Chapter 17 -

Assist.  Prof.  Dr.  İlkay  KALAY  Materials  Science  and  Engineering  Department  

Çankaya  University    

MSE  226    ENGINEERING  MATERIALS  

(SECTIONS  01  &  02)  

Corrosion  and  DegradaLon  

Chapter 17 - 2

STANDARD EMF SERIES • EMF series

Au Cu Pb Sn Ni Co Cd Fe Cr Zn Al Mg Na K

+1.420 V +0.340 - 0.126 - 0.136 - 0.250 - 0.277 - 0.403 - 0.440 - 0.744 - 0.763 - 1.662 - 2.363 - 2.714 - 2.924

metal V metal o

Data based on Table 17.1, Callister 7e.

mor

e an

odic

m

ore

cath

odic

metal o

• Metal with smaller V corrodes. • Ex: Cd-Ni cell

ΔV = 0.153V

o

Adapted from Fig. 17.2, Callister 7e.

-

1.0 M Ni 2+ solution

1.0 M Cd 2 + solution

+

25°C Ni Cd

Chapter 17 - 3

Passivition Passivity •  A number of metals and alloys passivate, or lose their

chemical reactivity, under some environmental circumstances. This phenomenon is thought to involve the formation of a thin oxide film on the metal surface which serves as a protective barrier to further corrosion.

•  the formation of a film of atoms or molecules on the surface of an anode so that corrosion is slowed down or stopped.

•  Passivity is displayed by Cr, Fe, Ni, Ti, and many of their alloys.

•  Examples: •  Stainless steels and aluminum alloys exhibit this type of

behavior. –  α-alumina film formed on Al surface –  Stainless Steels (contains Cr) forms CrO3 on SS surface

Chapter 17 - 4

Types of Corrosion

Types of corrosion can be classified according to the appearance of the corroded metal 1)  Uniform or general attack corrosion

2)  Galvanic or two-metal corrosion 3)  Pitting corrosion 4)  Crevice corrosion 5)  Intergranular corrosion 6)  Stress corrosion 7)  Erosion corrosion 8)  Selective leaching or dealloying

Chapter 17 - 5

• Uniform Attack Oxidation & reduction occur uniformly over surface.

• Selective Leaching Preferred corrosion of one element/constituent (e.g., Zn from brass (Cu-Zn)).

• Stress corrosion Stress & corrosion work together at crack tips.

• Galvanic Dissimilar metals are physically joined. The more anodic one corrodes.(see Table 17.2) Zn & Mg very anodic.

• Erosion-corrosion Break down of passivating layer by erosion (pipe elbows).

FORMS OF CORROSION

Forms of

corrosion

• Crevice Between two pieces of the same metal.

Fig. 17.15, Callister 7e. (Fig. 17.15 is courtesy LaQue Center for Corrosion Technology, Inc.)

Rivet holes

• Intergranular Corrosion along grain boundaries, often where special phases exist.

Fig. 17.18, Callister 7e.

attacked zones

g.b. prec.

• Pitting Downward propagation of small pits & holes.

Fig. 17.17, Callister 7e. (Fig. 17.17 from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.)

Chapter 17 - 6

Galvanic Corrosion Galvanic corrosion occurs when two metals or alloys having different compositions are electrically coupled while exposed to an electrolyte. The more anodic one corrodes. Zn & Mg very anodic

A number of measures may be taken to significantly reduce the effects of galvanic corrosion. These include the following: 1. If coupling of dissimilar metals is necessary, choose two that are close together in the galvanic series. 2. Avoid an unfavorable anode-to-cathode surface area ratio; use an anode area as large as possible. 3. Electrically insulate dissimilar metals from each other. 4. Electrically connect a third, anodic metal to the other two; this is a form of cathodic protection.

Chapter 17 - 7

Pitting

Pitting is another form of very localized corrosion attack in which small pits or holes form.

They ordinarily penetrate from the top of a horizontal surface downward in a nearly vertical direction.

It is an extremely insidious type of corrosion, often going undetected and with very little material loss until failure occurs.

The pitting of a 304 stainless steel plate by an acid-chloride solution

It has been observed that specimens having polished surfaces display a greater resistance to pitting corrosion. Stainless steels are somewhat susceptible to this form of corrosion; however, alloying with about 2% molybdenum enhances their resistance significantly.

Chapter 17 - 8

Pitting

A corrosion pit on the outside wall of a pipeline at a coating defect before and after abrasive blasting.

Chapter 17 - 9

•  Found in solid solution alloys and occurs when one element or constituent is preferentially removed as a consequence of corrosion processes.

•  Selective leaching may also occur with other alloy systems in which aluminum, iron, cobalt, chromium, and other elements are vulnerable to preferential removal.

Weld decay in a stainless steel. The regions along which the grooves have formed were sensitized as the weld cooled.

Selective Leaching

Chapter 17 - 10

Selective Leaching •  The most common example is the dezincification of

brass, in which zinc is selectively leached from a copper–zinc brass alloy.

Chapter 17 - 11

Selective Leaching

Chapter 17 - 12

Erosion–Corrosion Erosion–corrosion arises from the combined action of chemical attack and mechanical abrasion or wear as a consequence of fluid motion. Virtually all metal alloys, to one degree or another, are susceptible to erosion–corrosion. It is especially harmful to alloys that passivate by forming a protective surface film; the abrasive action may erode away the film, leaving exposed a bare metal surface.

Grooves Structure

Chapter 17 - 13

Erosion–Corrosion If the coating is not capable of continuously and rapidly reforming as a protective barrier, corrosion may be severe. Relatively soft metals such as copper and lead are also sensitive to this form of attack. One of the best ways to reduce erosion–corrosion is to change the design to eliminate fluid turbulence and impingement effects.

Grooves Structure

Chapter 17 - 14

Stress Corrosion Stress corrosion, sometimes termed stress corrosion cracking, results from the combined action of an applied tensile stress and a corrosive environment; both influences are necessary. Most stainless steels stress corrode in solutions containing chloride ions, whereas brasses are especially vulnerable when exposed to ammonia.

Impingement failure of an elbow that was part of a steam condensate line.

Photomicrograph showing intergranular stress corrosion cracking in brass.

Chapter 17 - 15

Stress Corrosion Best measure to take in reducing or totally eliminating stress corrosion is to lower the magnitude of the stress. This may be accomplished by reducing the external load or increasing the cross-sectional area perpendicular to the applied stress. Furthermore, an appropriate heat treatment may be used to anneal out any residual thermal stresses.

Impingement failure of an elbow that was part of a steam condensate line.

Chapter 17 - 16

Hydrogen Embrittlement Various metal alloys, specifically some steels, experience a significant reduction in ductility and tensile strength when atomic hydrogen (H) penetrates into the material. This phenomenon is aptly referred to as hydrogen embrittlement; the terms hydrogen-induced cracking and hydrogen stress cracking are sometimes also used. Hydrogen embrittlement is a type of failure; in response to applied or residual tensile stresses, brittle fracture occurs catastrophically as cracks grow and rapidly propagate.

Hydrogen Induced Cracks (HIC)

Chapter 17 - 17

Hydrogen Embrittlement High-strength steels are susceptible to hydrogen embrittlement, and increasing strength tends to enhance the material’s susceptibility. Martensitic steels are especially vulnerable to this type of failure; bainitic, ferritic, and spheroiditic steels are more resilient. Furthermore, FCC alloys (austenitic stainless steels, and alloys of copper, aluminum, and nickel) are relatively resistant to hydrogen embrittlement, mainly because of their inherently high ductilities.

To reduce hydrogen embrittlement include reducing the tensile strength of the alloy via a heat treatment, removal of the source of hydrogen, “baking” the alloy at an elevated temperature to drive out any dissolved hydrogen, and substitution of a more embrittlement- resistant alloy.

Chapter 17 - 18

Corrosive environments include the atmosphere, aqueous solutions, soils, acids,

bases, inorganic solvents, molten salts, liquid metals, and, last but not least, the human body.

Chapter 17 - 19

Corrosion Control

Chapter 17 - 20

• Self-protecting metals! -- Metal ions combine with O to form a thin, adhering oxide layer that slows corrosion. • Reduce T (slows kinetics of oxidation and reduction) • Add inhibitors -- Slow oxidation/reduction reactions by removing reactants (e.g., remove O2 gas by reacting it w/an inhibitor). -- Slow oxidation reaction by attaching species to the surface (e.g., paint it!).

CONTROLLING CORROSION Metal (e.g., Al, stainless steel)

Metal oxide

Adapted from Fig. 17.22(a), Callister 7e. (Fig. 17.22(a) is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Co., 1986.)

steel pipe

Mg anode

Cu wire e -

Earth Mg 2+

e.g., Mg Anode

• Cathodic (or sacrificial) protection -- Attach a more anodic material to the one to be protected.

Adapted from Fig. 17.23, Callister 7e. steel

zinc zinc Zn 2+

2e - 2e -

e.g., zinc-coated nail

Chapter 17 - 21

-- Use metals that passivate - These metals form a thin, adhering oxide layer that slows corrosion.

• Lower the temperature (reduces rates of oxidation and reduction)

CORROSION PREVENTION (i)

Metal (e.g., Al, stainless steel)

Metal oxide

• Apply physical barriers -- e.g., films and coatings

• Materials Selection -- Use metals that are relatively unreactive in the corrosion environment -- e.g., Ni in basic solutions

Chapter 17 - 22

• Add inhibitors (substances added to solution that decrease its reactivity) -- Slow oxidation/reduction reactions by removing reactants (e.g., remove O2 gas by reacting it w/an inhibitor). -- Slow oxidation reaction by attaching species to the surface.

CORROSION PREVENTION (ii)

Adapted from Fig. 17.22(a), Callister & Rethwisch 8e.

Using a sacrificial anode

steel pipe

Mg anode

Cu wire e -

Earth Mg 2+

• Cathodic (or sacrificial) protection -- Attach a more anodic material to the one to be protected.

Adapted from Fig. 17.23, Callister & Rethwisch 8e. steel

zinc zinc Zn 2+

2e - 2e -

e.g., zinc-coated nail

Galvanized Steel

e.g., Mg Anode

Chapter 17 - 23

Design 8 engineering design rules that are important to the reduction or prevention of corrosion are:

.

Allow for the penetration action of corrosion along with the mechanical strength requirements when determining the appropriate metal thickness

1

Weld rather than rivet containers to reduce crevice corrosion. If rivets are used, choose a rivet material that is cathodic to the materials being joined.

2

If possible, use galvanically similar metals for the entire structure. Avoid dissimilar metals that can cause galvanic corrosion. If galvanically dissimilar metals are bolted together, separate them with nonmetallic gaskets and washers.

3

Avoid excessive stress and stress concentrations in corrosive environments to prevent stress-corrosion cracking, especially when using susceptible materials such as stainless steels and brasses.

4

Chapter 17 - 24

Design… Cont., 8 engineering design rules that are important to the reduction or prevention of corrosion are:

Avoid sharp bends in piping systems to prevent erosion corrosion.

5

Design tanks and other containers for easy draining and cleaning.

6

Design systems for easy removal and replacement of parts that are expected to fail in service, such as pumps in chemical plants.

7

Design heating systems such that hot spots do not occur.

8

Chapter 17 - 25

CONTROLLING CORROSION Preventing the Corrosion of Iron To prevent the corrosion of iron, the interaction of iron surface with oxygen and water must be avoided. -  Surface covering with a paint or tin, greasing and oiling is a

commonly used way to prevent corrosion. However, if the coating is broken and the iron is exposed to oxygen and water, corrosion will begin.

-  Galvanizing in which the iron is coated with a thin layer of zinc is a commonly used technique to prevent the corrosion. The iron is dipped into hot liquid zinc and zinc forms an impervious oxide layer that further protects the iron. Galvanizing uses the electrochemistry to protect the iron even after the surface coat is broken.

Chapter 17 - 26

SUMMARY • Metallic corrosion involves electrochemical reactions -- electrons are given up by metals in an oxidation reaction -- these electrons are consumed in a reduction reaction • Metals and alloys are ranked according to their corrosiveness in standard emf and galvanic series. • Temperature and solution composition affect corrosion rates. • Forms of corrosion are classified according to mechanism • Corrosion may be prevented or controlled by: -- materials selection -- reducing the temperature -- applying physical barriers -- adding inhibitors -- cathodic protection

Chapter 17 - 27

Reading:

Core Problems:

Self-help Problems:

ANNOUNCEMENTS