The following, details basic causes of coating failures and Inspection Hold Points to help prevent failures. For more information on basic failure analysis techniques; See Failure Analysis. While these guidelines are presented for reference, they should be performed by a trained inspector or consultant. If you do not have one available, you may contact CCI Inspection Services at (281) 367-6740. Three Major Causes of Premature Coating Failure Improper surface preparation/coating application Mis-selection of the coating system Mis-formulation of the coating POTENTIAL CAUSES OF COATINGS FAILURES Poor applicator training Poor quality control Lack of qualifications by sub-contractor Lack of facilities to adjust environment Unsafe conditions Non-user friendly coatings system Prime in shop - Finish in field Specification non-compliance Improper surface preparation Recoat too quickly Too long between coats Lack of catalyst Wrong catalyst Improper mixing Lack of induction Improper storage Out of potlife Improper design Not enough millage Excessive profile Improper media Surface contamination Last in cycle Poor maintenance on equipment Wrong thinners Improper additives Water in lines Oil in lines Out of shelf life Excessive millage Personnel turnover Concealed areas Lack of time Poor paint quality Limited people Low bid syndrome Lack or system familiarity Hoses too long Low air pressure Keep on trucking Language barrier Wrong product for service Lack of disclosure Resistance to change Too long between surface preparation and prime Quality Assurance Inspection Check Points Substrate inspection Protective covering Ambient conditions Compressed air cleanliness Surface profile Surface cleanliness Mixing and thinning procedures Application procedures Coating thickness Intercoat cleanliness Recoat times Holiday testing Adhesion Gloss Quality Assurance Inspections Substrate inspection – Weld spatter removal – Edge preparation – Removal of grease/oil – Conducted prior to surface preparation Ambient conditions – Critical QA function Relative Humidity Amount of moisture in the air, expressed as a percentage of the amount of potential moisture Dew Point The temperature at which moisture in the air will condense on a surface Dew Point/Surface Temperature Relationship Surface temperature must be warmer than dew point temperature Industry guideline: Surface temperature at least 5°F (3°C) above dew point temperature Rules-of-Thumb - Ambient Conditions Measurements every four hours (more often if necessary) Measure conditions where work is being performed Document measurements including date and time of day Compressed air cleanliness – Blast cleaning air – Blow down air – Conventional spray atomization air ASTM D4285, “Evaluating the Cleanliness of Compressed Air Using a Blotter Test” SURFACE PREPARATION - The Key to a Successful Coatings Application Surface Preparation Surface profile (roughness) Surface cleanliness – Most difficult inspection – Visible and invisible contamination PURPOSE OF SURFACE PREPARATION The purpose of surface preparation is to remove mill scale on new steel, or / and remove existing coatings to provide a clean, uniform substrate, with a profile that will maximize the adhesion and service life of the installed system. SURFACE PREPARATION FACT: The service life of a coatings system is directly proportional to the degree of surface preparation employed. MYTH: Abrasive blasting eliminates the need to remove grease, oil from steel. MYTH: It is not required to remove mill scale before coating a steel structure Visible Surface Cleanliness SSPC-SP1 - Solvent Cleaning SSPC-SP2 - Hand Tool Cleaning SSPC-SP3 - Power Tool Cleaning SSPC-SP11 - Power Tool Cleaning to Bare Metal SSPC-SP 15, Commercial Grade Power Tool Cleaning SSPC-SP7/NACE No. 4 - Brush-Off Blast Cleaning SSPC-SP6/NACE No. 3 - Commercial Blast Cleaning SSPC-SP10/NACE No. 2 - Near-White Metal Blast Cleaning SSPC-SP5/NACE No. 1 - White Metal Blast Cleaning SSPC-SP 14/NACE No. 8, Industrial Blast Cleaning SSPC-SP12/NACE No. 5 - High/Ultra Pressure Water Jetting SSPC-SP13/NACE No. 6 - Preparation of Concrete SSPC-SP 14/NACE No. 8, Industrial Blast Cleaning SSPC-SP 15, Commercial Grade Power Tool Cleaning SP-1 Solvent Cleaning is a pre-treatment procedure that removes oil and grease by the use of solvents, emulsions, steam or similar methods. It does not remove rust, rust and mill scale, or old coatings. Principal problems occur in that solvent cleaning is an expensive process if properly performed, solvents become contaminated and redistribute it, solvents present fire, health and environmental hazards, and can leave detrimental residues behind. Solvent cleaning is the first step in every surface preparation by definition SP-1 Solvent Emulsion Cleaning utilizes a solution containing strong solvents and emulsifying agents. Upon application the solvents pick up the contaminants and then can be rinsed away, along with the contaminants. It is far superior to solvent cleaning, but can only be used where a water wash is possible. SP-2 Hand Tool Cleaning Is the oldest process used in preparing surfaces for painting, using wire brushes, sandpaper, scrapers, and chipping hammers to remove loose rust, scale, and paint. It is not intended to completely remove them. It is expensive due to the low square foot per hour rate, and ineffective in preventing corrosion due to contaminants. Hand tool cleaning can damage surfaces, and recommended only in instances where more effective methods cannot be used. SP-3 Power Tool Cleaning utilizes electric, or air driven wire brushes, impact tools, grinders, sanders, and other equipment incorporating a combination of methods. It is not intended to remove all, but only loose rust, mill scale, and paint. Efficiency is improved, when compared to SP-2 methods, however residual contamination and possible damage to the surface are still issues for concern. SP-5/NACE 1 White Metal Abrasive Blast provides a surface with a “gray-white (uniform metallic) color, slightly roughened to form a anchor pattern for coatings. This surface is free of all oil, grease, dirt, mill scale, rust, corrosion products, oxides, paint, and other foreign matter.” Note: It may be impossible to completely eliminate all traces of existing chloride contamination and that re-contamination of white metal surfaces can occur in a matter of minutes. SP-10/NACE 2 Near-White Metal Abrasive Blast provides a surface of which 95% has the appearance of a white metal blast. The remaining 5% can only exhibit light streaks, shadows, slight discoloration, or oxide bonded with metal. Note: While not perfect a near-white blast is substantially close. In many instances it may be impractical, or even impossible, to remove traces of oxide binder (from under mill scale) or retained oxide from previously rusted areas. SP-6/NACE 3 Commercial Abrasive Blast provides a surface of which 66.6% has the appearance of a white metal blast. The remaining 33.3% can exhibit slight streaks, shadows, or discoloration caused by mill scale oxide binder or rust stain. If pitted, slight residues of rust or paint are allowed in their bottoms.. Note: A commercial blast is the most utilized blast procedure in that it is relatively inexpensive, and suitable for most areas other then those of severe corrosion and immersion service. SP-7/NACE 4 Brush Abrasive Blast Is a surface from which oil, grease, dirt, and loose rust, mill scale and coatings are removed, but tightly adhering rust, mill scale and coatings are permitted to remain if they have been exposed to the abrasive blast so that flecks of the underlying metal are uniformly visible. Brush blasting is a good, low cost, field method of cleaning. It quickly removes loose, unwanted items, provides anchor profile, and the operator can pick up areas that are badly corroded. SP-11 Power Tool to Bare Metal Provides a surface, when viewed without magnification, that is free of all visible oil, grease, dirt, dust, mill scale, rust, paint, oxide, corrosion products, and other foreign matter. Pits, if present, may contain slight residues of rust and paint in their lower areas. Although producing surfaces that appear as SP-6, or SP-10, they are not necessarily equivalent to those produced by abrasive blast cleaning. SP-11 provides a means to improve the level of surface preparation in areas where blasting in not permitted or feasible. SP-12 High Pressure Water Cleaning LOW (1-3000 psi): also termed “power-washing” and is used to remove grease, oil, chemicals, and loose debris. It will not remove tightly adherent coatings. JETTING ( 3-5000 psi): provides improved efficiency but still will not remove tightly adherent coatings. HIGH (10-15000 psi): is most commonly used method and will remove tightly adherent coatings. ULTRA-HIGH (20-35000 psi): is most effective method of removing coatings. It will not remove tight rust, mill scale, or produce an anchor profile. TYPES OF BLASTING MEDIA Sand
- cheap, fast
- silicosis
- dust Steel Shot/Grit
- ideal for shops, recyclable
- eats equipment
- suited for fab shops, high
performance flooring Starblast - very fast rates - expensive · Baking Soda etc. - no danger of fouling compressors, gauges etc.. - no profile, lots of water Corn Cobs ,Walnut Shells, Sponges
- No static electricity
- No profile Aluminum Oxide
- good rates
- impingement Green Lighting - Naturally occurring
- slow rates Copper Slag
- low dust
- impingement on steel Gas, Frozen, CO2, Nitrogen Blasti Coal Slag - low dust
- soft
- impingement on steel Anchor Pattern or Profile SHOT: compacted and peened surface. GRIT: sharp, angular cut surface. SAND: finely cut, scoured surface. Rules of Thumb · · Profile height increases as the abrasive size increases. · · Profile height increases as the level of cleaning increases (SP-6 up to SP-5). · · Profile height increases as the angle of impingement increases (180 to 90 deg.). - · Profiles obtained from metallic abrasives tends to be higher and less “disturbed”.
TYPES OF ABRASIVE BLASTING · Media propelled by air · Media propelled by mech. force · Media propelled by water · Dry media wetted - removed · · Media propelled by air/ water vacuumed OTHER TYPES OF SURFACE PREPARATION · Chipping Hammers · Scrapers · Power Grinders, Disks · Needle Guns · 3M Roto-tools with Vacuum Attachments Surface cleanliness – Most difficult inspection – Visible and invisible contamination Material Receipt Inspection · Container conditions · Correct number of component · Correct manufacturer, product, color · Record of batch or lot number · Current shelf life Inspection of Mixing Procedures · Agitation of individual components · Mixing the components together · Straining · Induction time · Pot life Inspection of Thinning Procedures · Thinner type · Thinner amount · Viscosity measurements Inspection of Coating Application Procedures Application technique Wet film thickness measurements Mixing and thinning procedures – Most important – Commonly overlooked Limitations of Wet Film Thickness Measurements · Immediate measurements important · Unreliable measurements on fast dry or highly pigmented coatings – Zinc rich – Vinyl · Intercoat cleanliness · Recoat times · Hydraulic Results reported in psi |
