Ferrous Sulfate Treatment
Ferrous sulfate treatments are not absolutely essential to the successful performance of copper-nickel but can be viewed as a remedy when trouble has occurred or as a precaution if trouble is likely. Most ships in service have operated successfully without any ferrous sulfate dosing.
Ferrous sulfate treatment has been found to suppress corrosion rates of copper-nickel in seawater in both polluted and unpolluted conditions. For commissioning which can last from a few weeks to 3 months, the ferrous sulfate content of the cooling water can be set up to 2 to 3 ppm. As an alternative method to encourage good initial film formation during commissioning, a system can be filled with fresh water containing 5ppm ferrous sulfate and left in the system for 1 day.
After this initial treatment, ferrous sulfate (5 ppm concentration) should be added to the system and recirculated for 1 hour per day throughout the fitting out period. This practice is also useful when systems are either retubed or renewed. During normal service on ships, additional ferrous sulfate dosing is seldom required.
Chlorination treatment and ferrous sulfate treatment should not be carried out simultaneously because chlorine reacts with ferrous sulfate to form ferric sulfate. Since ferrous sulfate is consumed, it is not available to contribute to the development of protective films. The remaining ferric sulfate provides no beneficial effect. An alternative method of releasing ferrous ions into the system is by fitting iron anodes. This however, is more suited to maintaining a protective layer than initial oxide film formation and will also reduce the biofouling resistance as will be explained in the next section.
Although if exposure to known polluted water is going to occur (e.g. when entering port) a reasonable additional precaution would be to add 5ppm ferrous sulfate for one hour per day during the three days prior to entering while in and after leaving port. One treatment a week can be applied throughout prolonged voyages.
Other pretreatment chemicals have been studied or used with variable success. Dimethyldithiocarbomate has been used by the British Navy as well as that of Germany.
Copper Nickel for Seawater Corrosion Resistance and Antifoulin
90-10 and 70-30 Copper-Nickel Alloys
Corrosion Resistance
The Importance of the Surface
General Corrosion Rates
Localised Corrosion
Velocity Effects
Sand Erosion
Galvanic Properties
Handling Sulfides
Ferrous sulfate treatment
Biofouling Resistance
Ease of Biofouling Removal
Reasons for Biofouling Resistance
Boat Hull Experience
Offshore Sheathing
Conclusions
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