Chromate treatment programs

The gradual demise of strongly passivating chemical treatment programs, such as chromate, operating typically at pH < 8.0... 

Operating practices for chromate treatments are well established and it is recognized that the effectiveness of the program initially depends on ... 

In the 1970s various PBS/AM compounds, such as 2-phosphonobutane-1,2,3,4-tetracarboxylic acid, were introduced. These were marketed as scale and corrosion inhibitors and were often formulated with zinc and phosphates to provide, along with the lignosulfonate/zinc/phosphate programs, an alternative to chromate treatments. 

NOTE It is likely that in the United States the limitations placed on the discharge of Mo into wastewater systems by some authorities will ultimately prove to have been unnecessary and that molybdate will continue to be a most effective and viable alternative to chromate-based treatment programs. 

Acid leaks result in the deactivation of some chemical treatment programs, for example, P-PO4 is hydrolyzed to O-PO4, and chromate (in the presence of ferrous ions) is reduced to trivalent chromium. Also, depassivation of the inhibitor film and corrosion occurs. The correct action is to first stop the leak, then to increase the pH by bleeding off the low pH water and making up, to the maximum extent of the system. If the leak is so severe that the pH drops below 4.3, and free mineral acidity (FMA) occurs, soda ash should be used to restore the pH to 4.3 to 4.5, then discontinued. Caustic soda should not be added, as this action will almost certainly result in heavy fouling from precipitated ferric hydroxide. Dispersants or antifoulants may be required. When the system is stabilized, it should be repassivated and the reason for the acid leak resolved (faulty pH meter ). 

C. Piping and Instrumentation. Cooling water piping is usually carbon steel. This fact makes it essential for the plant operator to keep up with the water treatment program. Untreated cooling water is the most corrosive form. Most plants at least add inhibitors. These vary in composition, and it is difficult to generalize other than to say that chromates, once a standard, are now seldom used, because of their toxicity. 

The first methods of cooling tower corrosion control involved adding several hundred parts per million of sodium chromate, as chromate is capable of excellent anodic corrosion control at these dosages. However, these early programs were both inefficient and expensive. The advent of synergized zinc chromate-polyphosphate treatments not only made corrosion control more... 

NOTE The consumption of chemical treatment in a cooling system is directly proportional to total water consumption (for any given operating conditions, such as cycles of concentration), but the selection of a suitable type of chemical program (e.g., chromate, alkaline zinc) is based on a combination of factors, including system metallurgy, operating conditions, anticipated or actual... 

NOTE Acid dosing with chromate-based cooling water programs continues to be used in many countries around the world, even in some fully industrialized and developed nations, and most often quite safely, despite the general environmental concerns from this form of chemical treatment. Such concerns need to continue to be voiced, however, as there are some irresponsible operators who use chromates and discharge directly into water courses, contravening local prohibitory regulations (that also may not be adequately enforced). 

There are perhaps almost as many formulations for chromate programs available as there are water treatment service companies, and some subjectivity may be involved in their selection. Apart from purely cosmetic changes, there are, however, chromate programs designed for every quality of water and circumstance. 

Surface treatment methods are currently used to prevent corrosion and enhance adhesion between metals and coating materials. For these purposes, chromate-, zinc- and phosphate-based materials are employed to increase the corrosion resistance of surfaces as well as enhance coating quality. However, some of these materials are unacceptable from an ecological and human health point of view, and an alternative system must be considered to reduce these concerns. Recently, several research programs have focused on low-toxicity materials that could replace currently used materials. These new surface treatments should improve corrosion protection and also provide good adhesion between metal and paint surfaces. ... 

---