Part 4: Material Selection for Chemical Process Equipment – Liners & Coatings
Part 4 of the seven-part series on material selection for chemical process equipment focuses on liners and coatings – excluding paint and powder coating, which will be explored independently in Part 5 – that can significantly enhance the chemical resistance and/or other properties for base process equipment and process system/area materials. A broad range of liners and coatings are available from many of the materials discussed in Parts 1, 2, and 3, including metals, plastics, rubber and elastomers, FRP, advanced composites, and ceramics. Liners and coating systems can be factory or field-installed, depending on the process systems and project circumstances (please refer to these previous papers for material-specific acronyms used herein). For liners and coating systems, the method and conditions of surface preparation and application/installation are critical to the overall effectiveness in achieving desired resistance to physical damage and chemical attack. While these terms are sometimes used interchangeably, in this discussion (unless otherwise noted) liner systems are applicable to the surfaces inside of wet process equipment (e.g., tank interiors, pump and process equipment internals, etc.), and coating systems are applicable to exterior surfaces (e.g., tank and equipment exterior surfaces, process structural steel, concrete floors, etc.).
Surface Preparation
Effective surface preparation of materials is critical to render materials surfaces and profiles in the required state for control of both good adherence and thickness of coatings and for good fit and adherence of liners. Surface preparation includes a range of chemical and mechanical cleaning and surface treatments. Mechanical cleaning processes include sanding, grinding, and abrasive blasting. Additional surface preparation includes proper radiusing and deburring of sharp edges. It is important to complete chemical cleaning prior to mechanical treatments. Mechanical cleaning of oily surfaces can embed organic contaminants in material surfaces and impact coating and liner adhesion. For liner and coating systems, the surface preparation techniques should be acceptable by industry standards, such as joint Society for Protective Coatings (SSPC) and National Association for Corrosion Engineers (NACE), for the specific base material surface and coating system or liner application.
Liner and Coating Application
Liner and coating application processes (following proper, complete, and quality-verified surface preparation) include: hand or automated spray-on application in layer or layers followed by curing (thermal or chemical); setting material sheets or pieces and anchoring with specialty fasteners and hardware; trowel-on layers followed by curing, powder coating, thermal fusion, thermoplastic molding and welding, electroless plating and electroplating, thermal spray, physical vapor (vacuum) deposition, cladding; and many other techniques and processes.
The following sections provide examples of different liner and coating materials and applications.
Plastics
A range of plastics are used for liners and coatings. For proper applications consideration, a full range of process chemistry and application-specific conditions and life-cycle requirements should be considered for each candidate plastic. Common surface finishing process examples for plastic linings and coatings include:
Tank (and vessel) liners: Tank liners can typically be classified as flexible or rigid. Flexible liners range from lower cost “loose” liners to higher cost bonded liners. Loose liners, with welded seams providing elongation and tensile properties similar to the virgin material film, provide higher resistance to stress cracking compared to similar material bonded liners. One of the most typical flexible liners materials is PVC (e.g., Koroseal®). Other plastic liners are available, but less common than PVC, ranging up to PTFE for relatively high temperature and aggressive chemistry applications.
Steel and stainless-steel tanks can be factory-lined with plastics (e.g., PVC, CPVC, and PP) that are bonded to the surface. Higher temperature and chemical-resistant, fluoropolymer-bonded liner materials for metal tanks include PVDF, ETFE, ECTFE, FEP, PTFE, and PFA. Metal surfaces are prepared by grit-blasting and cleaning. Fusion-welded virgin fluoropolymer films are bonded to the prepared metal surfaces with proprietary manufacturer processes typically involving saturating or impregnating a fabric (e.g. glass fabric or polyester) with an adhesive system, positioning the fabric between the metal surface and the fluoropolymer film, and activating adhesive setting while laminating the layers with pressure or other techniques to produce the fluoropolymer-bonded, rigid liner metal tank. Rubber materials can also be adhesively bonded to tanks and other process equipment (e.g., filter housings).
Considering dual-laminate, PVDF-lined FRP tanks, the FRP provides cost-effective tank strength and application-specific external surface properties, and the rigid PVDF liner provides enhanced application-specific chemical resistance. PVDF-lined FRP tanks are custom-made for applications under carefully controlled factory processes and procedures by highly qualified manufacturers, where a backing fabric is laminated onto a plastic film and the film is overlaid with FRP. Other dual-laminate, FRP tank plastic liners include PVC, CPVC, PP, FEP, PTFE, and PFA.
Fluoropolymer-coated immersion heaters and heat exchangers: Aggressive chemistries can cause severe corrosion of heated metal surfaces. Highly chemical-resistant fluoropolymer coatings (e.g., PFA and PTFE) on base metal surfaces (e.g., stainless steel) provide much longer heater life without the use of much higher cost metals or superalloys that would not need coatings for an application. Fluoropolymer coatings also maintain purity of process solutions and reduce fouling and scaling (due to very good non-stick properties). Additional heater surface areas are required with the fluoropolymer coatings since they reduce the overall heat transfer coefficient, compared to non-fouled, non-corroded metal surfaces. The fluoropolymer coating thickness and processes are optimized by manufacturers to minimize coating thickness while maintaining good coating adhesion and coverage for long service life.
Plastic-lined pumps, valves, and other factory-manufactured process equipment: Considering plastic lining materials for wet process fluid surfaces, the Teflon plastics (PTFE, PFA, and FEP) provide for the combined highest temperature service (FEP has lower temperature ratings than PTFE or PFA) and excellent chemical resistance for a broad range of chemistries, while other materials, such as steel and stainless steel alloys, are used for the main process equipment structure material.
Metals
Metals can be deposited on properly prepared metal or non-metal process equipment surfaces to provide special lining or coating properties that extend the application of the base material, including enhanced chemical resistance and/or electrical conductivity. Metal deposition processes include electroless plating, electroplating, thermal spray, and various other physical and chemical deposition technologies. Example process equipment applications using metal coatings include:
Improved chemical resistance for electrodes in very aggressive process chemistries: Platinum group metals (Ru, Rh, Pd, Os, Ir, & Pt) are used to coat suitable lower cost base metals (e.g., titanium) to provide desired long service life electrodes for surface finishing process applications with aggressive chemistries. Preparation of the base electrode metal titanium for platinum group metal coating typically includes mechanical polishing followed by electropolishing in specific solvent/acid mixtures.
Specialty process reactor surfaces: Metals, ranging from nickel to precious metals, are used in non-proprietary and proprietary specialty process reaction vessels to provide corrosion resistance at process reaction temperatures or other desired process reactor surface metal properties.
Adding metal layers onto plastics, ceramics, and composites: These are important applications in the surface finishing industry. One process approach for adding metal layers to non-conductive base materials is by first depositing a base layer using electroless plating (chemical etching and then metal deposition from a non-electrified process solution), followed by electroplating to build up the desired metal layer thickness to provide desired conductivity or other properties. Metal-coated plastics, ceramics, and composites have relatively minor applicability to surface finishing chemical process equipment.
Other Materials
Many of the materials discussed in Part 3 have applications in surface finishing process equipment linings and coatings, including composite materials and advanced composites, technical ceramics, and rubbers and elastomers. To meet project application needs, other liners and coating materials may also include glass, amorphous carbon, and acid-proof brick; these have some common and some limited specialized applications for surface finishing process systems and plant areas. Some limited examples from a diverse range of other materials used for chemical process equipment and process area surfaces include:
A range of vinyl ester epoxy or other resin coating products with application-specific solids/fill materials are commonly applied in varied multi-layer systems, typically including a base layer, one or two coats above the base layer, and a topcoat layer. Specific resin coating systems are used for different process concrete areas (e.g., 100% solids polymer blend with epoxy novolac base topping for process area floors and tank pads, or as a top-coat lining for sumps and containment areas) and for process area structural steel applications specific to process chemistry exposure conditions ranging from constant immersion to short term containment/frequent spills to intermittent spills followed by prompt washdown.
Graphite particles blended into PFA powder are one example of a composite coating for heater and heat exchanger surfaces where the graphite provides for significantly increased thermal conductivity while maintaining the coverage integrity and chemical resistance of the PFA coating.
Glass-lined steel tanks are manufactured where glass materials are fused to the interior steel tank wall at high temperature (e.g., 1600°F) to provide a lining for specialty high-purity and corrosion resistant applications.
Next week, watch for Part 5: Material Selection for Chemical Process Equipment – Paint & Powder Coating.