Corrosion and Corrosion Prevention Metallurgy for the Non-Metallurgist

Learning Objectives After completing this lesson, students will be able to: Describe the principal types of corrosion including: uniform, galvanic, concentration-cell, pitting, selective leaching, intergranular, erosion, and crevice corrosion Explain the significance of the galvanic series in corrosion analysis and prevention List the ways to prevent or to minimize corrosion

Introduction: Corrosion and Corrosion Prevention Corrosive environments, temperature effect, films Examine principal types of corrosion Electrochemical nature of corrosion Galvanic series Corrosion prevention

Interactions may be specific to the material/environment combination. No problem Destruction Wet Chlorine Dry Chlorine Iron Titanium

Direct Corrosion Using the example of iron in an atmosphere containing sulfur dioxide (dry corrosion), the reactions are: Fe + SO2 = FeS + O2 2Fe + O2 = 2FeO

LEO Loss of electrons is oxidation GER Gain of electrons is reduction

Formation of ferrous (Fe2+) ions in the corrosion of iron

Water ionizes to some extent to form hydrogen (H+) and hydroxyl (OH–) ions.

Hydrogen ions accept electrons at the cathode and form hydrogen gas.

Polarization of a local cathode by a layer of hydrogen minimizes corrosion.

Corrosion of steel by water containing oxygen. When depolarization occurs (hydrogen and oxygen combine to form water) corrosion again proceeds.

Basic diagram showing requirements for corrosion of metals. In a metallic conductor, the electrons move in the opposite direction that conventional current is assumed to flow.

Complete circuit for current flow by means of an external wire, combining the reactions shown in Fig. 1 and 3.

Section of a dry cell or battery. Usually MnO2 is added as a polarizer for longer battery life.

Different types of corrosion

Common types of corrosion Uniform attack or general overall corrosion Galvanic or two metal corrosion Concentration cell corrosion Pitting corrosion Selective leaching Intergranular corrosion Stress corrosion cracking Erosion corrosion Crevice corrosion Corrosion fatigue Hot corrosion, oxidation, sulfidation

Effect of acid concentration on the corrosion rate of iron completely immersed in aqueous solutions of three inorganic acids at room temperature: (a) hydrochloric acid, (b) sulfuric acid, and (c) nitric acid. Note that the scales for corrosion rate are not the same for all three charts. (Source: M. Henthorne, “Corrosion Causes and Control,” Carpenter Technology Corp., Reading, PA, 1972, p 30)

Etched longitudinal section of a carbon steel steam tube that corroded on the inner surface more rapidly opposite the exterior heat-transfer fin than elsewhere along the tube. Original magnification: 3×

Passivation Not a permanent treatment Establishes good conditions for corrosion resistance, film renewal In stainless steels, passivation treatments remove tramp iron from the surface Performed in nitric or citric acid Ruined by dropping a steel washer into a passivated tank

(a) Sprinkler system in which a malleable iron deluge clapper latch failed from galvanic attack caused by contact with a copper alloy clapper in stagnant water. (b) Photograph of clapper latch, showing effects of galvanic attack at areas of contact (near top) and crevice corrosion (at lower left). (c) Micrograph of a cross section of the failure area on the clapper latch, showing the pattern of the corrosion and elongated grains in the microstructure (indicative of a ductile type of failure). Original magnification: 250×

Part of the propulsion system for a missile in which the aluminum alloy6061-T6 fuel line failed from galvanic attack because of contact with a type 301 stainless steel helium-pressurization line. (a) Setup showing proximity of the fuel line to the helium-pressurization line and in view A-A the point of contact (5 mm [0.2 in.] maximum separation) between the two components at which the failure initiated. (b) Macrograph of the 12.5 mm (½ in.) long crack at a 60° bend in the fuel line. Light area around the crack indicates lack of the protective chromate coating. Original magnification: 2× (c) Micrograph of a polished but unetched section through the crack, showing severe intergranular attack and extent of corrosion. Original magnification: 25×

Unetched section, through the bottom of a type 321 stainless steel aircraft freshwater storage tank that failed in service as a result of pitting, showing subsurface enlargement of one of the pits. Original magnification: 95×

Micrograph showing difference in dezincification of inside and outside surfaces of a plated copper alloy 260 (cartridge brass, 70%) pipe for domestic water supply. Area A shows plug-type attack on the nickel-chromium- plated outside surface of the brass pipe that initiated below a break in the plating (at arrow). Area B shows uniform attack on the bare inside surface of the pipe. Etched in NH4OH-H2O2. Original magnification: 85×

Copper alloy 270 (yellow brass, 65% Zn) air-compressor intercooler tube that failed by dezincification. (a) Unetched longitudinal section through the tube. (b) Micrograph of an unetched specimen showing a thick uniform layer of porous, brittle copper on the inner surface of the tube and extending to a depth of approximately 0.254 mm (0.010 in.) into the metal, plug-type dezincification extending somewhat deeper into the metal, and the underlying sound metal. Original magnification: 75×. (c) Macrograph of an unetched specimen showing complete penetration to the outside wall of the tube and the damaged metal at the outside wall at a point near the area shown in the micrograph in (b). Original magnification: 9×

(a) Schematic illustration of a fused-salt, electrolytic-cell pot of type 304 stainless steel that failed by intergranular corrosion as a result of metal sensitization. (b) to (f) Micrographs of corroded and uncorroded specimens taken from the correspondingly lettered areas on the pot shown in (a); specimens were etched in CuCl2. Original magnification: 500×

As-polished cross section through a stress-corrosion-cracked type 304 stainless steel part, showing branching of cracks as they proceed downward from the surface (top of micrograph). Original magnification: 100×

Micrograph of a nital-etched specimen of ASTM A245 carbon steel, showing stress-corrosion cracking that occurred in a concentrated solution of ammonium nitrate. Original magnification: 100×

Micrograph of a nital-etched section through corrosion fatigue cracks that originated at corrosion pits in a carbon steel boiler tube. Corrosion products are present along the entire length of the cracks. Original magnification: 250×

Micrographs of etched specimens that show corrosion on the inside and outside surfaces of a buried 356 mm (14 in.) diam low-carbon steel (0.20% C), schedule 40, water supply pipe. (a) Smooth surface produced by erosion-corrosion on the inner surface. (b) Pitting corrosion on the uncoated outer surface of the pipe. Both have original magnification: 115×

Prevention of Corrosion The major types of corrective and preventive measures are: Change in alloy, heat treatment, or product form Use of resinous and inorganic-base coatings Use of inert lubricants Use of electrolytic and chemical coatings and surface treatments Use of metallic coatings Use of galvanic protection Design changes for corrosion control Use of inhibitors Changes in pH and applied potential Continuous monitoring of variables

(a) Original design of a cathodic protection system for a buried steel tank that caused local failure of a nearby unprotected buried pipeline by stray current corrosion. (b) Improved design: installation of a second anode and an insulated buss connection provided protection for both tank and pipeline, preventing stray currents.

Standard anode or ground-bed installation. Note backfill in this case is coke breeze.

Inhibitor Systems The choice and concentration of inhibitor depend on the: Type of system Composition of the electrolyte Temperature Rate of movement of the metal and/or the electrolyte Presence of residual or applied stresses Composition of the metal Presence of dissimilar metals

Pourbaix diagram showing the theoretical conditions for corrosion, passivation, and immunity of iron in water and dilute aqueous solutions

Avoiding Corrosion Two major problems: design, maintenance Plan for uniform exposure, no crevices Coat cathode, not anode Flowing preferred to stagnant Smooth flow, not turbulent Remove deposits, maintain cleanliness Beware of C in SS weldments: 304 LC PREN: %Cr +3.3x %Mo + 30x%N Corrosion allowance: uniform only!

Summary: Corrosion and Corrosion Prevention Aqueous corrosion is electrochemical Consider both anode and cathode Behavior specific to metal/environment Design for drainage, avoid stagnation Beware stress corrosion cracking Protect by organic or metallic coating, anodic or cathodic protection, change of pH, use of inhibitors, deaeration