BGAS Painting Study Material Chapter 2 continuation

In the context of this course we are considering the following: -

a) Sand

It is not permitted to use sand.  SI 1657 states that any mineral used as an abrasive must release less than 1% free silica on impact.  (Silica causes preumonicosis or silicosis).  COSHH REGS does not allow the use of sand containing silica for dry blasting.  Sand itself is perfectly safe, but shattering on impact releases silica which can be inhaled. The amount of copper in the structure is extremely minute.

b) Copper Slag

Copper Slag

1. Minerals melted with the copper,

2. Liquefies and forms a protective cover over the molten copper to prevent reaction with the atmosphere. 

3. When the copper metal is run off the slag is rapidly cooled in cold running water

4. The material is supplied in grit form (random, sharp Edges, amorphous (no definite shape) and is very brittle), shatters into smaller pieces on impact, and should be used only once and then discarded and so classed as expendable.

Garnet

c) Garnet

A natural mineral classed as being “of a diamond type Hardness” can be either expendable or recyclable. Cleansing units are available to extract contamination so that the material can be reused, usually up to three times.  Doesn’t shatter on impact but does suffer some “Wear” Supplied in Grit form. 

d) Metallic Grit

Metallic Grit

Steel and Iron are both metallic.  Steel grit being the Softer of the two to round off on impact and loses its sharp edges.  Angular Chilled Iron chips off small slivers on impact to produce sharp cutting surfaces on its next cycle.  Metallic abrasives are recyclable because the particles reduce in size slowly. Hence it can be re-used many times and still perform a useful function in a '‘working mix’. A working mix is an accepted ratio of large and small particles, where the large particles cut the profile and the smaller particles clean out the troughs.

e) Metallic Shot

Metallic Shot

Shot is spherical and doesn’t shatter (otherwise it would form grit).  When supplied the particles are virtually uniform in size and shape, (not a working mix) But like the grit they wear down slowly in size. The particles are worn down eventually to finings, and are drawn out of the system during cleansing.

f) Metallic Shot and Grit Mixed;

A mix of shot and grit results in a more uniform profile. 

1. The grit cuts the profile

2. The shot, being unable to enter the troughs produced, controls the peak height and so greatly reduces the number of ‘rogue peaks.’

Metallic Grit & Shot Mix

A rogue peak; is one, which is well proud of the acceptable profile range, and if painted over due to contraction of the paint, will leave bare metal in contact with the atmosphere, thus allowing corrosion to occur.  When rogue peaks are in concentrated area the effect is of a rash, hence rust rashing or rust spotting.

A typical mix ratio of Shot to Grit as used in a pipe coating mill would be 70 – 80 % shot to 20-30% grit.

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