Surface Lamination

10.3.1 Surface Lamination

for future maintenance and help determine if Surface lamination is the most common

premature failures loom. surface defect on steel substrates. It is typi-

If the specification calls for repair of the cally caused by rolling the steel. These defects, then complete the repairs before fur- defects must be removed by grinding since ther work proceeds. Fabrication defects fall no coating system can effectively protect the into several broad categories. Some of them surface if they are left intact. Small surface are very similar to design defects in terms of laminations (even if they do not yet project the proper way to repair and document them. above the surface) may later curl upward

and penetrate or disrupt the coating system.

10.4.1 Imperfect Welds

10.3.2 Inclusions

10.4.1.1 Weld Spatter

Remove inclusions ( Figure 10.26 ) in all Weld spatter, also known as weld splatter, forms, such as rolled-in mill scale. Methods

( Figure 10.27 and Figure 10.28 ) describes include chipping and/or grinding. Some-

particles of molten metal from welding that times weld filling can restore the surface.

are thrown onto the surface adjacent to the weld. Sometimes weld spatter removes eas-

ily with an impact tool such as a chipping Fabrication errors should be addressed in the

10.4 Fabrication Errors

hammer, but generally removal requires job specification and discussed at the pre-job

grinding down with a disc or angle grinder meeting. Inspectors should ensure these

in order to remove it successfully. defects are corrected in accordance with the

Weld spatter may be up to 20 mils (500 µm) specification. If correction is not called for

or more in height over the substrate and is by the specification, make sure the workers

difficult to cover using only a coating of prepare and coat the defects as well as possi-

appropriate thickness. The resulting thin ble. Find and document each defect’s precise

coating film over the weld spatter may break down early in service and allow corrosion

Surface Preparation 10-13

extending the life of the coating generally more than offsets the extra cost of continu- ous welding. Report any cracks in welds to the owner for further evaluation before coat- ing.

The best method to seal the area is to make a continuous weld, clean and grind it smooth, then follow by stripe coating along with the regular coating. If continuous welding is not possible, use penetrating sealer or caulking

Figure 10.27 Fabrication Defect: Weld Spatter

in the overlap joints. Use a caulking com- pound that is compatible with the coating system over the prepared joint before stripe coating. Epoxy and silicone-based caulks have both been used with success.

Figure 10.28 Fabrication Defect: Close-Up of Weld Spatter

to develop, which can later spread beneath the coating film. Consequently, treatment of weld spatter is important. Remove by

Figure 10.29 Fabrication Defect: Skip Weld

mechanical means before blasting according to the specification. In tanks and vessels,

10.4.1.3 Rough Welds

where coatings are in immersion service and Grind rough welds ( Figure 10.30 ) smooth where frequent inspection is impractical, it

( Figure 10.31 ) or repair them to remove is most critical to remove all weld spatter to

sharp edges and other irregularities to pre- minimize or eliminate the opportunity for

vent the coating from bridging over voids in corrosion to occur.

the weld. Sharp ridges and spikes are diffi- cult to coat; the coating pulls back from

edges of the ridge and is much thinner at Proper welding technique is also important

10.4.1.2 Skip Welds

those points. Grind or repair all welds to cre- to avoid skip welds ( Figure 10.29 ). The

ate a smooth surface. Also fill undercuts and welding should be continuous rather than

pinholes with welding.

spot or intermittent. Continuous welding is more expensive than spot welding; however,

10-14 Surface Preparation

Figure 10.30 Fabrication Defect: Rough Weld Figure 10.32 Fabrication Defect: Lamination

Figure 10.33 Fabrication Defect: Gouge Figure 10.31 Welds Ground Smooth Prior to

Coating

10.4.4 Sharp Corners and Edges

10.4.2 Laminations

Sharp corners and edges cause surface ten- sion which causes coatings to pull back, par-

Correct laminations, scabs, rollovers, and ticular during drying and/or curing. Round

other defects of this type ( Figure 10.32 ) to all sharp edges. ( Figure 10.34 , Figure

expose areas that are otherwise inaccessible

for cleaning and coating. Be sure to remove slivers, laminations, and other defects that