Coating and Lining
Failure
DESCRIPTION
Coating failures are usually the result of problems associated
with either coating application (i.e. surface preparation,
primer, or topcoat), chemical or environmental durability or wear
and abrasion. Such failures are typically related to loss of
adhesion, cracking or wear.
PREVENTION OR REMEDIAL ACTION
- surface preparation
- surface must be properly cleaned to remove all
dirt, corrosion and contamination.
- surface must be roughened to provide an adequate
anchor pattern for coating.
- selection of primer
- provide corrosion inhibition
- provide additional barrier between metal
environment.n\
- selection of topcoat
- need adequate coating thickness to cover all of
the surface (free of defects or holidays). This
may require multiple coats.
- must have chemical or moisture resistance and
temperature resistance.
- in some applications, it must also have abrasion
resistance.n\
- applications involving cathodic protection
- must have resistance to cathodic disbonding.
STANDARD TEST METHODS
- NACE TM0175 - visual standard for surfaces of new steel
centrifugally blast cleaned with steel grit and shot.
- NACE TM0174 - laboratory methods for the evaluation of
protective coatings used as lining materials in immersion
service.
- NACE TM0183 - evaluation of internal plastic coatings for
corrosion control of tubular goods in an aqueous flowing
envrionment.
- NACE TM0375 - abrasion resistance testing of thin film
basked coatings and linings using the falling sand
method.
- ASTM G-8, G-80, G-95 - tests for cathodic disbonding of
pipeline coatings.
- ASTM G-42 - test for pipeline coatings subjected to high
or cyclic temperatures.
- ASTM B-117 - salt fog testing.
- ASTM G-85 - modified salt spray (fog) testing.
- conversion coatings: ASTM B-449 (Al-alloys); ASTM B-201
(Zn and Cd alloys); ASTM B-281 (Cu-alloys).
- ASTM G-7 - practice for atmospheric environmental
exposure of nonmetallic materials.
- water resistance of coatings in 100% humidity.
Evaluation of Coatings and Lining
In general, coatings and linings work to prevent corrosion by
imposing a dielectric barrier between the corrosive environment
and the material surface. Therefore, an evaluation of coating
performance is one of determining the chemical compatibility and
thermal stability of the coating in the service environment and
the adhesion of the coating on the substrate. Many coatings tests
use post exposure visual examination and/or mechanical
performance to rate its resistance to the corrosive environment.
Newer evaluation techniques such as electrochemical impedance
spectroscopy (EIS) can be used in-situ to assess changes in
coating parameters such as film capacitance and pore resistance
well in advance of the point where damage can be determined
visually. Another important aspect of coating evaluation is the
use of service life testing methodologies. In their simplest
form, these take the form of evaluations of coating performance
after multiple exosure period instead of the commonly used single
point testing. This allows for the tends in coating performance
versus time to be obtained. Additionally, the use of theoretical
and empirical models can be used to accelerate the test by
increasing temperature or by changing other parameters (e.g.
applied potential). Short term failures obtained at multiple
conditions can be used to predict longer terms limits of
serviceability in various service environments.
