Alkaline Burn

Alkali burn also known as PH Burn, is when the surface of the coating has patches or large areas where the colour of the paint film has changed. This change normally appears as lighter patches on the surface but can also be a complete visual colour change.

What is Alkali Burn?

Alkali, or pH burn, is a condition that occurs when high alkalinity in masonry substrates break down the paint’s binder resulting in eventual deterioration of the paint film by exposing the pigments of the coating. This deterioration is seen as a loss or change of colour and a blotchy appearance.

This breakdown is normally more pronounced in dark colours.

The Process:

Causes of Alkali Burn?

Soluble salts (like sodium or potassium carbonates), are salts present in masonry materials like brick, concrete or mortar. When water comes into contact with these materials, the salts dissolve and travel through the pores and eventually reach the painted surface. Rain water, ground water or even condensation can trigger this migration.

Water Absorption: If the masonry material still has a high moisture content due to retained moisture and is painted,or If the masonry material has a high level of moisture content due to retained moisture and the material is painted on; or if water seeps through the painted via cracks, immediately the pH of the plaster may increase as alkaline salts migrate towards the surface but are blocked by the paint film. Water gets absorbed into the masonry material, dissolving the soluble salts.

Salt Migration: The dissolved salts travel through the masonry pores towards the drier area, which is often the painted surface.

Evaporation: As the water evaporates from the paint surface, the salts crystallise behind the painted surface.

Photograph 1 shows the dissolved salts that have migrated to the surface of the substrate. On a painted surface these sit behind the paint film and attack the coatings binder resulting in alkali burn of the paint film. These accumulated salts which are deposited directly behind the applied paint film are highly alkaline, typically with a pH above 13. This high pH damages the paint film, causing various problems like:

• Discoloration, fading, yellowing, or blotchy patches 
•  Staining, white efflorescence (salt deposits)
•  Blistering and peeling, lifting and flaking of the paint 
•  Loss of gloss, dull and chalky appearance

This alkali burn process can be slow and progressive, taking months or even years to manifest visible damage. Certain types of masonry materials and paints are more susceptible to this problem than others.

Photograph 2 Shows alkali burn on the coating, the deterioration of the paint film is seen as a loss or change of colour and a blotchy appearance.

Rectification:

• Address the source of the moisture within the substrate. A professional water-proofer is recommended.
•  Professional cleaning may be required to remove the salts from the surface.
• Neutralizing the salts with an acidic solution. It can be brushed down and wiped with a dampened sponge using a 5-10% solution of white vinegar and water.

Once the source of the problem has been addressed repainting with a suitable Dulux Alkali Resistant Plaster Primer and suitable Dulux topcoat is recommended.

Conclusion

Early detection and proper treatment are key to mitigating the damage caused to the applied paint system by alkali burns. Consulting a professional to diagnose the root cause and recommend the best course of action is highly advised.