Inspection of Fluorescent Coating Systems

SSPC-TU 11December 1, 2010

1

SSPC: The Society for Protective Coatings

TECHNOLOGY UPDATE NO. 11Inspection of Fluorescent Coating Systems

1. Scope Protective coating systems, when properly selected and

applied, will protect the underlying substrate from corrosion and deterioration. Confirming that the coating has been applied correctly can require an extensive and lengthy inspection process. The use of fluorescent coatings in a coating system permits faster identification of holidays and areas with low film thickness. This technology update discusses the technique and the equipment required to inspect a coating system that incorporates fluorescent properties.

2. Description Fluorescent coatings, visually inspected with ultraviolet

and/or violet light, assist in quickly identifying holidays and areas with low film thickness. They can be used as both primer and finish coats. This technology may also enable the inspector to detect incomplete removal of coatings. General information on ultraviolet and violet lamps and safety precautions for their use is also provided.

3. Background on the Available Technology3.1 For many years, the marine industry has been trying

to extend the life of vessel tanks. The failure of the coating system and subsequent failure of the underlying structure is a leading cause of ships being taken out of service. In the most extreme cases, failure of ballast tanks has caused vessels to break apart in high seas with loss of life. Fluorescent coatings were originally developed for use in ballast tanks in large ocean-going vessels to assist in the inspection of applied coatings for holidays both during and after application. The proper use of this technology gives the ship owner further assurances that the

coating system has been applied correctly and will provide the expected service life. The United States Navy took a leading part in the advancement of this technology when in 2003 it included ultraviolet fluorescence as a requirement for Type VII (solvent free) tank linings meeting MIL-PRF-23236.(1),1

The same technology can be used for many other structures or pieces of equipment that are difficult to inspect. Virtually any type of coating can be made to fluoresce with the addition of fluorescent pigments or optically active additives. These addi-tives are similar to other colorants and optical effects additives in coatings. Depending on the application, the fluorescent effect is available either as a fluorescent effects product line formulated by a coatings manufacturer, or as a concentrated colorant formulated for field addition to a coating. The coat-ings applicator should consult the coatings supplier, coatings specification and asset owner to determine which products are appropriate for a specific project.

3.2 Ultraviolet (UV) radiation is a part of the electromagnetic radiation spectrum just outside the visible range and adjacent to the violet color of the visible spectrum. The UV-A range is between 315 nanometers (nm) and 400 nm. This is the typical UV range for fluorescent coating inspection. The UV-B range is between 280 nm and 315 nm. The UV-C range is between 180 nm and 280 nm. Visible light is generally considered to be between approximately 400 nm nanometers (nm) and 760 nm. In the visible spectrum, violet light is in the range of 400 to 420 nm. This is the typical violet light range for fluorescent coating inspection (see Figure 1).

(1) US Department of Defense Single Stock Point (DODSSP) for military standards may be accessed online at <http://dodssp.daps.dla.mil>.

Figure 1. Wavelength Chart. Wavelength (in nanometers) illustrating UV-A and violet ranges used in fluorescent coating inspection and their relationship to the visible light spectrum.

Visible Light


2

3.3 The intensity and color of fluorescence is related to the type and concentration of the fluorescent and other pigments used in the coating; the power and wavelength of the inspection lamp; and the standoff distance. Under a suitable inspection lamp, pinhole defects can be visible at a distance of about 3 meters (10 ft), depending on ambient lighting conditions. The work area does not have to be blacked out to perform the inspection. The visual effect can be captured clearly on camera.

3.4 The addition of fluorescent pigments to a coating has no effect on the application method typically used for applying that coating. However, the applicator’s progress and application technique can be monitored by using a suitable inspection lamp.

4. Inspection Lamp Requirements 4.1 Light wavelength must be selected to complement the

optically active additive and may need to account for other light absorbing additives as well. The coating manufacturer should be consulted to select the light wavelength that best activates the fluorescent property of their coatings. ASTM(2) E 25012 and ASTM E 22973 are example of industry standards that address specifying inspection lights.2

4.2 The most advantageous light intensity, throw distance and area illuminated depend on the size and type of the struc-ture being inspected.

4.3 Environmental and safety considerations can impose requirements for mercury-free or explosion-proof lighting. Inspection lamps that use bulbs may contain mercury in both the bulb and the lamp ballast.

5. Training Requirements5.1 A coating inspector with previous training and experi-

ence can use this technology with little additional training. The inspector should follow the instructions supplied with the lamp, especially recommended safety practices. Ultraviolet or violet lamps may require the use of appropriate contrast enhance-ment/safety eyewear.

5.2 Before inspecting fluorescing coatings for the first time, the inspector or inspection team and the coating supplier’s representative should examine sample panels under the lamp

(2) ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959. Standards are available online at <http://www.astm.org>

that will be used to perform the inspection to learn to recognize film defects. The coating supplier should have several panels prepared showing the following:

Fluorescent coat on a flat panel with pinholes• Fluorescent prime coat and nonfluorescent finish coat • on a flat panel with pinholes in the finish coatFluorescent coat on a weld seam, with cracks in the • coatingFluorescent prime coat and nonfluorescent finish coat • on a weld seam, with cracks in the finish coatFluorescent prime coat and nonfluorescent stripe • coat on an angleFluorescent coat applied below recommended dry • film thickness (DFT)Fluorescent prime coat applied correctly with next • coating applied below recommended DFT

5.3 In rare cases, individuals with certain forms of color blindness cannot see the fluorescence emitted from certain optically active pigments. The contrast enhancement provided by the fluorescence is not seen by these individuals. This possi-bility should be assessed during the training and orientation session for each new coating system encountered.

5.4 The inspection of fluorescent coatings with ultraviolet or violet lamps can speed up or enhance finding and interpreting many types of coating defects. Depending on the nature of the project and the types of defects encountered, it may be appropriate to emphasize to the coatings inspector that this process is not intended to replace other inspection methods like the use of white light or yellow lamps for visual inspec-tion, NACE SP-0188(3).4 for holiday detection, or SSPC-PA 25 for DFT readings.

6. PPE Requirements6.1 Eye Protection: UV radiation is that radiation just

outside the visible range, or under 400 nanometers (nm). There are three ranges of UV (see Table 1).

The UV-A lamps used in fluorescent inspection contain a significant amount of invisible radiant energy that can potentially cause eye and skin damage. UV blocking glasses should be worn when using UV-A lamps. Contrast-enhancement/glare-

(3) NACE International, 1440 South Creek, Houston, TX 77084. Standards are available online at http://www.nace.org.

Region Range in nm Hazard Potential

Damage Mechanism (High Exposure)

UV-A 315-400 Lowest Possible cataracts. Traditional “black lights” emit in the range of 340-390 nm

UV-B 280-315 Mid to High Cataracts, skin or eye burns. Increased risk of some types of skin cancer

UV-C 180-280 Highest Skin or eye burns.

TABLE 1TYPES AND HAZARDS OF UV LIGHT


3

reduction filters are appropriate for violet lamps. The beam from the UV-A or violet lamp should never be directed into the eyes, and needless exposure of the skin should also be avoided. The lamp manufacturer’s written safety instructions should be strictly followed.

The ASTM standard E 2297-044 is a resource for Personal Protective Equipment (PPE) for UV-A lamps. For guidance in specific situations for specific sources, ACGIH(4) publishes Threshold Limit Values (TLVs®)6 for exposure to UV and intense visible light that can be used to evaluate work exposure and PPE.

Some LED light manufacturers include an IEC(4) 624717

classification statement on the product or in the product tech-nical data sheets. This standard classifies the eye safety of a lamp source (including LED) device based on its wavelength and power. The statements provide guidance on what kind of eye protection (if any) is required when using the light. An “Exempt” lamp device is considered “safe under reasonably foreseeable conditions of operation” in this standard. LED light products classified before 2006 may include an IEC 60825-18

classification statement on the product or in the product tech-nical data sheets. In this standard, a Class 1 LED device was considered “safe under reasonably foreseeable conditions of operation.”

At the time of this writing there is no OSHA-mandated UV exposure limit in the workplace. This information may be found at the OSHA web address:

<http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=24755>

6.2 Skin Protection: Limited exposure to UV-A radiation can cause redness (erythema) on normal skin. Such redness is temporary and normally will not produce blistering. Inspectors and workers should be cautioned to be particularly cautious when taking medications that are photosensitizers and extra protection of the skin would be warranted.

However, UV exposure is not immediately felt. The user may not realize a hazard until after the damage is done. It is best to wear gloves and a UV-resistant face shield during long term inspection processes.

7. Post-Application Inspection 7.1 Visual Effect 1 – Fluorescent coating over non-

fluorescent substrate or coating: This scenario includes: fluorescent primer over steel;• fluorescent stripe coat over non-fluorescent primer;• fluorescent single coat systems, including multiple • passes of the same coating.

These are some of the effects an inspector may observe when the coating is illuminated by an ultraviolet or violet lamp:

Black or dark spots seen under the beam of the lamp • indicate holidays or undercoated areas. In some

(4) International Electrotechnical Commission, 3, rue de Varembé, P.O. Box 131, CH -1211 Geneva 20 Switzerland. Standards are available online at <http://webstore.iec.ch>

cases, black dirt or grit has a similar look and could be misinterpreted as a holiday. Areas of brighter than normal luminosity may indicate • higher film thickness. Less bright areas may indicate lower film thickness.Areas where the coating is glowing white may indi-• cate areas of paint detachment from the surface or hot-work damage.Organic dust and grit may show speckled bluish-white • bright spots under the light. Organic dust particles can come from rags, clothing, and protective fabric booties.

In general, if the area under the lamp is not a uniform brightness, closer examination is required to determine the nature and cause of appearance.

7.2 Visual Effect 2 – Non-fluorescent coating over fluorescent coating: This scenario includes:

nonfluorescent topcoat over fluorescent primer;• nonfluorescent stripe coat over fluorescent primer.•

The nonfluorescent coat has no fluorescent pigments and will not respond to the ultraviolet or violet lamp. These are some of the effects an inspector may observe when the coating is illuminated by an ultraviolet or violet lamp:

Prime coat fluorescence is clearly seen through • holidays, pinholes, and cracks in the topcoat (see Appendix A). Prime coating fluorescence may show through areas • of thin film build in the second coat. Organic dust from rags, clothing, and protective fabric • booties may show speckled bluish-white bright spots under ultraviolet or violet light. In some cases, this could be misinterpreted as a holiday.

7.3 Visual Effect 3 – Fluorescent coating of one color over a fluorescent coating with a different fluorescent color: This scenario includes:

a three-coat system with a fluorescent primer and • a fluorescent intermediate coat or fluorescent stripe • coat

This type of coating system is uncommon. It is recom-mended that coating applicators and coating inspectors who encounter this type of system rely on application-specific training with example panels per Section 5.

Given the availability of different combinations of prime coats and topcoats, coating applicators may encounter a 2-coat system with contrasting colors under normal white light that have the same fluorescent emission color under ultraviolet or violet light. These systems behave like a Visual Effect 1 system (see Section 7.1 above), not like the Visual Effect 3 system of this section. In this case, topcoat defects that are visible under white light can disappear under a UV-A or violet inspection light.


4

8. In-Process Application Inspection A fluorescent coating system can be used for in-process

inspection if the applicator is supplied with a suitable lamp (explosion-proof or intrinsically safe). The fluorescent pigment is reactive in the wet film, so the applicator can easily observe and repair holidays while applying the coating. This provides instant visual feedback to the operator and over time can improve worker skill by acting as a training aid.

9. Preconstruction Primer RemovalIf a fluorescent primer has been incorporated into the

preconstruction primer, the extent of removal of the primer can be determined by shining the ultraviolet or violet light on the surface. Any preconstruction primer remaining on the surface will be detected by the ultraviolet or violet lamp. If no fluores-cence is observed, it can be concluded that the preconstruction primer has been completely removed.

10. Benefits of the Use of Fluorescent Primers in Coating Inspection

The use of fluorescent primers provides owners and inspectors with an additional inspection option to ensure high quality coating application to tank interiors. The main benefits of using this technology are listed below.

10.1 Coating ApplicationReduces defects, re-work and job cycle time by • repairing defects during application.Improves the quality of the workmanship by providing • continuous instant feedback to the applicator.

10.2 Newly Applied Coatings Inspection Improves coating defect detection by visual inspec-• tion.For specially formulated coatings, provides capability • to identify thin film build by visual inspection.Improves inspection of inaccessible areas, as coating • condition can be assessed more remotely.Enhances ability to capture images of defects for • future comparison or remote assessment.

10.3 In-Service Coating Inspection Permits early determination of topcoat wear.• Damage to the topcoat clearly shows the fluorescent • primer underneath.Blisters broken to substrate will show halos of • fluorescent primer.

11. Disclaimer 11.1 This technology update is for information purposes

only. While every precaution is taken to ensure that all infor-mation furnished in SSPC technology updates is as accurate, complete, and useful as possible, SSPC cannot assume responsibility nor incur any obligation resulting from the use of any materials, coatings, or methods specified herein, or of the technology update itself.

11.2 This technology update does not attempt to address problems concerning safety associated with its use. The user of this technology update, as well as the user of all products or practices described herein, is responsible for instituting appro-priate health and safety practices and for ensuring compliance with all governmental regulations.

12. References 1. MIL-PRF-23236C (2003) Performance Specification;

Coating Systems for Ship Structures. (Washington, DC: US Department of Defense

2. ASTM E 2501-07. Standard Specification for Light Source Products for Inspection of Fluorescent Coatings (West Conshohocken: ASTM International, 2007)

3. ASTM E 2297-04. Standard Guide for Use of UV-A and Visible Light Sources and Meters used in the Liquid Penetrant and Magnetic Particle Methods. (West Conshohocken: ASTM International, 2004)

4. NACE SP0188, Discontinuity (Holiday) Testing of New Protective Coatings on Conductive Substrates. (Houston: NACE International, 2006)

5. SSPC-PA 2, Measurement of Dry Coating Thickness with Magnetic Gages. (Pittsburgh: SSPC)

6. 2004 TLVs® and BEIs®; Based on the Documentation of the Threshold Limit Values for Chemical Substances and Physical Agents & Biological Exposure Indices, (Cincinnati, OH: American Conference of Governmental and Industrial Hygienists®, 2004).

7. International Standard IEC 62471; Safety of lamps and lamp products -. Edition 1-2006-08. (Geneva: Interna-tional Electrotechnical Commission, 2006)

8. International Standard IEC 60825-1; Safety of laser products - Part 1: Equipment classification, require-ments and user’s guide. Edition 1.2 2001-08. (Geneva: International Electrotechnical Commission, 2001).

BibliographyBuckhurst, Miles and Bowry, Earl - “An Optically Active

Coating System for Coating Ballast Tanks,” Presen-tations: The Paint and Coatings Expo 2005, Paper T-44. (Pittsburgh, PA: SSPC, 2005).

U.S. Department of Labor, OSHA Technical Manual, Section III: chapter 6, “Laser Hazards.” Available online at <http://www.osha.gov/dts/osta/otm/otm_iii/otm_iii_6.html>

U.S. Department of Labor, Standard Interpreta-tions 02/26/2003 - Workplace exposure limits for ultra-violet radiation. Available online at <http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id= 24755>


5

White light inspection of fluorescing first coat. The arrow points to a pinhole that is distorted and obscured by shadows and flashlight glare.

Violet LED light inspection of fluorescing first coat. The holiday is easily distinguished from coated areas using a violet LED flashlight. Because the digital camera sensor is more sensitive to violet light than the human eye, the inspector would not see the flashlight glare.

Figure A1. Weld pinhole under white light and violet LED light.

APPENDIX A. EXAMPLES OF DEFECTS EXPOSED BY INSPECTION OF FLUORESCING PRIMER COATINGS

Note: These examples of optically active coating systems represent a variety of coatings from different manufacturers. Due to the range of avail-able formulations, the colors of the visual effects vary when viewed by the human eye or when photographed.

Images courtesy of National Surface Treatment Center


6

Figure A3. Non-fluorescent second coat over fluorescent first coat, holiday in pit. Appearance under white light shown at left. Blue crescent that appears under violet LED light at right indicates fluorescent first coat showing through second coat.

Figure A2. Contamination on fluorescent first coat inspected by UV light. Black marks are grit from a blasting operation in adjacent tank. Irregularly shaped “halo-like” patches are caused by solvent being deposited onto the film while still wet.

Image courtesy of Luminous Technololgies, Ltd.

Image courtesy of National Surface Treatment Center


7

Figure A5. Microcracking of a non-fluorescent second coat over fluorescent first coat. Horizontal ‘bright’ lines are first coat with fluorescing pigment viewed under UV. Second coat without fluorescing pigment has cracked, exposing first coat. Thicker vertical lines are caused by damage to second coat.

Figure A4. Wrinkling. UV light inspection shows wrinkling of non-fluorescent second coat exposing fluorescent first coat. Wrinkling is due to application of second coat over inadequately cured first coat.

Image courtesy of Luminous Technologies, Ltd.



8

Figure A6. Poor edge retention and damage to non-fluorescent second coat over fluorescent first coat. Bright arc from left to center right shows poor edge reten-tion where the first coat with fluorescent pigment is exposed through non-fluorescent second coat. Irregular bright area in center and surrounding small pinholes show first coat exposed due to damage.


Inspection of Fluorescent Coating Systems

Documents

Transcript of Inspection of Fluorescent Coating Systems