Pressure vessels vary immensely, but as their name indicates these vessels are under pressure, with the addition of chemicals and temperature; externals need to tolerate heat and care needs to be taken to avoid corrosion under insulation.

Corrosion challenges for pressure vessels

Great care must be taken to understand the make-up of the contents of the pressure vessel, not least at the phase changing time. Intermediate components can be more destructive than the initial and final products.
 
In addition to the direct resistance, pressure plays a part in coating lifetime. Not the actual pressure but the time used to both increase the pressure and more so the drops in pressure. In case of pressure drop it is wise to use time over this in order to 'protect the coating'. Although some coatings will behave well in rapid pressure drop they are not always suitable due to the chemical/temperature resistance.
 
In most cases it is necessary to discuss with Jotun’s technical department.

HPI brochure