and the scope of application, GMA Garnet™ 's superior toughness and shatter resistance enables it to be recycled up to five times.

Humidity

Metallic abrasives are highly moisture sensitive and absorbent. Increased humidity levels can cause corrosion in the abrasive and make the abrasive unusable. In addition, metallic abrasive blasted surfaces can have blasting media embedment or residues of metal dust. Substrates should ideally be coated within four hours of blasting. Embedment by corroded metallic abrasives increase the likelihood of spot corrosion or weak points in the coating system, therefore reducing this timeframe and increasing production pressure to not leave substrates uncoated for extended periods.

Being of a natural mineral composition, GMA Garnet™ is not adversely affected by moisture or humidity and does not corrode. Even though wet garnet may be used after it is allowed sufficient time to dry out, it is highly recommended that GMA Garnet™ be stored indoors and in a dry place to maintain it optimal performance and that it is ready for use at moment’s notice.

The GMA Garnet™ product range includes abrasives to suit most blasting applications. We offer customers the flexibility to use the same abrasive on multiple substrates, while improving production efficiencies and reducing downtime.

Get the better abrasive for your blasting projects. Contact our technical teams today

By Steffen Glade, GMA Europe,
Sara Madavamani & Alan Godinho
from GMA Asia Pacific