1. Corrosion and Corrosion Prevention
Metallurgy for the Non-Metallurgist

2. 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

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

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

5. 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

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

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

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

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

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

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

12. 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.

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

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

16. Different types of corrosion

17. 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

18. 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)

19. 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×

21. 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

22. (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×

23. 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×

24. 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×

26. 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×

27. 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 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×

28. (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×

29. 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×

30. 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×

31. 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×

32. 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×

33. 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

35. (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.

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

37. 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

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

39. 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!

40. 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