Material Matters PVC linings provide better corrosion protection with improved welding technique

In the process industry, such as metal finishing, PVC linings are com- monly used as corrosion barriers to protect the interiors of steel tanks or concrete pits, which typically contain chemicals that are extremely corrosive, says NACE International member Cur- tis Goad, president and owner of GOAD COMPANY, a provider of custom-designed tank lining systems for the surface finishing and chemical pro- cessing industries. Open rectangular tanks and pits, as well as secondary containment areas, are typically used in surface processing such as electroplat- ing, he says, and often are lined with high-performance plasticized (flexible) PVC sheets that are bonded with adhe- sive to side walls and bottoms and sealed at the seams and corners by thermoplastic welding. The thermoplastic welding process melts the welding material (rod or strip) to its molten state as well as preheats the PVC lining sheets, and applies pressure to both the welding material and the sheets to allow the molecules to fuse into a solid when cooled. Welding techniques for include hot gas/hot air welding, where a hand-held welding device uses heated nitrogen gas or air to soften both the An operator uses a thermoplastic hot welding technique de- rod or strip and the gas/hot air extrusion welding machine veloped by GOAD sheets to be joined while an operator to join PVC lining sheets. Photo COMPANY (Ellisville, moves the device along the seam. courtesy of Curtis Goad. Missouri) improves the In one method of hot gas/hot air welding, known as hand welding, an thermoplastic welding operator uses a welding gun to lay a processA for bead of material along the seam where (PVC) linings and helps prevent the base materials touch. The size and aggressive chemicals from seeping shape of the weld bead are determined by the tip on the welding gun nozzle, behind a plastic lining and attack- and the operator controls the preheat ing the underlying substrate. temperature, welding speed, and Information on corrosion control and prevention

strate’s pores and voids as well as joins the PVC sheet linings, he explains. Rather than extending the sheet linings on a side wall and bottom of a tank or pit so that they meet at the 90-degree joint, the sheet linings are shortened to leave a small gap at the joint. This al- lows the molten thermoplastic weld material to flow into and fill the gap between the lining sheets and penetrate the joint to the substrate. An infused weld area is created that eliminates channels behind the weld seams, re- duces the probability of leaks, and in- The photos show PVC lining sheets joined with thermoplastic welds made with creases the service life of the tank or pit. (a) the hand welding technique and (b) the extrusion machine welding method. Should a leak happen, the weld blocks Photo courtesy of Curtis Goad. solution from flowing behind the lining. Inner bottom corners, where three intersecting lining sheets must be joined, amount of pressure that is applied to machine along the seam. The weld is are typical problem areas and a frequent the materials. For thicker gauge sheets, formed in a single pass, which eliminates source of early leaks and premature lin- or joints with 90-degree angles and the heat stresses associated with multiple ing failures, Goad says. “It is difficult to corners where two side walls meet the passes. perform a high-quality weld in a corner. bottom, the operator can lay several Advances in tank linings have focused You need perfect speed, temperature, beads of welding material on top of on machine welding, which provides a and pressure as you move the welding each other through multiple passes of more consistent, higher-quality thermo- machine along the joint; but when you the welding gun. One drawback with plastic weld, says Goad, commenting that come to a corner, you can’t preheat the hand welding is that excessive stresses the welding parameters are more tightly lining sheets because the machine stops,” from alternating heating and cooling controlled and the resulting welds are he notes. To address this problem, the cycles can occur with multiple passes. proven to be stronger and more tolerant company has devised molded thermo- Additionally, the quality of the hand of stresses. His company has developed a plastic corner inserts that enable the weld directly depends on the skill of the hot gas/hot air extrusion welding ma- machine to weld continuously in the operator. chine technique that overcomes disad- corners. Another hot gas/hot air welding vantages that are characteristic of hand Goad comments that the extruded method uses a hand-held machine that welding, such as channels and gaps that welding machine technique with plasti- extrudes molten thermoplastic weld ma- can form behind welded seams and allow cized PVC also can be used with rigid terial through a the solution in a tank or pit to flow behind PVC and chlorinated PVC (CPVC) (PTFE) shoe that is shaped to fit the size the lining and corrode the substrate if a fabrications. Although steel and concrete of the desired weld. The melting chamber leak occurs. are the major substrates used for process- and base material preheat temperatures This patent-pending technique ing tanks and pits, the PVC linings and are set prior to welding and are digitally modifies the preheating process and machine welding technique can be used controlled and monitored by the ma- delivery of the weld material, Goad with wood, fiberglass, and other sub- chine, and the operator ensures the says. The welding machine extrudes a strates that require corrosion protection. proper speed is maintained and adequate high-performance, plasticized PVC Contact Curtis Goad, Goad CoMPANY— pressure is applied while moving the weld material that infuses into the sub- e-mail: [email protected].