Industrial Water Engineering, Inc.

CHEMICAL WATER TREATMENT

toll free 1-800-543-6519

GSA # GS-10F-7884A (1/31/2007)

DOD Cage # 0MV90

 

 

Cooling water chemistry must be maintained to prevent corrosion.  Corrosion can occur from several factors.  Galvanic corrosion can occur when two dissimilar metals are connected and a strong electrolyte (tower water) is present.  The anode corrodes and electrons flow to the cathode where hydrogen gas is generated.

Water treatment may contain two types of corrosion inhibitors, anodic and cathodic.   Anodic inhibitors are called dangerous inhibitors.  If inhibitor concentration is low then parts of the anode are not protected.  The unprotected anode will experience severe corrosion.  Some anodic inhibitors are orthophosphate, chromate, nitrite, orthosilicate, and ferrocyanide.

Cathodic inhibitors protect the passive half-cell.  If underfed, pitting corrosion on the anode will not occur.  Cathodic inhibitors are safe inhibitors and include polyphosphate, zinc, molybdate, and polysilicate.

Specific metal inhibitors are used to protect commonly found metals.  These include the specific copper and yellow metal alloy inhibitors such as tolytriazole (TTA), benzotriazole (BZT), and mercaptobenzotriazole (MBT). BZT and TTA are typically used at 1 to 2 ppm active.  MBT is used at 10 ppm active. MBT is degraded by oxidizing agents such as chlorine.  BZT and TTA are resistant to normal levels of oxidizing agents.

Corrosion can be measured by using metal strips called corrosion coupons. These coupons are preweighed and inserted into a corrosion coupon rack that has a controlled flow rate of 3 feet per second. The coupons are exposed to the actual water in the system.  After a set time period, the coupons are removed, cleaned, and weighed.  Normal exposure is 30 to 90 days.  The weight loss is used to determine the corrosion rate in mils per year. The type of corrosion is noted. Pitting corrosion is never acceptable.   The following guidelines apply:

Visual inspection is important.  Tower mild steel coupons should be clean to lightly encrusted.  Heavy oxides indicate excessive corrosion.  Copper coupons should be shiny to some minimal discoloration.  If the treatment program uses a manganese inhibitor, heavy discoloration or "desert varnish" is acceptable. **Galvanized layer should be intact. If tower is operated at pHs above 8.3, then zinc carbonate or white rust will form. Galvanization will be lost.  If galvanization is to remain intact, acid feed is recommended as well as pretreatment before start-up.

Instantaneous corrosion rates can be measured electrically by a corrator probe. Corrator data must be interpreted with corrosion coupons to be meaningful.