Corrosion andCorrosion andCorrosion PreventionCorrosion Prevention

Metallurgy for the Non-Metallurgist

Learning ObjectivesLearning Objectives• After completing this lesson, students will be able

to:

o Describe the principal types of corrosion including: uniform, galvanic, concentration-cell, pitting, selective leaching, intergranular, erosion, and crevice corrosion

o Explain the significance of the galvanic series in corrosion analysis and prevention

o List the ways to prevent or to minimize corrosion

Introduction: Corrosion and Introduction: Corrosion and Corrosion PreventionCorrosion Prevention

• Corrosive environments, temperature effect, films

• Examine principal types of corrosion• Electrochemical nature of corrosion• Galvanic series• Corrosion prevention

No problemDestruction

Wet ChlorineDry Chlorine

DestructionNo problemIron

Titanium

Interactions may be specific to the material/environment combination.

Direct CorrosionDirect Corrosion

Using the example of iron in an atmosphere containingsulfur dioxide (dry corrosion), the reactions are:

Fe + SO2 = FeS + O2

2Fe + O2 = 2FeO

LEOLoss of electrons is oxidation

GERGain 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 depolarizationoccurs (hydrogen and oxygen combine to form water) corrosion again proceeds.

Basic diagram showing requirements for corrosion of metals. In ametallic conductor, the electrons move in the opposite direction that conventionalcurrent 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 polarizerfor longer battery life.

Different types of corrosion

Common types of corrosionCommon 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 immersedin aqueous solutions of three inorganic acids at room temperature: (a) hydrochloricacid, (b) sulfuric acid, and (c) nitric acid. Note that the scales for corrosionrate are not the same for all three charts. (Source: M. Henthorne, “CorrosionCauses and Control,” Carpenter Technology Corp., Reading, PA, 1972, p 30)

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

PassivationPassivation• 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 themicrostructure (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 301stainless 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 thefuel line. Light area around the crack indicates lack of the protective chromate coating. Original magnification: 2× (c) Micrograph of a polished but unetchedsection through the crack, showing severe intergranular attack and extent of corrosion. Original magnification: 25×

Unetched section, through the bottom of a type 321 stainless steelaircraft 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 outsidesurfaces of a plated copper alloy 260 (cartridge brass, 70%) pipe fordomestic 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 theplating (at arrow). Area B shows uniform attack on the bare inside surface ofthe 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 stainlesssteel 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 type304 stainless steel part, showing branching of cracks as they proceed downwardfrom 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 solutionof 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. Corrosionproducts are present along the entire length of the cracks. Original magnification: 250×

Micrographs of etched specimens that show corrosion on the inside and outsidesurfaces of a buried 356 mm (14 in.) diam low-carbon steel (0.20% C), schedule40, water supply pipe. (a) Smooth surface produced by erosion-corrosion on the innersurface. (b) Pitting corrosion on the uncoated outer surface of the pipe. Both haveoriginal magnification: 115×

Prevention of CorrosionPrevention of Corrosion

• The major types of corrective and preventive measures are:o Change in alloy, heat treatment, or product formo Use of resinous and inorganic-base coatingso Use of inert lubricantso Use of electrolytic and chemical coatings and surface

treatmentso Use of metallic coatingso Use of galvanic protectiono Design changes for corrosion controlo Use of inhibitorso Changes in pH and applied potentialo Continuous monitoring of variables

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

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

Inhibitor SystemsInhibitor Systems• The choice and concentration of inhibitor depend

on the:

o Type of systemo Composition of the electrolyteo Temperatureo Rate of movement of the metal and/or the electrolyteo Presence of residual or applied stresseso Composition of the metalo Presence of dissimilar metals

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

Avoiding CorrosionAvoiding 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 Summary: Corrosion and Corrosion PreventionCorrosion 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