The most common parameters to be considered when choosing an abrasive are:
- Shape – angular or round
- Hardness – hard or soft
- Density – heavy or light
- Size – large or small
- Type of surface – new, light or heavy rust, coated, steel, aluminum, etc.
- Profile required – light or heavy
- Coating system to be used
- Degree of cleaning required
- Environmental constraints
Unfortunately, choosing the type and size of abrasive that will economically and effectively produce the desired surface finish is not an exact science.
Some of the most common types of abrasives being applied are:
1. Cast Steel – shot and grit
2. Naturally Occurring – mineral and silica sand, fl int, garnet and zircon.
3. By-Product–slags (boiler, copper, nickel), walnut shells, peach pits and corn cobs.
4. Manufactured – silicon carbide, dry ice, baking soda, sponges, aluminum oxide and glass bead
In case of a recycling process, the most commonly used abrasive is cast steel with different sizes of shot and grit. In cases where recycling of the abrasive is difficult to obtain, alternatives like silica, mineral sand and boiler and copper slags are used.
A general rule of thumb is to select the smallest size abrasive that will produce the specified degree of cleaning and surface profile. Normally the smaller size abrasives will clean faster and with lower costs compared to the larger sizes.
You may expect from the supplier of abrasives that they will support you to provide specific information on the abrasive. Additional information that might be relevant is the presence or absence of (specific) contaminants.
Surface Profile
The Surface profile or an anchor pattern is a measurement of the roughness of a surface which results from abrasive blast cleaning. The profile or anchor pattern is measured from the bottom of the lowest valley to the top of the highest peaks. The depth of profi le will be directly related to the size, type and hardness of the abrasive as well as its velocity and angle of impact and the hardness of the surface being cleaned. The maximum allowable depth (height) will depend on the coating system dry fi lm thickness. A rule of thumb states that profi le depth (height) should not exceed 1/3 of the coating systems dry total film thickness.
Rust Back
Flash rust, or Rust back occurs when freshly cleaned steel surfaces are exposed to high humidity, rain or a corrosive atmosphere. The time involved in getting rust back can vary tremendously ranging from minutes to weeks.
As a rule is it always advisable that an abrasive blast cleaned surface is coated within eight hours. Under no circumstances should the steel be allowed to rust before coating is applied regardless of the time elapsed. One exception to this rule, however, would be the use of surface tolerant coatings which are designed for application over rusted surfaces. Abrasive blast cleaning should not be conducted when the surface temperature is less than 3°C above the dew point. Moisture can condense on the surface if it is colder than the surrounding ambient air temperature and rust back can occur.
Rust Inhibitors: When using water blasting or wet abrasive blasting, the cleaned surface will rust very rapidly. It is therefore essential that a “rust” inhibitor be applied to the surface itself immediately after cleaning or mixed in the water used during the actual cleaning process. Use of any “rust” inhibitor should always be cleared through the coating manufacturer to insure that incompatibility problems between the inhibitor and coating to be used do not occur.
Rust Inhibitors: When using water blasting or wet abrasive blasting, the cleaned surface will rust very rapidly. It is therefore essential that a “rust” inhibitor be applied to the surface itself immediately after cleaning or mixed in the water used during the actual cleaning process. Use of any “rust” inhibitor should always be cleared through the coating manufacturer to insure that incompatibility problems between the inhibitor and coating to be used do not occur.
