Pitting and Crevice Corrosion

19.2.4 Pitting and Crevice Corrosion

In environments containing appreciable concentrations of Cl − or Br − , in which stainless steels otherwise remain essentially passive, all the stainless steels tend to corrode at specifi c areas and to form deep pits. Ions such as thiosulfate

( SO 2 2 − 3 ) may also induce pitting. In the absence of passivity, such as in deaerated alkali - metal chlorides, nonoxidizing metal chlorides (e.g., SnCl 2 or NiCl 2 ), or oxidizing metal chlorides at low pH, pitting does not occur. This holds even though, in acid environments, general corrosion may be appreciable.

Stainless steels exposed to seawater develop deep pits within a matter of months, with the pits usually initiating at crevices or other areas of stagnant electrolyte ( crevice corrosion ). Susceptibility to pitting and crevice corrosion is greater in the martensitic and ferritic steels than in the austenitic steels; it decreases in the latter alloys as the nickel content increases. The austenitic 18 – 8 alloys containing molybdenum (types 316, 316L, 317) are still more resistant to seawater; however, crevice corrosion and pitting of these alloys eventually develop within a period of 1 – 2.5 years.

Stainless steels exposed at room temperature to chloride solutions containing active depolarizing ions, such as Fe 3+ , Cu 2+ , or Hg 2+ , develop visible pits within hours. These solutions have sometimes been used as accelerated test media to assess pitting susceptibility.

Many extraneous anions, some more effective than others, act as pitting inhibitors when added to chloride solutions. For example, as mentioned in Section

6.6 , addition of 3% NaNO 3 to a 10% FeCl 3 solution completely inhibited pitting

STAINLESS STEELS

of 18 – 8 stainless steel, as well as avoiding general attack, over a period of at least

25 years. Hence, so long as passivity does not break down at crevices because of dissolved oxygen depletion, or for other reasons, localized corrosion does not initiate in inhibited solutions no matter how long the time. Similarly, in neutral chlorides (e.g., NaCl solutions), addition of alkalies inhibits pitting. In aerated 4% NaCl solution at 90 ° C, at which temperature the pitting rate of 18 – 8 is highest, addition of 8 g NaOH per liter was found to eliminate pitting [40] . In

refrigerating brines, addition of 1% Na 2 CO 3 was effective for at least 5 years [41] . Pits develop more readily in a stainless steel that is metallurgically inhomo- geneous. Similarly, the pitting tendency of an austenitic steel increases when the alloy is heated briefl y in the carbide precipitation (sensitization) range. Pitting resulting from crevice corrosion is also favored whenever a stainless steel is covered by an organic or inorganic fi lm or by marine fouling organisms, which partially shield the surface from access to oxygen. The movement of fl owing seawater tends to keep the entire surface in contact with aerated water and uni- formly passive, reducing any tendency for localized corrosion.

Pitting corrosion is usually considered to consist of two stages:

19.2.4.1 Theory of Pitting.

1. Initiation, in which the integrity of the passive fi lm is breeched at localized areas and pits form.

2. Propagation of the pit, at a rate that is often found to increase with time, because of the increasing acidity inside the pit.

The initiation of pits on an otherwise fully passive 18 – 8 surface, as discussed earlier (Section 6.5.2 ), requires that the corrosion potential exceed the critical potential [0.21 V(S.H.E.) in 3% NaCl]. The oxygen potential in air at pH 7 (0.8 V) or the ferric – ferrous potential ( φ ° = 0.77 V) is suffi ciently noble to induce pitting. However, the stannic – stannous ( φ ° = 0.15 V) and the chromic – chromous poten- tials ( φ ° = − 0.41 V) are too active; hence, pitting of 18 – 8 stainless steels is not observed in deaerated stannic or chromic chloride solutions. In suffi cient concentration, the nitrate ion shifts the critical potential to a value that is noble to the ferric – ferrous oxidation – reduction potential; hence, pitting in

3% NaNO 3 + 10% FeCl 3 is not observed. Other anions shift the critical potential similarly, with their effectiveness in this regard decreasing in the order OH − > NO 3 − > SO 4 2 − > ClO − 4 . Increasing amounts of alloyed chromium, nickel, molybdenum, and rhenium in stainless steels also shift the critical potential in the noble direction, accounting for increased resistance to pitting.

In addition to the critical pitting potential (CPP) (noble to which pits initi- ate), critical pitting temperatures (CPT) have been measured. Stable pitting occurs at temperatures above the CPT [42] . Below this temperature, stable pitting does not take place at any potential. The CPT has been explained as the

352 ALLOYING FOR CORROSION RESISTANCE; STAINLESS STEELS

temperature below which the rapid dissolution required for pitting cannot take place.

Once a pit initiates, a passive – active cell is set up of 0.5 − 0.6 V potential dif- ference. The resultant high current density accompanies a high corrosion rate of the anode (pit) and, at the same time, polarizes the alloy surface immediately surrounding the pit to values below the critical potential. Through fl ow of current, chloride ions transfer into the pit forming concentrated solutions of Fe 2+ , Ni 2+ , and Cr 3+ chlorides, which, by hydrolysis, account for an acid solution (Fig. 19.5 ). The measured pH of the confi ned anodic corrosion products of 18 – 8 stainless

steel in 5% NaCl at 200 A/m 2 (0.02 A/cm 2 ) is 1.5 [43] . The high Cl − concentration and low pH ensure that the pit surface remains active. At the same time, the high specifi c gravity of such corrosion products causes leakage out of the pit in the direction of gravity, inducing breakdown of passivity wherever the products come into contact with the alloy surface. This accounts for a shape of pits elon- gated in the direction of gravity, as is often observed in practice. An 18 – 8 stain- less - steel sheet exposed to seawater for one year was observed to develop an elongated narrow pit reaching 60 mm (2.5 in.) from its point of origin (Fig. 19.6 ). The mechanism of growth was demonstrated in the laboratory [43] by continu-

ously fl owing a fi ne stream of concentrated FeCl 2 solution over an 18 – 8 surface slightly inclined to the vertical and totally immersed in FeCl 3 , resulting in forma- tion of a deep groove under the FeCl 2 stream within a few hours (Fig. 19.6 ). A similar groove does not form on an iron surface, because a passive – active cell is not established.

A pit stops growing only if the surface within the pit is again passivated, bringing the pit and the adjacent alloy to the same potential. Extraneous anions, such as SO 2 4 − , have no effect; on the other hand, dissolved oxygen or passivator ions (e.g., NO − 3 ) reinitiate passivity on entering a pit. Successful repassivation depends on factors such as pit geometry and stirring rate. The factors accounting for rates of corrosion at crevices follow the same principles as those described for pit growth. The higher the electrolyte conductiv- ity and the larger the cathode area outside the crevice, the higher the rate of attack at the anode. The initiation of crevice corrosion, however, does not depend

Figure 19.5. Passive – active cell responsible for pit growth in stainless steel exposed to chlo-

STAINLESS STEELS

(b) Figure 19.6. ( a ) Elongated pit in 18 – 8 stainless steel specimen 75

(a)

× 125 mm (3 × 5 in.) exposed to Boston Harbor seawater for 1 year; pit began at crevice formed between bakelite rod and interior surface of hole. ( b ) Artifi cial elongated pit formed by fl owing 50% FeCl 2 in a fi ne

stream over 18 – 8 stainless steel immersed in 10% FeCl 3 , 4 hr.

on exceeding the critical pitting potential. Instead, it depends only on factors infl uencing the breakdown of passivity within the crevice. This breakdown may occur, for example, by oxygen depletion in the crevice caused by slow uniform alloy corrosion, followed by setting up a differential aeration cell that results in accumulation of acid anodic corrosion products in the crevice. These changes in electrolyte composition eventually destroy passivity, thereby establishing a still larger potential difference between active metal in the crevice and passive metal outside, analogous to cells operating in pitting corrosion. This mechanism for initiation of crevice corrosion indicates that chlorides are not essential to its occurrence, accounting for the observation that crevice corrosion occurs in solu- tions of sulfates, nitrates, acetates, and so on, as well as of chlorides.

Cathodic protection effectively avoids crevice corrosion, provided that the alloy surrounding the crevice is polarized to the open - circuit potential of the active (nonpassive) alloy surface within the crevice. This contrasts with the more lenient requirement of polarizing below the critical potential in order to avoid pit initiation.

Alloy additions that are effective in helping retain passivity in the presence of both low dissolved oxygen concentration and acid corrosion products help reduce or avoid crevice attack. Additions of molybdenum to 18 – 8 stainless steel (type 316) and palladium additions to titanium (see Fig. 25.2 , Section 25.2 ) are

354 ALLOYING FOR CORROSION RESISTANCE; STAINLESS STEELS

There are four princi- pal ways of reducing or avoiding pitting corrosion:

19.2.4.2 Reducing or Avoiding Pitting Corrosion.

1. Cathodically protect at a potential below the critical pitting potential. An impressed current can be used; or in good conducting media (e.g., sea- water), stainless steel can be coupled to an approximately equal or greater area of zinc, iron, or aluminum [44] . Austenitic stainless steels used to weld mild - steel plates, or 18 – 8 propellers on steel ships, do not pit.

2. Add extraneous anions (e.g., OH − or NO 3 − ) to chloride environments.

3. Reduce oxygen concentration of chloride environments (e.g., NaCl solutions).

4. For type 304, operate at the lowest temperature possible. *

There are three ways to reduce or avoid crevice corrosion:

19.2.4.3 Reducing or Avoiding Crevice Corrosion.

1. Avoid crevices between metals or between metals and nonmetals. Periodi- cally remove contaminating surface fi lms using alkaline cleaners with stainless - steel wool or the equivalent.

2. Avoid stagnant solutions. Circulate, stir, and aerate electrolytes in contact with stainless steels. Ensure uniform composition of electrolyte at all regions of the metal surface.

3. Cathodically protect, ideally to the potential of corroding active metal at the crevice. Approaching such a potential reduces the corrosion rate, but not to zero. In seawater, use of sacrifi cial iron and similar less - noble - metal anodes have proved useful [45] .