Coating Reference Handbook - Hempel

HEMPEL
CHECKPOINT WELDS P 2b
WHY?
Welds are irregular areas along more even areas. Important checks for suitability
to be protected are:
* IRREGULARITIES * UNDERCUTTING
* POROSITY
* Irregularities, i.e. wire residues, protrusions etc. are not removed totally by
abrasive blasting. The contours may produce a local low dry film thickness
upon paint application resulting in premature local rusting and blistering on
submerged areas.
* Undercutting produce a deep sharp edged valley in the steel next to the weld.
Similar to irregularities this cannot easily be covered with sufficient paint.
* Porosities may contain millscale and weld flux residues, that are not cleaned by
mechanical methods incl. blasting. Paint cannot penetrate and cover such
porosity.
CORRECTIVE ACTIONS:
* Irregularities must be ground smooth, so that they do not feel sharp or
protruding, by feeling with your finger, or as specified in the working specification.
* Undercutting must be ground smooth or rewelded if too deep.
* Porosity must be removed by grinding, or rewelding. If later exposure is of
low corrosivity, a suitable filler is acceptable.
PREVENTIVE ACTIONS:
Excessive undercutting, porosity and irregularities are often caused by the weld
operators working too fast and with wrong weld parameters. You cannot instruct
them, but talk to paint foreman or QC-Department about consequences for
surface prep workers.
Welding in shopprimer may be the cause of porosity, when MIG/MAG welding.
Grinding to reduce dft or remove shopprimer at weld lines may be a solution.
Some manual weld positions (vertical) will cause irregular welds.
HOW TO DETECT:
Visual and by touch.
At tankcoating jobs it may be advantageous to have the welds
blasted before the inspection of the steel surface. Some porosity
and undercutting do not show up until after blasting.
ISO has developed a standard for steel surface condition in connection with
coatings.
This standard is ISO 8501-3.
INSPP2b ed3 21/11/2006 EMi