Importância da Limpeza Adequada para Ensaios por Líquido Penetrante

A proper cleaning before starting the test is essential to obtain a true result. It is easy to notice that if the part is not completely free of foreign substances, there may be a masking of the discontinuities that may exist, resulting in their non-visibility.

Any trace of oil, grease, paint, varnish, oxidation, dust, moisture, etc., must be removed from the surface of the part in order to provide the capillary effect of the penetrant.

It is important that whatever pre-cleaning process is used, the part is left completely clean, dry and undamaged.

The main processes used for pre-cleaning are:

Detergent Cleaning

Immersion tanks with heating or not are a common way to perform the cleaning required by the penetrant liquid test. The detergents used have the property to penetrate, emulsify and saponify various types of surface contaminants. Care should be taken so that they do not corrode the parts, since they can be acid or alkaline.

Steam Degreasing Cleaning

Steam degreasing cleaning is particularly effective in removing oil, grease and similar organic contaminants, leaving the part clean and dry

Solvent Cleaning

The solvent cleaning can be carried out in an immersion tank or by rubbing the surface and then wiping it with a cloth that does not shed.

Ultrasonic Cleaning

Ultrasonic agitation is usually combined with a detergent or solvent to improve the cleaning efficiency and decrease the cleaning time. This technique is particularly useful for cleaning small parts.

Scale and oxidation removal

Commercially available acid or alkaline strippers or acid pickling solutions can be used.
If an acid is used for surface etching, an alkaline solution should be used as a neutralising agent.

Chemical attack and etching processes typically use dip tanks or can be applied by scrubbing.

Paint Removal

Special paint removers can be used by immersion or not. Any chemical process is preferable to a mechanical process.

Cleaning Processes Should be Avoided

Sand or grit blasting, abrasive liquids, emery wheels, steel brush and metal scrapers.

These methods tend to close (mask) surface discontinuities. Steel brushes should only be used to remove rust, general oxidation and scale, when other means are not sufficiently effective.