Before initiating surface preparation or coating operations, the temperatures (air and surface), dew point, relative humidity, and wind velocity must be checked to ensure that they conform to specification requirements. SSPC-PA 1 provides information on proper conditions for shop and field painting. Since ambient and steel temperatures may change quickly, they should be measured periodically throughout the day. ASTM E 337 dictates that the ambient condition test or environmental test should be done: “before, during, and after” the application and they must be monitored at least every four hour interval, even more when the condition are unstable.
The application of a coating system shall occur only when the air & substrate temperature is within the range indicated by the manufacturer’s written instructions for both application and curing. A rule of thumb, no work shall be done when air temperature below 50C and surface temperature less than 30C above dewpoint temperature.
Dewpoint is defined as the temperature at which moisture will condense. Dew point is important in coating work because moisture condensation on the steel surface will cause freshly blast cleaned steel to rust, or a thin, often invisible film of moisture trapped between coats may cause premature coating failure.
Due to curing of coatings may be adversely affected by humidity that are too low or too high, no coating shall be applied unless the supplier or manufacturer’s written technical requirements for humidity are met. High humidity may cause moisture to condense on or react with uncured coating films to cause blushing or other adverse effects. However, for certain inorganic zinc and one-package, moisture-curing polyurethane coating, require a minimum humidity for curing, but for most organic coatings, the rule of thumb, no work shall be carried when relative humidity above 85%.
For field or open air application, wind velocity may blow airborne contaminants to work surfaces and coating materials. It also contributes to dry spray, dusty-spotted effects to the coated surface and accelerates solvent evaporation time which may cause immature drying. No work shall be done in the open air field when the wind velocity above 24 km/hour.
AMBIENT TEST INSTRUMENTS
Surface Magnetic Thermometer is used to measure steel substrate temperature. Must be allowed to stabilize on surface to be measured for at least 5 minutes. Must be used at actual location, avoid direct sunlight, and must be calibrated often.
Sling Psychrometer is used to measure wet and dry temperatures. These information are then used to calculate dewpoint and relative humidity (some latest instrument has dewpoint and relative humidity scales).
Dewpoint calculator is used to calculate dewpoint temperature and relative humidity. Prior to use this instrument, data must be first obtained from the Sling Psychrometer.
Anemometer is used to measure wind velocity.
PRE-SURFACE PREP. INSPECTION
Before the start of surface preparation for coating, all necessary construction or modification of items requiring coating should have been completed. This includes grinding of welds and sharp edges and filling of pits. Likewise, the surface must be free from all contaminants. Also, the job site must then be inspected for complete readiness (i.e., all required operational and support equipment is present, and access for inspection of work is available). This includes safety aspects such as ladders and scaffolding, power, and traffic control, so that the inspector can safely perform his duties.
All new mineral and slag abrasives must be inspected for physical and chemical properties as described in SSPC – AB 1. Recycled ferrous metal abrasives must be checked for cleanliness and fines as described in SSPC – AB 2. The abrasives should be properly labeled for identification. Even if a sieve analysis (ASTM C 136) is provided by the supplier, it is prudent to run a check at the job site or retain a sample for later analysis should cleaning rates be lower or profile heights other than anticipated.
A simple test can be conducted for contaminants or fines in the abrasive. A spoonful of abrasive is placed in a vial of distilled water and shaken vigorously. It is then checked for:
• Oil or grease that forms a surface sheen
• Fines suspended in or at the surface of the water
• Color or turbidity from dirt
• Soluble salts by conductivity or deposition upon evaporation
• Acidity or alkalinity with pH paper
BLASTING EQUIPMENT CHECK
All air compressors and blasting equipment should be checked for proper size, cleanliness, operation, and safety. Hand or power tools should also be checked for operation and safety, and should be used only as specified in their standard operating procedures. These checks should be made before the start of abrasive blasting and periodically thereafter, especially after a change of abrasive. Air and blast hoses should be checked for damage and constrictions and should be as short and of as large a diameter as practical to reduce frictional losses of air pressure. The blast hose should have a static grounding system. Couplings should be of the external fit type, secured well, and safety-wired.
Blast nozzles should be of the venturi type, with a flared exit to allow more rapid and uniform cleaning. An orifice gauge should be used to check the nozzle size (inches) and air flow (cfm at 100 psi). This wedge-shaped instrument or bore-nozzle inserted into the rear of the nozzle has a measuring range of 1/4 to 5/8 inch and an air flow range of 81 to 548 cfm. Nozzles should be discarded after an increase of one size (e.g., 1/16 inch is the difference between a #6 and a #7 nozzle). All nozzles must have a deadman control that will automatically shut off the flow of air and abrasive when released.
The compressed air used in abrasive blasting must be checked to determine whether oil and water traps have completely removed contaminants. This is done by the blotter test described in ASTM D 4285. A clean, dry, white blotter or cloth is held about 18 inches (450 mm) in front of the blast nozzle with the air flowing for one to two minutes. Oil and water contaminants are detected visually on the blotter or cloth surface.
Abrasive blasting is usually done at pressures between 90 and 100 psi for efficient blasting. Higher blasting pressures may produce even higher blasting rates. A pocket-sized air pressure gauge with a hypodermic needle can be used for determining cleaning pressure at the nozzle. The gauge is inserted in the blasting hose just before the nozzle in the direction of the flow. Instant readings can be made up to 160 psi.
POST-SURFACE PREP. INSPECTION
Steel surface cleanliness requirements for abrasive blast cleaned steel (i.e., SSPC levels of surface preparation) can readily be determined using SSPC-VIS 1 photographic standards. SSPC surface preparation standards define cleanliness in terms of visible contaminants such as rust, mill scale, paint, and staining.
Two commonly used methods for determining the profile (average peak-to-valley depth) of blasted steel surfaces are described in ASTM D 4417. The Testex Press-O-Film Replica Tape method is preferred, because it is easy to conduct, accurate, and produces a permanent record. The tape consists of a layer of deformable plastic foam bonded to a Mylar backing. The tape is rubbed onto the blast-cleaned surface with a plastic swizzle stick to produce a reverse replicate of the profile. The tape profile is then measured with a spring micrometer. The micrometer can be set to automatically subtract the two-mil (50 µm) thickness of the non-deformable Mylar backing.
An alternate procedure, in which a surface profile comparator is used, is available for determining surface profile. Comparators include ISO, Clemtex, and Keane-Tator instruments. Basically, they use a five-power illuminated magnifier to permit visual comparison of the blast-cleaned surface to standard profile depths. Standards are available for sand, grit, and shot-blast cleaned steel.
Another concern are the non-visible contaminants such as soluble salts, (e.g., chlorides and sulfates). These salts are deposited from the environment, e.g., marine air, and industrial pollutants. They can cause problems such as flash rusting of steel or blistering of applied paint films. These contaminants are not removed by abrasive blast cleaning (or other mechanical methods). A good indication of salt contamination on blast-cleaned steel is the rapid rerusting of the steel in the absence of condensing moisture.
ASTM D 4940 provides a water extraction test procedure for determining salt concentration. Extraction methods include swabbing, rigid limpet cell, and Bresle cell procedures. After extraction, the water is tested for conductivity and/or specific salt ions. Test kits for analysis of chloride, sulfate, and ferrous ions, as well as pH, are commercially available from suppliers of coating instruments. They contain strips, swabs, papers, and operating instructions for simple chemical testing.
Abrasive blast cleaned steel surfaces should be checked to determine if all the residual abrasive has been removed by vacuuming, brushing, or blowing. Detection of residual abrasive can be done by pressing a piece of transparent cellophane (Scotch) tape onto the cleaned steel and then pulling it off. If any abrasive is visually detected on the piece of tape, further removal of abrasive is required.
All blasted steel surfaces should be primed as soon as possible after cleaning, and always on the same day except in dehumidified spaces. If not primed soon enough, particularly on humid days, flash rusting of the steel may occur. If any flash rusting is observed, the steel must be reblasted.
• Coating storage conditions
• Mixing procedures
• Thinning materials and amounts
• Tinting, or color verification
• Straining of coatings to remove large particles
• Spray equipment check
INSPECTION OF COATING APPLICATION
Inspection during and after coating application consists chiefly of checking for:
• Induction time and pot life
• Wet and dry film thicknesses
• Cosmetic and film defects
INDUCTION TIME AND POT LIFE
For coatings that cure by chemical reaction (thermosetting), the inspector should check to see that the manufacturer’s induction time and pot life requirements are met.
WET FILM THICKNESS
Wet film thickness (WFT) measurements should be made immediately after paint application to determine if the coating is sufficiently thick to obtain the desired dry film thickness (DFT). Measurement is less accurate on highly pigmented (e.g., zinc-rich) and quick-dry coatings. Since measurement of WFT destroys the film integrity, the coating must be repaired after the measurements have been completed. The most widely used type of WFT gauge, described in ASTM D 4414, consists of a thin rigid metal notched gauge, usually with four working faces. Each of the notches in each face is cut progressively deeper in graduated steps. The face with the scale that encompasses the specified thickness is selected for use.
To conduct the measurement, the face is pressed firmly and squarely into the wet paint immediately after its application. The face is then carefully removed and examined visually. The WFT is the highest scale reading of the notches with paint adhering to it. Measurements should be made in triplicate. Faces of gauges should be kept clean by removing the wet paint immediately after each measurement.
DRY FILM THICKNESS
DFT measurements are made after complete curing of coatings to determine if specified thicknesses have been met. Calibration of gauges and measurement of DFT by magnetic gauge are described in detail in SSPC-PA2. Magnetic gauges are normally used for determining coating DFT on steel surfaces. They rely on the fact that the thicker the coating, the smaller the magnetic field above the coating. Typical measurement error may be 3–10 percent.
There are several factors that adversely affect DFT measurements with magnetic gauges. These include:
• Roughness of steel surface (deeper blasted surfaces result in higher measurements)
• Steel composition (high alloy steels may have erroneous measurements)
• Thickness of steel (there is a minimum thickness for gauge accuracy)
• Curvature of steel surface (measurements may be erroneous)
• Surface condition (contaminated coating surfaces may cause high readings; “pull-off” magnets may adhere to tacky surfaces; probes may indent soft paints)
• Orientation of gauge (must be held perpendicular to surface)
• Other magnetic fields (strong magnetic fields from direct current welding or railway systems may interfere)
All magnetic thickness gauges should be calibrated before use. It is also good practice to check the calibration during and after use. Gauge suppliers provide a set of standard-thickness, nonmagnetic (plastic or nonferrous metal) shims to cover their working ranges. The shim for instrument calibration should be selected to match the desired coating thickness. It is placed on a bare steel surface with the same profile that will be used for the coating application, and the gauge probe is placed on it for calibration. If the instrument does not agree with the shim measure, it should be properly adjusted. If adjustment is difficult, the reading for bare steel can be added or subtracted from field readings to determine actual thicknesses. The steel surface used for calibration should be a masked-off area of the steel being painted or an unpainted reference panel of similar steel, if possible.
Another calibration system utilizes a set of small, chrome-plated steel panels of precise thickness, available from the National Institute of Standards and Technology (formerly the National Bureau of Standards). These standards are expensive but very accurate. SSPC-PA 2 presents detailed information on the calibration and use of both pull-off and fixed probe gauges.
Newly coated structures on which the coating integrity is important (particularly linings or coatings in immersion conditions) should be tested with a holiday detector to ensure coating film continuity. A holiday (sometimes called discontinuity) is a pinhole or other break in the film that permits the passage of moisture to the substrate. This allows substrate deterioration to begin. Holidays are not easily detected visually, and must be located with electrical instruments called holiday detectors. Holiday detectors are available in two types, low and high voltage, as described in ASTM D 5162.
Low-voltage (30 to 70 volts) holiday detectors are used on coatings up to 20 mils (500 µm) in thickness. These portable devices have a power source (a battery), an exploring electrode (a dampened cellulose sponge), an alarm, and a lead wire with connections to join the instrument to bare metal on the coated structure. A wetting agent that evaporates on drying should be used to wet the sponge for coatings greater than 10 mils (250 µm) in thickness. The wetted sponge is slowly moved across the coated surface so that the response time is not exceeded. When a holiday is touched, an electric circuit is completed through the coated metal and connected wire back to the instrument to sound the alarm. Holidays should be marked after detection for repair and subsequent retesting.
High-voltage (above 800 volts) holiday detectors are used on coatings greater than 20 mils (500 µm) in thickness. The exploring electrode may consist of a conductive brush or coil spring. The detector may be a pulse or direct current type. It should be moved at a rate not to exceed the pulse rate. If a holiday or thin spot in the coating is detected, a spark will jump from the electrode through the air space to the metal.