Cleanliness, Coverage and Roughness of Metal Before Coating

 

Why Prepare Surfaces?

When coating surfaces with paint, rubber, metal, Rislan, etc., correct preparation ensures optimal adhesion and resistance (particularly to corrosion).

The type of preparation depends on several factors:

                                 *  base material (carbon steel, stainless steel, light alloy, etc...),   
                                 *  composition and thickness of coating,
                                 *  blasting method (wheel machine or compressed air),
                                 *  type of abrasive (rounded shot, grit, etc...).

Each surface state is identified by customer specification and technical specifications provided by the abrasive, equipment or paint supplier.

How is Cleanliness Defined?

Cleanliness means removing all contaminants, (oxidation, scale, and welding or coating residues) from the surface to be treated.

The surface state is most widely defined by the "Swedish standards" describing the initial and final states.

Initial state:
A.
  Practically no rust.  Scale film.
B.  Beginnings of rust. Scale begins to come away.
C.  Scale film begins to disappear as a result of rust.  Cavities  form.
D.  Uniform rust.  Numerous cavities visible to naked eye.

Final state:
The final state is the cleanliness value or the evaluation of the quantity of contaminants remaining after cleaning (e.g.: 96 % cleanliness = 4 % residual contaminants).

Swedish 
Standards
Corresponding Cleanliness Values
SA 3
SA 2.5
SA 2
99 % ("complete cleaning")
96 % ("very careful cleaning")
80 %

 

Swedish French British American
SA 3 Ds 3 1st quality White metal
SA 2.5 Ds 2.5 2nd quality Near white
SA 2 Ds 2 3rd quality Commercial
SA 1 Ds 1   Brush off

Cleanliness is therefore expressed as a value with respect to an  initial surface state, e.g. cleanliness value SA2.5 for a steel plate of surface state C = "C SA 2.5".

NB.:   The greater the degree of cleanliness required, the more important it is to blast with dry, grease-free air.  Because metal absorbs oxygen, it soon oxidizes in damp atmospheres.  Coatings must therefore be applied quickly - within an hour - fir metallisation to be most effective.

HOW IS ROUGHNESS DEFINED?

A roughness profile is a regular series of peaks and troughs, intended to ensure optimum adhesion of the coating to the base material.

The most important and widely used criterion in defining roughness is the arithmetic mean roughness, Ra, expressed in microns or mils (thousandths of an inch).  It is the arithmetic mean of all profile deviations (trough depths and peak heights) with respect to the mean surface profile.

The second useful defining criterion is the maximum roughness, Rmax, expressed in microns or mils.  It is the greatest deviation between a trough and a peak over a given evaluation distance.  This is the criteria for defining the thickness of the deposit.

The main way of defining roughness is therefore to define the Ra and Rmax ranges.  Depending on the working conditions of the finished part, other series of criteria of varying complexity may be used to analyse all roughness characteristics in detail.

The general rule is to achieve as many evenly spread peaks and troughs as possible.  This creates the maximum number of anchor  points and hence optimal coating adhesion.

INSPECTIONS:

Cleanliness

This requires visual comparisons with reference photographs normally issued by standards organisations, e.g. Swedish Standards booklet.

Coverage

Sample plates are treated in the blasting machine and compared visually with a reference sheet. (TA Method)

Roughness

Surface finish testers are used to measure surface roughness.  A sensor reads the roughness profile.  A display (dial, numerical readout, profile curve plot) gives the roughness values Ra, Rmax, etc...

Various models are available, ranging from the pocket tester to the full site computer.  It is also possible to carry out a simple visual inspection.