Corroded metal parts can be repaired with thermal spray coating instead of having to be replaced, to return them to original dimensions.
Assess Whether the Part Is a Good Candidate
Every corroded part can be repaired in due course, but first the base material needs to be checked for soundness. This means checking for cracks, deep pitting in load-bearing parts of a component, intergranular corrosion etc. Surface corrosion and some material loss are typical repair items. Through-section corrosion is usually beyond repair.
Prepare the Surface Properly Before Spraying
Crevices, as with any surface preparation work, must be cleared of all corrosion products prior to coating to prevent weak spots in the coating layer. This surface must be prepared in a similar way to that used for a number of other coating methods (i.e. cleaned and grit-blasted) to give a suitable rough surface for coating to bond to.
Choose the Right Coating Material
In some cases a single corrosion environment requires a different material such as a Zinc or Aluminium coating to offer sacrificial protection to Steel in moist or saline environments. In chemical applications Stainless Steel or Nickel-Alloy coatings are used. Further information on how the thermal spray processes are classified and matched to the application conditions is given in TWI’s technical knowledge pages.
Build Back Lost Dimensions
Thermal spray coating can be deposited in layers to build up corroded areas within parts and then complete the repair by finishing grinding or machining to the required tolerance. Thermal Sprays is covered in more detail at https://www.poeton.co.uk/surface-treatments/thermal-metal-sprays/.
Verify the Repair Before Return to Service
Pull-off tests can check adhesion of the new coating. The dimensions of the part must be re-checked with a micrometer prior to issue. Whilst a simple visual check is first carried out, in most cases, a subsequent hardness test and possibly salt spray test will confirm that the repaired part has a coating that is of adequate design to meet the requirements of the original part.
When all has been said and done, a part that has been repaired with a thermal spray coating can go on to serve up a comparable service life to a ‘brand new’ part, having been repaired for a fraction of the cost of a new part.
