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Type 316

Type 316

Type 316

Type 316

Type 316

Type 316

0 20 40 60 80 100 Acid by weight (%)

■ <1 Mils/year 1 <0.03 mm/year 11-5 Mils/year 0.03-0.13 mm/year

5-30 Mils/year 0.13-0.75 mm/year >30 Mils/year >0.75 mm/year

Figure 6. Corrosion resistance of 304 and 316 stainless steels to mineral acids. Source: Reproduced by permission of British Steel Pic.

In the case of stainless steels, although there are several ionic species that will initiate the attack, by far the most common are solutions containing chloride. The presence of salt in virtually all foodstuffs highlights the problem. Low pH values also enhance the propensity for initiation of attack.

Other environmental factors such as temperature and the oxygen or dissolved air content of the process stream all play a role in the corrosion process.

Because the presence of oxygen is a prerequisite for the onset of crevice corrosion (and many other forms of attack), in theoretical terms complete removal of oxygen from a

Figure 7. Crevice corrosion at the interplate contact points of a heat-exchanger plate.
Figure 8. Photomicrograph of a section through a site of crevice corrosion. Note the deep undercutting which is typical of chloride-induced attack on stainless steel.

process stream will inhibit corrosion. In practice, however, this is difficult to achieve. Only in equipment where complete and effective deaeration occurs, such as a multiple effect evaporator operating under reduced pressure, will the beneficial effect of oxygen removal be achieved.

Stainless steels containing molybdenum (316,317) have a much higher resistance to crevice corrosion than do alloys without this element (304, 321, 347). The higher the molybdenum content, the greater the corrosion resistance. For particularly aggressive process streams, titanium is often the only economically viable material to offer adequate corrosion resistance.

Pitting Corrosion. As the name implies, pitting is a form of corrosion that leads to the development of pits on a metal surface. It is a form of extremely localized but intense attack, insidious insofar as the actual loss of metal is negligible in relation to the total mass of metal that may be affected. Nevertheless, equipment failure by perforation is the usual outcome of pitting corrosion. The pits can be small and sporadically distributed over the metal surface (Fig. 9) or extremely close together, close enough, in fact, to give the appearance of the metal having suffered from general attack.

In the case of stainless steels, environments that will initiate crevice corrosion will also induce pitting. As far as the food industry is concerned, it is almost exclusively caused by chloride containing media, particularly at low pH values.

Many theories have been developed to explain the cause of initiation of pitting corrosion (10), and the one feature they have in common is that there is a breakdown in the passive oxide film. This results in ionic migration and the development of an electrochemical cell. There is, however, no unified theory that explains the reason for the film breakdown. Evans (11) for example, suggests that metal dissolution at the onset of pitting may be due to a surface scratch, an emerging dislocation or other defects, or random variations in solution composition. However, propagation of the pit proceeds by a mechanism similar to that occurring with crevice corrosion. Like crevice corrosion, the pits are often undercut and on vertical surfaces may assume an elongated morphology due to gravitational effects (Fig. 10).

The onset of pitting corrosion can occur in a matter of days but frequently requires several months for the development of recognizable pits. This makes the assessment of the pitting propensity of a particular environment very difficult to determine, and there are no short-cut laboratory testing techniques available. Methods and test solutions are available to rank alloys; the best known and most frequently quoted is ASTM Standard G48 (12), which employs 6% ferric chloride solution. Another chemical method involving ferric chloride determines the temperature at

Figure 9. Pitting corrosion of a stainless-steel injector caused by the presence of hydrochloric acid in the steam supply.
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