Once a tank shutdown has been scheduled, a contractor and linings applicator has been selected and the lining system specified, it is good practice and greatly improves the likelihood of success if a pre-project meeting is held. Regarding the lining application, the pre-project meeting is a good opportunity to ensure there is agreement in how the lining system will be applied and inspected.
Prior to applying a lining, the critical stage of surface preparation must take place. Even the highest quality linings will struggle to perform as anticipated if applied to a poorly prepared substrate. Conversely, a lower quality lining applied to a correctly prepared substrate may perform better.
For linings to perform well they need to have a chemical and mechanical bond to the substrate, and this requires the steel to be clean, reactive and rough. Reactive steel is clean and not oxidized (rusted). A rough, clean and reactive surface is best created using abrasive grit blasting. There are many standards for surface preparation, most widely used are those from ISO, NACE and SSPC. It is recommended that the relevant standard(s) are read alongside this course to fully understand the extent of work required.
The areas where the lining is to be applied need to be prepared to ensure the lining can be applied at the correct thickness; sharp edges should be radiused, corners filleted, pitting filled and uneven areas such as welds may need to be smoothed over to create a smooth and continuous surface ready for application.
In general, immersion grade linings are recommended to be applied to white or near white metal, with a roughness of 50 to 125 microns. The cleanliness and roughness of the steel will be given by the lining supplier and will depend on the lining product used, as well as the service environment. Surface preparation of new steel can be relatively straightforward due to the fact the steel is not damaged or contaminated.
In maintenance, preparation can be much more difficult as the steel may have suffered physical damaged from pitting corrosion and can also be contaminated; a common issue is salt contamination of the steel. The contamination should be reduced to acceptable levels and various methods are used depending on the contaminant (e.g. fresh water for salts, solvents for oils). Blasting corroded and contaminated steel back to white metal in a maintenance situation is not always possible, which may need to be accepted by both the lining supplier and the tank owner (along with the possible reduction in service life).
Inspection of the prepared surfaces is strongly recommended, and it is often good practice to have an independent inspector who can resolve any issues between the tank owner and the contractor. Inspectors should be qualified (NACE, SSPC, FROSIO and other organisations hold inspector training courses) to ensure they understand what they are looking for and the correct equipment to use. Testing can be carried out to check surface cleanliness, roughness and contamination.
After surface preparation has been carried out, it is important to maintain the prepared surface until the lining is applied. It is recommended to start application as soon as possible, to reduce the chance of degradation occurring. If this is not possible then steel blast holding primers or environmental control equipment can be used. For example, in the case of a closed tank, using de-humidification can allow the creation of a dry environment where the steel can not corrode.