Boiler corrosion oxygen scavengers

The dibydrochloride salt is used as a photographic developer. It also is employed as an intermediate in the manufacture of fur dyes, in hair dyeing, as a reagent in testing for ammonia and formaldehyde, and as an oxygen scavenger in water to prevent boiler corrosion (173). 

It is therefore not surprising to find that where boiler systems operate under these constraints, they suffer rapid and extensive oxygen-initiated corrosion to the internal surfaces of the FW system, the feed lines, the boiler shell, and all internal heating surfaces. With poor oxygen scavenging, heavy pitting corrosion and tuberculation is found, especially on the tubes and at the waterline of the FW tank and boiler shell. 

Thus, a useful method of corrosion control is to slow down the driving force of the corrosion cell by reducing the difference in potential at the cathode (cathodic polarization). In practice, this can be achieved by maintaining reducing conditions at the boiler surface (typically by dosing an oxygen scavenger), which increases the rate of cathodic polarization and drives down the corrosion rate. 

Chemical treatment programs are often individually designed for particular boiler plant systems but usually contain oxygen scavengers, pH boosters, and corrosion inhibitors. In addition, the formulations employ materials specifically designed to limit the degree of deposition and control the mechanisms of deposition. 

For these medium to large operations, apart from taking measures to ensure the continuous efficiency of deaeration and oxygen scavenging, little more can be said because all that can be done practically to remove DO is being done. The problems of inadequate FW deaeration, the resultant corrosion risks entailed, and the consequent need for constant vigilance primarily affects those owners and operators of small to midsize steam-raising boiler plants where mechanical deaeration is not available. 

It should be noted, however, that oxygen corrosion of operational boilers supplied with mechanically deaerated FW, supplemented by the use of an appropriate oxygen scavenger chemical, and under a constant load is relatively rare. This position is not necessarily the same in idle boilers, low-load boilers, or chemically cleaned boilers, and despite all best efforts oxygen corrosion may take place. 

In these simpler types of boiler plants, corrosion protection generally is provided solely by the use of chemical treatments containing anodic passivators and alkali. Under conditions in which the (oxygen-containing) MU water requirement is expected to be very low, no specific oxygen scavenger is typically employed. This is often not the case in practice, however, for various reasons such as inadequate preventative maintenance, poor BD practice, the infiltration of air or oxygen, or other such reason. 

Anodic programs without a separate oxygen scavenging function are entirely suitable for all these system types, provided makeup (MU) requirements remain consistently low. Where MU water volumes are too high, the risk of oxygen corrosion processes is also high. Classically, this is evidenced by rapid pitting corrosion in boiler tubes. 

Where copper corrosion occurs, the problem usually can be traced back to an excess feed of hydrazine, DEHA, or similar product, coupled with inadequate post-boiler oxygen scavenging. 

Erythorbates are safe products and there are no harmful breakdown products, although when early formulations utilized ammonia as a PH buffer (and neutralizer for part of the carbon dioxide), copper corrosion problems resulted. However, erythorbates are not steam-volatile,and consequently there is no post-boiler oxygen scavenging potential available. Thus, in the event of complete breakdown of the product at high pressure, oxygen-induced, ammonia corrosion of copper may continue unchecked. 

The FW oxygen scavengers mentioned earlier are volatile and can therefore provide additional protection against post-boiler section corrosion induced or enhanced by oxygen in-leakage. These particular scavengers also break down under pressure to produce some level of volatile ammonia, which can neutralize any carbon dioxide found in the steam-condensate system. 

NOTE Where MU rates are high, a supplementary oxygen scavenger may also be required to avoid the problem of pitting corrosion in the boiler and associated pipe work. 

A further longer term wet lay-up alternative is through the use of volatile corrosion inhibitors (VCIs) such as dicyclohexylamine acetate. These are dissolved in the water at a temperature below 60 °C, and the water is circulated for 4 to 5 hours. The boiler does not need to be completely filled because the VCI migrates to all parts of the boiler and reaches equilibrium in each of the void spaces. With traditional lay-up chemicals, the oxygen scavenger may become depleted easily (which is why the reserve usually is so high) and corrosion protection is quickly lost however, with VCI programs, there is always a volatile buffer available that maintains equilibrium and hence corrosion protection. 

Rudolph, John W. Reid, Robert W. Cuisia, Dionisio G. New Oxygen Scavenger for Boiler Systems. NACE Corrosion 83 . National Association of Corrosion Engineers, USA, April 1983. 

Shimura, Y. Uchida, K. Sato, T. Taya, S. The Performance of New Volatile Oxygen Scavenger and Its Field Application in Boiler Systems. Kurita/NACE Corrosion 2000. NACE International, USA, April 2000. 

BWT. [Stewart Hall] Boiler compd. for steamline corrosion prevention, oxygen scavenging. 

Mekor . [Drew Ind. Div.] Volatile oxygen scavenger, metal passivator, corrosion inhibitor for boiler water treatment... 

Uses Reducing agent corrosion inhibitor wastewater treatment electrolytic plating redox reactions polymerization catalyst organic hydrazine derivs. rocket propellant corrosion inhibitor, oxygen scavenger in boiler... 

Both sulfite and hydrazine are available in catalyzed form to promote more rapid reaction rates. A boiler designed to be operated with a catalyzed oxygen scavenger must never be operated on the uncatalyzed grades, or severe corrosion will be encountered in the economizers or even the steam drum. 

Sulfite ion, SOj, is found in some industrial wastewaters. Sodium sulfite is commonly added to boiler feedwaters as an oxygen scavenger where it reacts to remove potentially corrosive dissolved molecular oxygen fi om the water ... 

Boiler Feedwater Treatment. The strong reducing characteristic of hydrazine makes hydrazine an oxygen scavenger for the treatment of boiler water to reduce corrosion, with the addition of 0.1 ppm. 

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