First consider whether the metal is generally corroded over all its exposed surface or whether the attack is localized, with some surface undamaged. Corrosion products may have to be removed before it is evident that the metal underneath is damaged. Particularly in flowing systems, rust or other solid corrosion products may deposit on areas that are unattacked, while corroded areas may be out of sight. Most corrosion, however, is general in nature, with all exposed metal surface corroding.
Example: An engineer was asked to select a better material for a critical stainless steel heat
exchanger that had had to be replaced four times. Looking at one of the exchangers on the scrap heap, he saw the tubes were plugged with rust, but under the rust was bright, uncorroded stainless steel. The rust had come from carbon steel equipment upstream. He replaced this steel with a more resistant alloy and the plant operated with scrapped, cleaned heat exchangers for many years.
Types of Corrosion
Corrosion is usually electrochemical, and corrosion in aqueous environments is nearly always electrochemical. Corrosion of metals in most organic liquids cannot be electrochemical, however, because the organic is not an ionic conductor; it is not an electrolyte. In this case the organic molecules exchange electrons directly with metal atoms at the metal surface. The same direct process can occur with metal in some molten salts. Dissolution of a solid metal in a molten metal is another obvious example where the environment is not reduced at a cathode site on the metal surface.
Corrosion in gases is quite complex. A gas molecule may react directly with a metal but if the oxide product builds up on the metal surface, the oxide acts as electronic and ionic conductor, and also as a cathode. All these special corrosion processes will be discussed in later chapters: corrosion in molten salts, molten metals, and gases in Chapter 9, and organic liquids in Chapter 10.
In electrochemical corrosion, if anode sites move around so that the entire surface is fairly evenly removed, the result is uniform attack or general corrosion.
Cathode Processes
The corrosion of a metal surface that is sometimes anode, sometimes cathode, is commonly controlled by the reaction rate of the cathode reaction. Cathode processes are rather involved: the reactant in
the environment (dissolved oxygen, for example) must first diffuse through the solution to the cathode surface, with diffusion rates commonly much slower than reaction rates. Secondly, this reactant must adsorb on the metal, a process involving a slight electron rearrangement for both the metal surface and the adsorbing species, so that the reactant sticks to the surface.
Next comes the reaction itself: transfer of electrons from the metal to the adsorbed species and rearrangement of chemical bonding (for O2, breaking of the O-O bond and forming O-H bonds with
hydrogens of adjacent water molecules). Desorption, the release of adsorbed species (OH- ions for O2
reduction), may then follow. If H+ is being reduced, once two H atoms combine, the H2 molecule can
bubble off immediately. The steps in cathode reaction are shown schematically for oxygen reduction in Figure 4.1. M M M M M M a. b. c. d. e. f. H O H H O H H O H H O H H O H H O H H O H H O H H O H H O H H O H H O H H O H H O H H O H O HO O H H O H O H O H O O O O O O O O O e e
Figure 4.1 Steps involved in oxygen reduction at cathode. (a) Dissolved oxygen diffuses toward cathode surface.
(b) Oxygen molecule becomes physically attached to cathode surface. (c) Oxygen-oxygen bond weakens; bonding electrons in metal adjust.
(d) Oxygens receive electrons from metal, thus attracting hydrogens in water. (e) Hydroxide ions form.
(f) Desorption of hydroxide ions.
Anode Processes
Meanwhile, elsewhere on the surface the anode reaction is occurring: simply the release of metal ions from the metal surface, leaving the valence electrons behind in the metal. As a metal ion leaves the surface it is immediately surrounded by water molecules, the negative (oxygen) ends of the molecules being attracted to the positive metal ion. This is shown schematically in Figure 4.2.
Question: Could diffusion ever become the rate-controlling process for the anode reaction?
Answer: Seldom, but possible. In a restricted environment such as a crevice, a large concentration of
metal ions around the anode might inhibit further anode reaction until they can diffuse away; or solvating water molecules may have to diffuse through porous rust to get to the anode.
M M
M
a. b. c. H O H H O H H O H H O H H O H O H H e e M M H O H H O H H O H e M MFigure 4.2 Steps in metal corrosion at anode. (a) Metal surface in electrolyte prior to oxidation.
(b) Metal ion leaves surface; its valence electron(s) stay behind. (c) Water molecules shield metal ion.
Localized Corrosion
If corrosion products tend to shelter some surface so that water molecules can't reach the surface, the anode reaction is prevented at that site. Then only localized corrosion occurs.
Localized corrosion, whether pitting, crevice corrosion, intergranular corrosion, or some other concentrated attack, quickly becomes a much more serious problem than uniform corrosion. Intensifying attack at one or just a few locations will greatly weaken the metal very quickly. A pinhole in a pipe will not have removed much metal but a flammable liquid spraying out is prelude to disaster.
Localized corrosion is much more difficult to detect than uniform attack. Examining the general condition of the inside of a pipe is awkward; examining every square inch inside for signs of localized attack is impractical in most cases. Often the first indication of trouble immediately becomes a panic situation.
Electrochemical cells that set up so that one place on the metal surface is continually anodic while other places are cathodic becomes the cause of localized attack. These cells can be of two types: (1) the metal differs from one place to another so that some metal is anodic and corrodes while other metal is less corrodible and becomes the cathode, assisting the anode to corrode, or (2) one uniform metal is in contact with two different electrolytes with one more corrodible than the other. The environment differs from place to place on the metal surface.
These two types of localized attack will be discussed separately because the remedies involved in thwarting these two types of cells can be quite different.
Mechanical Action
The worst situation that corrosion personnel must face, however, is the combination of corrosion (usually localized) along with mechanical action. Stresses operating along with corrosion processes may cause sudden failure, even though the stresses may have been below the metal's yield strength and the corrosion may have been minimal. Corrosion fatigue and erosion-corrosion are examples of synergistic action of stresses with corrosion that can bring extremely rapid, unexpected failure.
Question: Is combination with mechanical action, such as corrosion fatigue, caused more by fatigue
or by corrosion?
Answer: It may be 90% fatigue, or 50-50, or 90% due to corrosion, but the result is much more rapid
Because the rate usually changes with time, you must specify the duration of any corrosion test. For example "...uniform corrosion of 0.2 mm/y in a two-month test." This same material could very well have a corrosion rate of 0.1 mm/y in a one-year test.