1. Corrosion Basics Chapter 6 – NACE Book Protective Coatings
PTRT 1309
Corrosion Basics
Chapter 6 – NACE Book
Protective Coatings
Prepared by Dr. Capone

2. Introduction
Coatings are applied to a surface to protect it from the surrounding environment
For decorative or aesthetic purposes they are usually called paints
In this chapter we limit our discussion to protective coatings which are often multi-layer systems
However, there is clearly some overlap between the two

3. Coating Examples

4. Definition of Coating Relatively thin material
Usually applied as a liquid or powder
Upon solidification is firmly adhered to the surface being protected
Prevents the environment from contacting the surface

5. Requirements
In order to meet the definition of a coating the film must be:
Relatively flexible
Impact resistant
Chemically resistant to the environment to which exposed
Resistance to permeation by moisture
Good adhesion and cohesion
Temperature resistant
Selection must also be based upon cost factors

6. Coating Classifications
Service environment
Atmospheric
Immersion
External pipe service
Each has different requirements and can be further subdivided requiring different surface prep for example
Thick films or thin films

7. Service Environment Atmospheric Coatings Open or housed
Must be stable in changing weather conditions
Stable on UV exposure
Stable against moderate physical damage
Surface prep is more critical for open exposure than for housed
Immersion Coatings
Can be very demanding service
Air-dry, forced cure, baking, fusion-bonded materials are all available
Surface preparation is critical for these applications

8. Service Environment Underground (Buried) Coatings
Often referred to as “pipeline coatings”
Improved resistance to physical damage
Resistant to stresses in the soil
Resistant to cathodic disbondment from cathodic protection systems (Note: this results from discharges between the coating and the protected surface)
These coatings can be similar to other coatings, e.g. FBE
Generally are quite different often applied as tapes or extruded onto the pipe

9. Coating Thickness
Dry film thickness is important from several standpoints
Optimum thickness range depends on the coating material
Excessive thickness can cause debonding
Too thin will not provide adequate protection
Required thickness will determine the required anchor pattern
Thin-film Coatings – typically 5 – 8 mils thick ( mils for pipelines)
Thick-film coatings – typically mils thick (>100 mils for pipelines)

10. Chemical Composition Two general chemical categories
Organic – most common materials used to protect facilities and equipment
Inorganic – anything not organic
Both are typically liquid applied coatings sprayed onto the surface and allowed to dry
Table 6.1 provides a listing of chemical types in use

11. Curing Methods
Curing or drying refers to the mechanism of the coating converting from the wet as-sprayed film to the hardened final protective film
Air-dried – most common form in which solvents evaporate at ambient temperature
Force-cured – typical for internal tank coatings in which air-drying is accelerated by heating (Note: Not applicable for all coatings since excess drying rates can affect bonding)
Baked – coatings are actually baked in an oven (some coatings use UV light to accomplish this same task)
Fusion-bonded – applied as a powder to a hot metal surface and are becoming more prevalent due to lower VOC emissions

12. Coating Formulation
Pigment – solids suspended in the coating used for a variety of reasons

13. Coating Formulation
Vehicle – consists of the various resins, solvents and plasticizers in the formulation (also called the carrier)
Resin is used as a binder
Binds the pigments together
Wets the surface of the metal (or previous coat)
Serves as the primary barrier to the environment
Maintains integrity in corrosive environment
Solvents dissolve the binder and create a useable liquid
Many binders are solids until dissolved in solvent
Typically mixtures of various organic materials
Reducing VOC emissions is driving solvent reformulation at a rapid pace

14. Coating Systems Normally not single coats of materials
Atmospheric coatings are typically two or three coat systems
Primer and top coat
Primer, intermediate, top coat
Some systems have as many as 5 coats

15. Primers Most important part of the coating system
Purposes of the primer coat
Effective bond to metal substrate
Tie (inter-coat bond) to subsequent coats
Provide stronger resistance to corrosion
Can act as an inhibitor
Can provide sacrificial metal (zinc-rich primers)
Often the primer is applied at the factory and must provide protection during transport and prior to application of top coats.

16. Types of Primers Barrier Primers Inhibitive Primers
Variations on the formulations of the other coats (e.g. same resin)
Usual primer type for immersion systems
Inert and strong adhesion
Inhibitive Primers
Formulated with inhibitive pigments (red lead)
Phosphates and similar non-toxic substitutes being used
Sacrificial Primers
High concentration of zinc dust
Acts as sacrificial anode to protect bare steel when coating is damaged
Zinc-rich primers can be easily top coated unlike galvanized metal

18. Intermediate Coats Often called the Barrier Coat or Body Coat Purpose
Provides thickness
Strong chemical resistance
Resistance to moisture vapor transfer
Increased electrical resistance
Strong cohesion (within the coat)
Strong adhesion (to the primer coat AND the top coat)
Typically the most critical coat in aggressive environments
Many off-shore systems utilize three-coat systems

19. Top Coats Sometimes called the “finish coat” Purpose
Provides resistant seal for the coating system
Initial barrier against the environment
Resistance to chemicals, water, weather, and UV
Touch and wear-resistant surface
Pleasing appearance
Normally has a lower pigment to vehicle ratio (smooth finish)
Typically a different color than the intermediate coat to improve judgment of the coverage

20. Coating Application
Poor coating properly applied will often outlast the best coating improperly applied
Quality of the application relies of several factors
Design for Coating
Surface preparation
Coating application
Inspection
Technology cannot fix a bad coating application

21. Design For Coating Design must allow for
Proper cleaning
Good preparation
Uniform coating
Inspection
Sharp edges, skip welds, blind areas, crevices are all design faults to be avoided
Result in coating failure even before the coating is applied
Table 6.6 lists design considerations

22. Design Considerations

24. Surface Preparation Two major areas
Preparation of a clean smooth surface
Preparation of a proper anchor pattern
Cleaning
Washing to remove dirt and/or grease
Grinding to remove weld spatter and smooth rough welds
Final step involves grit blasting to remove rust and/or scale and establish the correct anchor pattern

25. Effects of Poor Surface Prep

26. Blast Cleaning Most common method of surface prep
Referred to as sand blasting or bead blasting
Sand is less common due to concerns over silica
Variety of Blast Cleaning Specs
NACE No. 1/SSPC-SP 5 White metal Blast cleaning
Free of contaminants
NACE No. 2/SSPC-SP 10 Near-white metal blast cleaning
Random limited to 5% of each 3”X3” area
NACE No. 3/SSPC-SP 6 Commercial blast cleaning
Random limited to 33% of each 3”X3” area
NACE No. 4/SSPC-SP 7 Brush-Off blast cleaning
Free of loose materials, tightly adhering material may remain

27. Grit Blasting Media
Glass
Garnet

28. Blast Cleaning – Visual Comparison

29. NACE Visual Comparator for welds

30. Anchor Pattern
Term used to describe the profile of the metal’s surface roughness
Average distance between peaks and valleys created by blast cleaning
Proper pattern is critical for good adhesion

31. Anchor Pattern Figure 2. Surface Profile for shot blasted surfaces.
Figure 3. Surface Profile for grit blasted surfaces.

32. Verification Replica tape method Visual comparators
Optical scanners either read the surface directly or read the replica tape

33. Coating the Structure Conventional and airless sprayers
Airless uses paint pump to pressurize the flow of paint through an atomizer
Spray technique is critical to coating success
Pressure
Spray pattern
Distance
Coverage area

34. Inspection Large jobs often handled by third-party inspectors
Often these are required to be NACE certified
Inspector must have authority to enforce the specs
Conducts whatever tests are necessary to verify the spec has been met (see example spec, Appendix 6C)

35. Inspection Timing
Initial inspections usually occur prior to surface preparation
Environmental conditions are factored in
Inspector assures that Table 6.8 and spec requirements are being met
Dried coats are holiday checked for voids or damaged areas prior to continuing
Inspection points
Pre blast cleaning
Post blast cleaning
Following each coat
After final cure
Final inspection

36. Inspection Tools Typical inspection tools
Surface cleanliness comparators
Routine inspection equipment
Mirrors, magnifiers, depth gages
Film Thickness (wet and dry) gages
Holiday Detectors

37. Film Thickness Gages Used to evaluate the film thickness
Notch-type thickness gage for wet film
Magnetic “pull-off” type gage for dry film
Force required to pull off the gage is proportional to the thickness of the dry film
Electronic and electromagnetic gages are also available

38. Thickness Gages