Part C Effective Corrosion Control Measures Inhibitors Design Materials Selection Cathodic Protection Protective Coatings Inhibitors A corrosion inhibitor is a substance added to the environment that decreases the rate of attack by that environment on a metal. Use of chemical inhibitors is usually restricted to water circulation systems, reservoirs or cooling towers. Since most inhibitors work in a water solution, their use in the field is very limited. Inhibitors Inhibitors are sometimes used as additives to prevent steel from flash rusting during wet abrasive blasting or UHP waterjetting operations. Inhibitors The amount of the inhibitor added when wet blasting or UHP waterjetting is critical. Too much inhibitor can have a deleterious effect upon coating performance. Always consult Technical Service regarding the suitability and/or selection of inhibitors. Over-Design Over-design of structures refers to the common use of heavier structural members or thicker plates than actually required in anticipation of corrosion losses. The disadvantages of over-design include higher cost, increased weight, and uncertain life expectancy. Over-Design In addition to over-design, other design factors may be employed to minimize corrosion losses. “Smart-design” includes avoidance of back-to-back angles, skip welds, sharp edges, inaccessible sections, etc, when designing a structure. Design can influence resistance to corrosion. Either good or bad. Skip welding Ð Ð Ð Ð Ð Ð Hard to reach angles…….. Î Carbon Steel Hot dipped galvanized steel Design Roof area of a waste water treatment tank No seal welding Crevice Corrosion Moisture + H2S Poor welding technique Dissimilar Metals Design Ð Î Ï Carbon steel to stainless Carbon steel braces Í Stainless steel tube for level indicator float Materials Selection Selection of “slow corroding” metals & avoiding the use of dissimilar metals. Know your metals! Corrosion Resistant Materials Alloys  Copper  Nickel Chromium  Thermal Spray Coatings (Metallizing)  Polyethylene  PVC  Stainless Steel  Thermal Spray Coating Metallizing / Thermal Spraying / Arc Spraying Ï Wire Cathodic Protection Use of a metal which corrodes instead of the structure. Reduces corrosion of the structure to be protected by making it a CATHODE. Cathodic protection is often used…… ship hulls, tank bottoms in underground environments on steel in concrete underwater pipelines Cathodic Protection Reduction or elimination of corrosion by making a metal (iron, steel*) a cathode. Sacrificial anodes Sacrificial coatings Impressed current system Sacrificial Anodes Made of some easily corroded material and corrode instead of the structure to which they are attached. The most common metal used is ZINC or other metal alloyed with zinc such as magnesium. Sacrificial anode used to protect pipeline Î Sacrificial Anodes Í Wire to test station Coke can Metallic pathway Ð Anode Cathode Electrolyte Sacrificial Anodes Strip Anodes Ô Ð Zinc anodes for brine waste water tank Zinc anode Cathodic Protection Before After Sacrificial Anodes Ò Ó Í Í Sacrificial Anodes Ò Ñ Sacrificial Anodes Sacrificial Anodes Í Í Í Í Í Í Í Cathodic Protection Impressed Current System Uses DC current to protect a structure. A strongly anodic metal is connected to the structure to be protected. The positive (+) terminal of the power source is always connected to the anode. AC Power Source Î - + DC Rectifier Pipe Anode DC Current Rectifier Ô Sacrificial Coatings Inorganic Zinc (IOZ) Does NOT contain organic resin Zinc corrodes to protect the steel substrate Over millage will mudcrack Acid intolerable Sacrificial Coatings Zinc anode and steel cathode in contact with same electrolyte. Zinc primer pigmented with sufficient zinc to maintain high electrical conductivity. Electrolyte Zinc Primer - Anode Steel substrate Cathode Not recommended for immersion without a topcoat. Sacrificial Coatings Inorganic Zinc Î Sacrificial Coatings Î Í IOZ Primer Over millage of an inorganic zinc can result in “mudcracking” Sacrificial Coatings Zinc Rich aka Zinc Rich Epoxy  Contains zinc dust in organic resin  Less temperature resistant than IOZ’s  Combination barrier coat and sacrificial coat  Not prone to “mudcracking”  Contain carbon making it sunlight sensitive (UV) Zinc Rich Epoxy No Zinc With Zinc Barrier Coatings Topcoat - Polyurethane Intermediate Coat -- Epoxy Primer Coat - Epoxy Steel Substrate Inhibitive Coatings Aid in decreasing attack on metals. Epoxies with additives – Amercoat 385PA Steel Three Little “Pigs”
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