inspection and engineering unit 5.1

7/31/2019 Inspection and Engineering Unit 5.1

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INSPECTION AND ENGINEERINGUNIT 5.1

COATING FAILURES

Objective:

The objective of this training unit is to give an overview of the types of coating failures that maybe encountered immediately after application and failures which may be encountered over aperiod of time.

ABRASION

Description:

Approximately vertical scratches in a cargo hold coating often Xmas tree like in appearance andoften with a build up of coating at the base.

Possible cause:

The coating is pushed down the bulkhead during settlement of a hard angular cargo, such as coal,after loading, and the effect is exacerbated by hot cargoes and solvent entrapment in the coating.

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ADHESION FAILURE

Flaking / Delamination

Description:

Flaking, generally accepted as detachment of the paint system from the substrate, anddelamination, used to describe intercoat detachment in a multicoat system, are both forms of

adhesion failure where a coating has failed to adhere to the substrate or underlying paint coating.

Flaking / Delamination

Probable Causes:

Adhesion failures can be either as a result of internal stress alone or internal stress plus theexacerbating effect of one or more other factors.

The contributory factors could be related to formulation, inadequate surface profile, surfacecontamination, exceeding overcoating times, application to a glossy paint surface, amine bloom, and



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incorrect surface preparation.

ALLIGATORING

(also known as Crocodiling)

Description:

Very large (macro) checking or cracking which resembles the skin of an alligator or crocodile.Cracks may penetrate through to the undercoat but generally not down to the substrate.

(also known as Crocodiling)

Probable Causes:

Internal stresses in the coating where the surface shrinks faster than the body of the paint film. Can

be caused by excessive film thickness and limited paint flexibility and on paint systems having aflexible undercoat with a hard topcoat.



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BLISTERING

Description:

Dome shaped projections or blisters in the dry paint film through local loss of adhesion from theunderlying surface. The blisters may contain liquid, gas or be dry.

Probable Causes:

Osmoticblistering is commonly associated with the presence of soluble salts,soluble pigments,retained solvents or the absorption and retention oflow molecular weight water miscible solvents.

Non-osmoticblistering, commonly described as cathodic blistering, is often seen asa circular patternof blisters around a coating defect where thesubstrate is exposed. Non-osmotic blistering can alsooccur due to acold wall effect producing condensation.



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CRACKING

Description:

Paint coatings with visible cracks that generally penetrate to the substrate. Cracking comes inseveral forms and ranges from minor cracking to severe cracking.

Probable Causes:

Cracking is generally related to internal stress within the body of the coating. Factors involved incracking are; formulation, high film thickness, thermal cycling, substrate geometry, substratemovement, ageing, absorption / desorption of water or chemicals and inadequate surface profile.One or a number of the contributory factors exacerbates the internal stress in the coating.



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DELAMINATION

Description:

Loss of adhesion between coats of paint.

Probable Causes:

Provided compatible paint materials have been used, delamination defects are generally related tocontamination between coats including amine bloom, exceeding overcoat times, or application to aglossy surface.

FLAKING

Description:

A form of adhesion failure where the paint flakes from the substrate which is a familiar sight whenhighly polished substrates have been coated.



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Probable Causes:

Incorrect paint system used. Either none or incorrect pretreatment used for certain substrates suchas non-ferrous or galvanized. Coatings applied to a highly polished surface often produced by theuse of rotary wire brushes.

GRIT INCLUSIONS

Description:

Particles of grit and dust embedded within and/or on the surface of the coating system.

Probable Causes:

Failure to remove used blast cleaning abrasive from the surface prior to application of the paint.Contamination of the wet primer or undercoat with abrasive from other blast cleaning operations,wind blown grit particles adhering to the topcoat, or poor housekeeping.

INCORRECT COATING SYSTEM



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Description:

An unsuitable or incompatible coating system has been used which results in rapid failure.

Probable Causes:

Use of incompatible or unsuitable coating materials.

IMPACT DAMAGE

(Star Cracking)

Description:

Cracks, which radiate from a point of impact.

Probable Causes:

Impact damage to a relatively brittle coating, and/or a highly stressed coating.



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MISSES/SKIPS/HOLIDAYS

Description:

Exposed areas of substrate or previous coat when the intention was to coat the entire area. Couldbe confined to a single coat in a multi-coat paint system.

Probable Causes:

Poor application techniques or lack of quality control, or both.

MUD CRACKING

Description:

The dried paint film has the appearance of a dried-out mud bath where the cracks can vary in size

and amount but form a network of cracks.

Probable causes:



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Generally over application of heavily pigmented primers such as inorganic zinc silicates or waterbased coatings although it can occur with other systems that are thickly applied.

PEELING

Description:

Similar to flaking, although peeling tends to involve soft and pliable coatings which can be pulled

away from, or spontaneously flake away from the substrate or from between coats due to loss ofadhesion.

Probable Causes:

Peeling is due to a reduction in bond strength of the paint film because of inadequate surfacepreparation, contamination or incompatibility of coats.

PINHOLES

Description:



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The formation of minute holes in the wet paint film during application and drying due to air or gasbubbles which burst giving rise to small craters or holes which fail to coalesce before the film dries.

Probable Causes:

Solvent or air entrapment within a paint film that is a common problem when coating poroussubstrates such as zinc filled primers, zinc silicates and metal-sprayed coatings. Pinholes can alsobe caused by incorrect spray application or incorrect solvent blend.

RUST SPOTTING

Description:

Fine spots of rust, which appear on a paint film, usually a thin primer coat which frequently starts aslocalized spotting but can rapidly spread over the surface.

Probable Causes:

Where the rust spotting emanates from the substrate, low film thickness, pinholes and defects in the



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steel such as untreated laminations, or too high a surface profile causing penetration of peaksthrough a paint film are probable causes.

The photograph shows a rogue peak that could be the cause of rust spotting.

Probable Causes:

Where the rust spotting does not penetrate to the substrate it is probably from metallic

contamination of a coated surface by ferrous grinding debris.

RUST STAINING

Description:

A red/brown staining on the surface of the paint.

Probable Causes:

Water run-off from a rusty surface adjacent to a soundly coated surface. Rust staining occurs when



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the rust is wetted-out and contaminated water runs over and discolors other items or locations. It isusually more of an eyesore than a defect where the coating itself may not be defective, only stained.


inspection and engineering unit 5.1

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