Document intercorr (en anglais)
| Effet du chlore sur la corrosion des inox dans de l’eau potable | ||||
| Résiduel de chlore | Profondeur maximale de l’attaque Localisé et généralisé Type 304 et Type 316 |
|||
| mg/l | Moyenne | Localisée | ||
| 304 (µm/an) | 316 (µm/an) | 304 (µm/an) | 316(µm/an) | |
| 0 | 0 | 0 | 0 | 0 |
| 0,8-1,0 | 0 | 0 | 0 | 0 |
| 2 | 0 | 0 | 0 | 0 |
| 3 à 5 | 300 | 100 – 400 | 1000-50000 | 300-1000 |
These data indicate that in the 3-5 mg/1-residual chlorine range, Tape 304L becomes vulnerable to crevice corrosion if the surface is not maintained clean and free of deposits, heat tint scale, embedded iron and other injurious defects. Type 316L can be expected to be more resistant to -corrosion in the 3-5 mg/l range of residual chlorine. Since residual chlorine may be up to 1.8 mg/l in finished water at the PWTP in order to meet the 0.2 mg/l minimum at point of use, Type 304L is a good material for finished water and distribution systems.
Chlorine can also have an indirect effect on stainless steel performance that is associated with the precipitating of iron and manganese discussed earlier. In a few inland power plants where there can be substantial amounts of the gallionella a strain of bacteria, the combination of gallionella-manganese-chlorine has led to extensive pitting of stainless steel tubing in a few heat exchangers and condensers. Although this type of corrosion has not been reported in PWTPs, special precautions would be warranted when substantial quantities of the rather uncommon gallionella strain of bacteria are known to be present.
Chlorine, but not ozone, has another important effect. In the moist vapors just above the water line, chlorine concentrations may reach concentrations that stain and even pit Type 304 and Type 316 stainless steel. This is more of a cosmetic problem than structural. One water district has been using stainless steel structurals in clearwells since the late 1950s and has not yet encountered the need for replacements. On long exposure, the staining is followed by shallow general pitting over the whole exposed surface. Vapor side staining and general pitting attack have been reported in several PWTPs where pipe galleries were not well ventilated near the chlorine injection area.
- The system be placed in standby at some future date.
- Slope horizontal runs sufficiently so they will drain without leaving a water heel between support points.
- Other guides for the operation and maintenance of stainless steel systems includes the following practices:
- Establish a schedule to flush out sediment and debris on horizontal runs of raw water piping.
- During downtime, drain completely and dry or, alternately, circulate water for one hour daily.
- Dissolve calcium hypochlorite granules before introducing them into or onto the surface of stainless steel piping for the initial disinfecting. Calcium hypochlorite granules dissolve slowly and have created a severe micropitting environment when resting on stainless steel.
- Provide proper ventilation to avoid high chlorine vapor areas.
Copyright IRH Environnement et Jean-Louis ROUBATY 2001
