INSPECTION LOGBOOK
Coating Inspections - Inspection of Steel Surface Preparation, Part 1 Massimo Cornago NACE International Certified Coating Inspector, NACE CIP PEER Reviewer
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cornago@ipcm.it
n our previous article, “Basic Inspection Practice”1, we analysed
1. Soluble salt contamination
the responsibilities of a Certified Coating Inspector (CI) during the
The CI must check that the surface is free from contamination to
preliminary inspection phase (Preliminary Inspection Responsibilities,
the extent specified, especially in terms of chlorides, sulphates,
paragraph 1), with ample space devoted to the necessary work
iron ions, and dust. Chloride salt contamination, in particular, can
planning activities. We will now delve into the inspections to
be tied to different sources, such as airborne industrial chemicals,
be carried out during the surface preparation phase (Surface
salt contained in a marine atmosphere (in areas near or on the
Preparation Inspection, paragraph 2), particularly those relating to
sea), direct contact with sea water, defrosting of bridges and/or
surface contamination. Although often project specifications do not
highways on which salt has been thrown to prevent freezing, or
mention this, it is good practice that, during the pre-job meeting, the
even already contaminated abrasives. The latter are often invisible
CI requires two steps to be carried out. These include contaminant
to an inexperienced eye and they are generally not removable by
removal, as contaminating agents can affect the adhesion of coatings
abrasive blast cleaning.
thus leading to the system’s premature failure due to corrosion, and
Although exact standards of acceptability have not been established
the achievement of proper roughness of the metal surface to be
in terms of allowable amounts of contamination that may remain
treated, in order to guarantee good adhesion.
and not affect coating performance – of course, each paint product
There are many ways to obtain proper surface preparation, including
has a specific value – there are some “conventional” parameters of
solvent cleaning, water cleaning, hand tool cleaning, power tool
reference for both new construction and maintenance works. In
cleaning, abrasive blast cleaning, and centrifugal blast cleaning;
general, research and experience show that removing any chemical
all the inspection procedures required for each of these methods
contamination helps extend the service life of coatings over time.
are governed by international standards, such as NACE, SSPC, ISO,
The standard NACE5/SSPC-SP12 is a valid help for this purpose. In
etc. Surface preparation is preceded by a control phase on the
fact, it is almost always mentioned in project specifications as the
surface itself (steel preparation). Here, the CI assesses all aspects
official reference document. Alternatively, a specifier drawing on
of the metal surface to be prepared to coating, including welding,
personal experience and/ or the support of the paint manufacturer
cutting, grinding of welds and sharps edges, weld spatters, etc., in
can give some directions and decide which are the minimum values
compliance with the reference standard regulations. Moreover, in
of acceptability.
case chemicals are used, which are generally very alkaline and/or
With the exception of acid/basic contaminants, which can be easily
contain thickeners, before surface preparation the CI must check
identified by measuring the surface pH, all other contaminants can
that the operation is carried out in full compliance with the project
only be determined through extraction, by swabbing or deionised
specifications and, above all, with the instructions issued by the
water-based cleaning with an aqueous solution. The latter can be
paint manufacturer. Indeed, if they are not properly removed, these
then analysed with different certified methods, such as indicator
products will remain on the surfaces long enough to alkalise and
strips (ASTM-D4262), conductivity meters, or other tools that can
attack the coatings.
determine a sample’s chloride ion content.
We will now describe in detail the tests to be carried out before the
The standard NACE5/SSPC-SP12 includes a table defining three
surface preparation phase.
levels of soluble salt contamination: 1. SC-1 – Surface free of all detectable levels of contaminants
M. Cornago, Coating Inspections – Basic Inspection Practice, in ipcm® Protective Coatings no. 35 (October 2020), pages 40-41.
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N.36 - 2020 DECEMBER - ipcm® Protective Coatings
(water-soluble chlorides, iron-soluble salts, and sulphates), as determined using available field test equipment.