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Common Coating
Failures: Causes and Remedies
Condensed
from: R.A. Burgess and M.M. Morrison
Chemical
Engineering Progress, Sept. 1995
Coating failures in industrial
applications can be very costly, leading to premature deterioration
and failure. Many times blame is placed on the coating materials when,
in fact, most failures occur in applications where specifications for
surface preparation, undercoating or coating application have not been
prepared or are just not followed. For example, improper or incomplete
cleaning and removal of rust, mildew, abrasive or other surface
contamination can lead to incomplete adhesion of the coating to the
metal surface. Likewise, inadequate surface profile will also limit
the usefulness of a normally acceptable coating system. Additionally,
inadequate curing between coats and improper mixing of multi-component
systems tend to occur too often.
However, several common types of
coating failures can be categorized as indicated below along with
their causes and remedies:
CHALKING
A powdery material on the surface of
the coating. This form of surface deterioration often results from
inadequate resistance of the coating to ultraviolet (UV) light. The
remedy is to utilize a topcoat with greater resistance to UV
radiation.
EROSION
Removal of the coating by contact with
surrounding elements. This loss of coating material reduces the
thickness. Reformulation or selection of alternative materials with
higher hardness should reduce erosional effects.
BLISTERING
Formation of small to large, broken or
unbroken bubbles, under or within the coating (See Figure 1). Most
common blistering results from improper solvent, oil or moisture
contamination, surface contamination with salts (osmotic blistering)
or by excessive cathodic protection. To minimize blistering problems,
greater attention is needed on surface preparation to remove
contamination and enhance adhesion between the coating and the
substrate.
ORANGE PEELING
Formation of "hills and valleys" on
the coated surface resembling an orange peel. This problem typically
results from excessively high viscosity during application or solvent
evaporation rate. Orange peeling can be eliminated by changing coating
conditions or solvent. Once formed, it can often be removed by sanding
and applying another coat of material.
PIN-HOLING
Occurrence of small holes which
provide a path of exposure to the substrate from improper spray
atomization or segregation of the resin from the pigment in the
coating. In thin film epoxy coatings, excessive heat can cause similar
effects by foaming the resin. Its effects may be minimized after
application and curing by application of additional coating under more
optimum conditions or by recoating depending the system.
UNDERCUTTING
Blistering and peeling where exposed
substrate corrodes. Propagation of corrosion under the coating causes
disbonding. It can be reduced or eliminated by enhancing
coating/substrate adhesion and through the use of inhibitive primer on
the substrate prior to applications of top coat.
This article appeared in
Chemical Engineering Progress, Sept. 1995 pp 63-69.
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