The potential life of a coating system can be realized if it is correctly applied to a suitably prepared surface, under the correct environmental condition. Preparation and the subsequent coating application are labor intensive and therefore subject to operator abuse. The process itself is susceptible to adverse environmental influences throughout all stages of the work. Inspection is an important requirement to ensure the success of the coating operation. This article is intended as a guide to operations that can be carried out in paint inspection, and is not a comprehensive guide to inspection procedures. The required data sheet for in-process inspection is provided in this Article (Attachment D).
INSPECTION OF PAINT WORK IN PLANTS AND INDUSTRIES
Surface Preparation
Before any preparation operations commence, the surface must be free of oil and grease, substrate defects and where possible, sharp edges removed. After the cleaning operation the procedures for inspection are mainly visual, with the degree of cleanliness and surface character evaluated with standards or comparators.
Surface Contamination
Visual testing is to ensure that dust or dirt has been removed from the surface. Use can also be made of adhesive tapes, especially on blast cleaned surfaces, which will pick up any contamination from the profile.
Residual Mill Scale
Visual examination will normally indicate the presence of residual mill scale. If necessary, however, a copper sulfate test can be carried out on new steel that has been blast cleaned. The copper sulfate will plate out with a bronze deposit on steel, but not on mill scale. Good test on new steel, but not effective on old, well rusted steel.
Soluble Iron Salts
A qualitative test can be carried out in accordance with BS 5493, Appendix G. The test consists of potassium ferricyanide papers, which are yellow in color, being pressed onto a dampened surface. If blue spots occur then the soluble iron salts are present. The test is simply a go/no go procedure, and no concentration is given as to the level of iron salts.
Standards of Cleanliness
Most common method is visual comparison of the steel surface with the photographic standards included in Swedish Standard SIS 05 5900 (1967), and the Steel Structures Painting Council Guide to Visual Standard No. 1. To carry out the comparison, the reference disc should be placed on the surface and examined with an illuminated magnifier. The reference section most closely matching the profile of the surface being tested is selected.
Wet Film Thickness
Estimates of the ultimate dry film thickness can be obtained by measuring the wet film thickness applied, and is therefore a good method of controlling the application. Measurement is most commonly carried out using a comb gauge.
The gauge consists of a number of calibrated teeth which are pressed firmly into the wet coating material at 90° to the surface. The wet film thickness is the last tooth to be wetted by the coating material. Gauges are available with different combinations of calibration, and should be selected in the range of the correct wet film thickness. It is important that the calibrated teeth are kept clean and are free from dry paint.
The most satisfactory approach is to establish the application technique required to achieve the specified wet film thickness at the start of the operation. This is commonly done to establish how many passes of the spray gun are necessary. Spot checks can then be carried out during the course of the operation to ensure the thickness is being maintained.
Wet film quoted on data sheets will only provide the correct dry film thickness on a smooth surface. The first coat on blasted steel should have additional wet paint applied depending upon the blast profile. As a rule of thumb, multiply half the average profile height by 100 and divide by the volume solids. A 3 mil profile and a 50% volume solids paint would require an additional 3 mils wet (3/2 x 100/50) to compensate for paint “losses” into the profile.
Dry Film Thickness
After the film is sufficiently dry or cured for inspection the dry film should be examined. The initial inspection is visual, and an accurate appraisal of the quality of work can be quickly established. Visual inspection will also reveal application defects such as over spray, misses, dirt inclusion,
blisters, sags, runs or other defects.
The dry film thickness can be obtained by either destructive or non-destructive methods. The most popular instruments for non-destructive testing employ the magnetic principle. A permanent magnet is mounted at the end of a balanced, pivoted arm assembly with a coil spring attached to the pivot and to a calibrated rotatable dial. When used the instrument must be firmly placed on the substrate and the rotatable dial moved forward until the probe sticks to the surface. Variations in film thickness above the steel substrate will alter the attractive force of the magnet. When the dial is rotated slowly backward a tension will be applied to the spring. When the spring tension exceeds the magnetic force, the magnet breaks contact with the coated surface, and the film thickness is shown on the calibrated dial.
These types of instruments are known as “Banana” gauges due to their shape, with the most common instrument, the Inspector Gauge manufactured by Elcometer, and the Mikrotest manufactured by ElectroPhysik. These types of gauges have an accuracy of ± 10%. Care must be taken to inspect the hemispherical tipped magnet for dirt, paint or wear before use. With older instruments, the film thickness must be recorded as the magnet breaks contact.
Another version of the magnetic principle involves instruments which utilize a magnetic reluctance technique. The probe contains a permanent magnet as a flux source, which when placed on the substrate forms a magnetic current the size of which is relative to the thickness of the coating. This is displayed on the meter scale. This type of gauge is portable and battery operated, the most common of which is the Minitector manufactured by Elcometer, and the Mikrotest manufactured by Elektro Physik.
Holiday Detection
When protective coatings are applied there is a possibility that flaws have occurred due to the presence of trapped air, or voids and pinholes. Of these defects only the largest can be detected visually. A series of battery operated field instruments are available.
1. Low Voltage Detectors
The wet sponge method is most commonly used. This is suitable for coatings up to approximately 375µ (15 mils). The method of operation is to draw the moistened sponge over the surface. Where a defect has occurred a small current will flow and activate an audible alarm. Excessive wetting of the sponge should be avoided.
2. High Voltage (Spark) Detectors
For thicker film a higher voltage is required for flaw detection. Two types are used, either with AC or DC voltages. The AC type is not totally satisfactory, as the coating is subject to cyclic stresses which can cause fatigue and subsequent breakdown of some coatings. The DC types produce a continuous current. Care must be taken with high voltage (spark) detectors to ensure that the test voltage selected is not excessive for the coating thickness. It
is possible to destroy the coating and is some cases the retained solvent is conductive enough to provide a current flow through the paint film.
Degree of Cure
An indication regarding the cure of two component materials, such as zinc silicates or epoxies, can be obtained by solvent swabbing. If no coating is removed after scrubbing the surface with relatively strong solvents, the cure will be well advanced. Your paint supplier should be consulted regarding the solvent composition and test method.
Environmental Conditions
The processes of surface preparation and paint application are all influenced by environmental conditions. In order to ensure the success of the operation the following should be monitored.
Steel Temperature
Various types of coating have differing minimum curing/drying or film forming temperatures. In each case, the surface temperature should be above the minimum at the time of application. If the steel temperature is below the specified minimum problems can occur with solvent entrapment, retardation of the cure with two component products, etc.
Also, with low temperature the minimum and maximum re-coat intervals will be extended. High temperatures present different problems such as dry spray, solvent boiling, etc., however, these can often be solved at site. It must be noted that minimum and maximum recoat intervals
will be reduced when steel temperature is high.
Two methods are commonly used to measure steel temperature, namely a battery operated thermocouple, or a contact thermometer that is simply clamped to the surface with a magnet. With both instruments it is important that time must be allowed for the device to reach a stable temperature. This could be up to 15 minutes for the contact type. Also a representative area should be selected.
Relative Humidity and Dew Point
There is a variable amount of water vapor in the atmosphere. The maximum amount air can hold before precipitation occurs is dependent on temperature and atmospheric pressure. The warmer the air, the more water it can hold. Relative humidity takes into account these factors, although from a practical viewpoint atmospheric pressure is taken as a constant. If the substrate is of a lower temperature than the air, the air in contact with the substrate will be cooled. A point can be reached where the air in contact with the substrate is at such a low temperature that it cannot hold its water vapor. This temperature is known as the dew point.
As the dew point is a temperature where water is spontaneously deposited, the substrate temperature must always be a minimum of 3°C (5°F) above the dew point if paint is to be applied to a dry surface. This even refers to coating with moisture tolerant properties, because these materials displace moisture at the time of application, and then lose it to the atmosphere.
Many types of instruments exist which will give readings of relative humidity, the most practical of which is the Whirling Hygrometer. This instrument consists of two thermometers mounted in a frame, with one thermometer attached by a wick to a water container. Prior to use, the water container should be filled with distilled water, ensuring the wet bulb is properly wetted.
The instrument should then be whirled again until succeassive readings are equal. The wet/dry bulb temperatures can then be used to calculate relative humidity and the dew point from either psychometric charts or a dew point calculator. As stated previously, the dew point is the temperature at which spontaneous condensation occurs, often referred to as “sweating”. Inaccuracies due to hygroscopic surfaces require a safety margin of dew point plus 3°C (5°F).