This month, the questions being answered by our corrosion technology experts relate to acid storage tank linings and the deployment of non-intrusive corrosion monitoring devises for pipelines and process pipework.

What is the best lining for a hydrochloric acid storage tank? DD

Equipment fabricators looking to offer corrosion protection for the storage of hydrochloric acid have a number of materials at their disposal. Whatever method is selected this should be able to create a safe environment, without compromising the tank integrity as well as maintain continuous operation and avoid adverse effects on the cargo itself.

At a very basic level solutions can be divided into inorganic and organic. The inorganic solutions may include; metallic solutions, such as alloys which are capable of resisting a wide range of acids including hydrochloric, and ceramic solutions, such as bricks /tiles, often used in conjunction with an organic solution such as vinyl esters or furans in the joints and behind the brick system. The organic solutions may include; thermoplastics which are bonded or physically adhering to the metal substrate, and reactive thermoset types which cross-link to firm an acid resistant coating barrier.

Whilst resin coating types are widely used in acid secondary containment areas, their use in tanks and vessels is considered a higher risk, however they remain attractive due to the relatively low cost of installation when compared to metallic or ceramic options. Phenolic or Novolac epoxies can provide suitable resistance to hydrochloric acid (though they are less resistant to more oxidising acid species) provided that they are formulated with a suitable hardener, with amine hardeners generally being more resistant than amides, though still susceptible to acid hydrolysis and subsequent degradation.

Vinyl esters have high resistance towards both oxidising and non-oxidising acids and offer a suitable solution for hydrochloric acid providing both good chemical resistance and ease of application. Their good overall resistance arises from the extensive presence of aromatic groups within the resin and the presence of methyl groups which have the effect of stabilising the ester linkage.

However, colour change of the coating material is a commonly found phenomenon with acid materials, and can result in adverse effects on the acid cargo itself (colour tainting) which may not be acceptable where stringent quality checks or high purity requirements are to be found. SD

Consideration of all the above factors will greatly influence successful applications/outcomes in this growing area of the CM equipment market. ST
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