An electrochemical corrosion cell can be created with a single, homogeneous piece of metal if it contacts a corrosive solution that differs in concentration at different places on the metal surface.
The situation can be sketched as a laboratory cell, shown in Figure 6.1a. The semipermeable membrane shown in the figure is simply a laboratory device to prevent mixing of the two solutions while allowing ionic conductivity. In real situations its equivalent is lack of stirring of the solutions, perhaps with a difference in density that tends to keep the two solutions separated (Figure 6.1b).
corroding metal
electrolyte 1
semipermeable membrane electrolyte 2
a.
electrolyte 2
corroding tank
electrolyte 1
b.
Figure 6.1 Electrochemical corrosion cell consisting of one metal in two connected electrolyte concentrations. (a) laboratory version; (b) more realistic situation.
Concentrati
Concentration on DifferencesDifferences
The most common cathode reactions in aerated solutions involve oxygen reduction. A corrosive solution with high oxygen concentration in one region and a lower oxygen concentration elsewhere will set up a localized oxygen concentration cell with the cathode being the metal in contact with the high-oxygen solution. The metal corrodes in the low-oxygen region.
A bare pipe buried at about the level of the ground's water table is an example. The upper part of the pipe contacts well-aerated soil while the lower part, in waterlogged soil, corrodes. A film of moisture coats all the soil particles and serves as the electrolyte.
Unmixed salt solutions can show the same effect. A heavy brine in the bottom of a tank has very low solubility for oxygen, but a dilute solution at the top of the tank will dissolve oxygen and become the cathode reactant. The anode will be the metal contacting the dense brine.
On the other hand, unstirred solutions of oxidizing salts (FeCl3, K 2Cr2O7, and the like) will make a cathode of the metal contacting the stronger oxidizer solution because the cathode reaction will be
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70 Environmental Effects Chapter 6Environmental Effects
reduction of the oxidizer. Similarly with unmixed acids: metal contacting the more concentrated acid will be the cathode.
Question
Question: A pipe carrying plant waste always runs about half full. The pipe is found to be severely thinned inside at the 3 and 9 o'clock positions with very little rusting at top and bottom. Explain.
Answer
Answer: Oxygen pickup at the liquid surface has created a cathode region. The pipe wall just below the cathode will be the anode region (the distance effect puts anode very near cathode). This situation is often called "waterline corrosion."
Crevice Corrosion Crevice Corrosion
Over time, crevices develop extremely corrosive conditions where attack is concentrated. Crevices can be created in three different ways:
• geometric crevices: metal-to-metal contact where solution can get between the two pieces of metal, such as in threaded connections.
• metal/absorbent nonmetal: the absorbent material holds moisture continually against the metal surface. Examples are wood bolted to metal, and sponge rubber gaskets in pipe joints.
• deposit corrosion: deposits of sediment, lime, rust, and the like on metal surfaces create a crevice.
Deposits that are porous and absorbent often create the worst conditions.
As Figure 6.2 schematically illustrates, with moisture initially in the crevice the corrosion occurs everywhere on the metal surface, in and out of the crevice (Figure 6.2a).
When all oxygen has been used up in the crevice (Figure 6.2b), the production of metal ions can still continue. At this point cathode and anode areas separate: the anode continues inside the crevice, while the cathode reaction occurs on the outer surface where oxygen is plentiful. The situation inside the crevice continues to change, first with precipitation of insoluble corrosion products, such as metal hydroxides, that tend to restrict oxygen diffusion into the crevice even more, and secondly by the migration of Cl- ions into the crevice, attracted by the large concentration of metal ions there.
Hydrolysis of the metal chloride solution then occurs. By reaction with water,
MCl + H2O→ HCl + MOH→ H+ + Cl- + MOH↓ , the Cl- is released to form more metal chloride as acidity increases in the crevice. The acid encourages faster anode reaction by cleaning the surface of corrosion products.
Crevice corrosion gradually develops as the oxygen concentration cell becomes established. Chloride ions are not essential to creation of acidity inside the crevice, but because they are so common and diffuse faster than any other anion (except OH- which eliminates itself by precipitation as an insoluble corrosion product), Cl- is usually involved. An example of crevice corrosion is shown in Figure 6.3.
Preventing crevice corrosion usually simply involves preventing crevices. Weld two pieces of metal together instead of riveting or bolting, seal crevices with soldering or caulking, use nonabsorbent gaskets, use continual agitation to prevent solids from settling, and frequently clean equipment.
Chapter 6 Environmental EffectsEnvironmental Effects 7171
a.
d.
b. c.
Cl
Cl
Cl
-M
M
M M
OH
OH
OH
OH
OH OH
O H
OH Cl
Cl M MOH
C l -M +
M+ M+
H + Cl
C l M M
M H O2
O2
O2
O2
O2
O2
O2
O2
O2
Figure 6.2 Development of crevice corrosion.
(a) Initial situation: oxygen-containing solution inside crevice.
(b) Depletion of oxygen after corrosion begins.
(c) Migration of Cl- into crevice and formation of solid corrosion products.
(d) Hydrolysis of metal chlorides to create acidic conditions in crevice.
Figure 6.3 Crevice corrosion beneath an absorbent gasket on stainless steel valve. (ref. 11).
Pitting Pitting
Pitting occurs when a protective surface layer is damaged so that the underlying metal is attacked locally. By some definitions a pit must be at least half as deep as it is wide (hemispherical or deeper),
76
76 Environmental Effects Chapter 6Environmental Effects Deposit corrosion:
Deposit corrosion: crevice corrosion under sediments, microbial colonies, corrosion products, lime, and other such nonmetallic precipitates and debris.
Dew point corrosion:
Dew point corrosion: see Condensate corrosion.
Hydrolysis:
Hydrolysis: reaction of a compound with water where the anion of the compound bonds to the H+ of the water and the cation bonds to the OH-.
Microbial corrosion:
Microbial corrosion: localized corrosion under deposits of bacterial colonies, algae, or fungi.
Oxidizer:
Oxidizer: chemical substance which by a reduction reaction facilitates oxidation of another substance.
Oxygen concentration cell:
Oxygen concentration cell: a higher concentration of dissolved oxygen in one region creates a cathode at the metal surface while a lower concentration at another region creates an anode.
Rebar:
Rebar: reinforcing bar, usually steel, embedded in concrete for added resistance to fracture.
Semipermeable membrane:
Semipermeable membrane: a porous material allowing diffusion of ions through its structure but blocking ionic transport by flow.
Stray current corrosion:
Stray current corrosion: corrosion caused by discharge of direct current that was accidentally received by a metal.
Tubercle:
Tubercle: knob of rust shielding a colony of iron-oxidizing bacteria.
Questions Questions
1. If a bare, buried pipe had a pit on the outside at the 12 o'clock position, how could that have happened?
a. A lump of clay touches pipe at 12 o'clock.
b. The pipe is nearly but not quite full of corrosive solution.
c. Splashing or condensation left droplets of corrosive solution at 12 o'clock.
d. A piece of wood or tree root contacts pipe at 12 o'clock.
e. Soil is more aerated at 12 than at 6 o'clock.
2. Couldn't metal contacting a nonabsorbent nonmetal create a crevice corrosion situation?
a. It doesn't; however, most nonmetallic materials are absorbent.
b. Absorbent nonmetallics will hold moisture against the metal longer.
c. Absorbent nonmetallics will wick away moisture from the metal.
d. Absorbent nonmetallics release some H+ into solution.
e. Absorbent nonmetallics create a tighter crevice.
3. Can hydrogen reduction be the cathode reaction in crevice corrosion?
a. Yes. H+ becomes depleted in crevice which then serves as anode.
b. Yes. H+ becomes depleted outside crevice, with reduction continuing in crevice.
c. No. H+ becomes depleted outside crevice, with reduction continuing in crevice.
d. No. H+ diffuses so fast it never is depleted in crevice.
4. Can noble metals pit?
a. Yes. Passive films could become damaged locally.
b. Yes. Crevice corrosion could create pits.
c. Yes. Droplets of condensate could create pits.
d. No. True pitting requires passive film.
e. No. Passive films are too tenacious.