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

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

Coating Examples

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

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

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

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

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

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 (20-100 mils for pipelines) Thick-film coatings – typically 12-25 mils thick (>100 mils for pipelines)

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

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

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

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

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

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.

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

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

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

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

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

Design Considerations

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

Effects of Poor Surface Prep

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

Grit Blasting Media Glass Garnet

Blast Cleaning – Visual Comparison

NACE Visual Comparator for welds

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

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

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

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

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)

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

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

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

Thickness Gages