The old adage “the devil is in the details” certainly applies when selecting chemical-resistant lining and coating systems for aboveground storage tanks in hydrocarbon service. Among the considerations for selecting an internal lining are whether water is present, the condition of the tank bottom and walls, compatibility of the lining with the stored products, operating temperatures, and a thorough evaluation of various lining options.
“Corrosion may occur when a layer of water settles to the bottom of a crude oil, intermediate product, or finished product storage tank,” according to the American Petroleum Institute (API) Recommended Practice (RP) 652. “This water, which may enter the tank with the product, through the seals, or during ‘breathing’ of the tank, often contains corrosive compounds. For example, crude oil may contain salt water and sediment that settles out on the bottoms of storage tanks. Chlorides and other soluble salts contained in the water may provide a strong electrolyte that can promote corrosion.”
The API RP 652 offers recommendations on lining the bottoms of aboveground storage tanks based on them type of corrosion present, its location and effect on the steel substrate. “An internal tank bottom lining may be deemed necessary if inspection shows that the minimum thickness of the bottom steel plate is less than 0.100 in. (2.5 mm), or if corrosion is expected to proceed so that the steel thickness may reach this minimum thickness prior to the next scheduled inspection,” according to the API RP 652.
Other API criteria for selecting a tank bottom lining are chemical resistance or compatibility with the stored product, as well as resistance to moisture permeation from the layer of water that is typically found on the floor of the tank. “All lining materials absorb moisture over time and this absorption can ultimately result in its failure,” the API noted.
Tank linings are classified by API RP 652 as thin-films (with a dry film thickness less than 20 mils) and thick-films (with a dry film thickness of 20 mils or more). “Recent advancements in technology have produced coatings with zero VOCs (volatile organic compounds),” API RP 652 reported. “These 100 percent solid coatings provide reduced safety concerns during the applications, can be applied in a single coat and offer a reduction in the tank turnaround schedule. With exceptions, typically these coatings are classified as thick-film linings.”
“Generally, corrosion due to immersion exposure creates a surface that is rough and pitted, and it is often difficult to completely coat and protect a corroded steel bottom with a thin-film lining system,” the API RP 652 stated. The API also noted that high solids linings offer better edge retention with reduced material shrinkage.
“The industry is shifting more and more to 100 percent solids technology,” acknowledged Gary Zinn, sales director, Industrial Markets for Tnemec. “Because you don’t have to apply multiple coats, you save labor costs. And thick-film linings offer longer service life due to their additional thickness, which also provides greater resistance to moisture permeation.”
Tank Armor 100 percent solids epoxy linings from Tnemec are spray applied at 20 to 60 mils dry film thickness (DFT) in a single, high-build application for lower labor costs compared to multi-coat thin-film or reinforced thick-film linings. The low-odor and VOC-compliant technology offers fast curing for return to service within 24 hours and is available in multiple formulations that can accommodate numerous commodities and operating conditions.
Series 330 Tank Armor, a thick-film reinforced epoxy internal lining, offers resistance to ethanol blends up to 30 percent, crude, gasoline, diesel, aviation fuels, motor oils and select chemicals, while Series 350 Tank Armor, a modified phenolic epoxy, is formulated for 100 percent ethanol. “We’ve also successfully tested Series 350 with B100, which is pure biodiesel manufactured from vegetable oils, animal fats, or recycled restaurant greases,” Zinn noted. “The B100 is used to blend with No. 2 diesel fuel for B20.”
For aggressive chemical immersion, 100 percent solids novolac epoxies are available. When spray-applied at 20 to 60 mils DFT, Series 365 Tank Armor exhibits an excellent range of chemical and solvent resistance, including 98 percent sulfuric acid. Series 390 Tank Armor applied at 20 to 50 mils DFT offers corrosion resistance for select acids, fuels and oils stored at elevated service temperatures to maintain low viscosity of the products being stored.
Proper surface preparation is critical to the application of thick-film linings for immersion service. “Continuous immersion is a severe exposure,” the API RP 652 emphasized. “Inadequate surface preparation is a major cause of lining failure. Surface preparation is performed to provide the appropriate combination of surface cleanliness and surface profile, or the anchor pattern required to establish good chemical and mechanical adhesion of the lining resin to the steel.”
In order to establish good chemical and mechanical adhesion of the lining to the steel, the API RP 652 specified abrasive-blast cleaning in accordance with SSPC-SP5/NACE No. 1 White Metal Blast Cleaning. “The anchor pattern required for linings is typically 1.5 to 4.0 mils and generally increases with the thickness of the lining,” the API RP 652 stated. “To achieve adhesion necessary for long-term performance, it is important that the anchor pattern is sharp and angular.”
Citing the importance of maintaining the specified level of cleanliness until the lining is applied, the API RP 652 indicated, “If the surface is degraded or contaminated subsequent to surface preparation and before lining application, the specified level of cleanliness should be restored before the lining application.”
Holding primers, such as Tnemec’s Series 61 Tneme-Liner, a tightly cross-linked thin-film amine epoxy, are often specified to hold the blast in large fuel storage tanks. “The criteria for selecting a holding primer is it’s resistance to the fuel or chemical being stored, as well as temperature conditions of the steel and whether the tank is insulated or uninsulated,” Zinn shared. “By assessing each project for these criteria, we have had great success when Series 61 has been used as a holding primer.”
Proper surface preparation also involves the removal of weld spatter, burrs or protrusions and the rounding of sharp edges. “It is common to use caulk to seal lap joints between riveted plates and around rivets to provide a continuous surface for the lining application,” API RP 652 stated. A polysulfide novolac blend polymer, Series 351 Tank Armor, is available as a flexible repair and base coat putty used for seams and chime areas.
Fast-curing Tank Armor 100-percent -solids epoxy linings enable required discontinuity (holiday) testing within 24 hours of application. “Holiday testing is critical,” Zinn acknowledged. “Holidays or voids in the film allow whatever is in the tank to attack and corrode the substrate causing delamination of the lining material.”Discontinuity testing of thin-film linings should be performed with a low-voltage detector, according to API RP 652.
A detailed list of references dealing with this topic is included in API RP 652, including NACE International (www.nace.org) RP0188 Discontinuity (Holiday) Testing of New Protective Coatings on Conductive Substrates; RP0287 Field Measurement of Surface Profile of Abrasive Blast Cleaned Steel Surfaces Using a Replica Tape; The Society for Protective Coatings (www.sspc.org) PA1 Shop, Field and Maintenance Painting; and SSPC PA2 Measurement of Dry Paint Thickness with Magnetic Gages.