Understanding Corrosion in Water Pipelines: A Guide for Pipeline Designers


Paint Saponification

Last updated: July 10, 2018

What Does Paint Saponification Mean?

Paint saponification is the process of a hydrolytic breakdown of ester bonds within a paint due to alkaline conditions and moisture. It is a type of paint failure and results in weakness and uncovering of the paint, exposing the underlying material. Paint saponification renders the paint mixture soft and water-soluble. Paints are particularly prone to saponification when covering certain metal surfaces such as zinc, magnesium, copper, lead and tin, which can generate alkaline conditions from corrosion. Generally paint saponification occurs when the pH is higher than 10.

Because paints are used to protect metal materials from environmental dangers such as corrosion, the risk of failure due to paint saponification should be considered.


Corrosionpedia Explains Paint Saponification

The mechanism of action of paint saponification is the same as any saponification process (including the production of soap). Under alkaline conditions, the concentration of hydroxide (OH) ions is high. These ions attack ester bonds (R-CO2-R’) to form carboxylate ions (R-CO2) and alcohols (HO-R’). This chemical transformation breaks down large molecules into smaller components and produces molecules that are more polar than esters. The polar components are prone to interaction with water molecules, making the paint mixture more soluble in water and weakening the paint's ability to adhere to a surface. The observation that the paint become sticky during this process is due to these polar molecules as well, which tend to interact well with other polar surfaces.

Corrosion products of certain metal surfaces such as zinc (galvanized) surfaces increase the amount of reactive hydroxide ions that can react in saponification. Paint saponification can thus be associated with corrosion. Paint failure can also lead to further corrosion by exposing the metal surface.

As there is no reversal to the paint saponification process, the entire painted surface needs to be prepared, cleaned and dried, after which a new paint with alkali-resistant properties can be applied.


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