Return Line Corrosion

Obrecht, Malvern F. PhD. Steam and Condensate Return Line Corrosion. Factors That Accelerate It, Retard Tt, with Emphasis on Oxygen. Heating, Piping Air Conditioning, USA, August 1964. 

Addition of Inhibitors. It is possible to add inhibitors for controlling two kinds of corrosion in boiler systems, namely, stress-corrosion cracking and return-line corrosion. The first can be minimized by addition of phosphates, as mentioned previously. 

Return Line Corrosion Condensate (especially when cold line feedwater makeup treatment is used) Is normally quite corrosive to steel piping systems. Stainless steel and some of the aluminum alloys are a better choice for piping materials. 

The differing distribution ratios of the volatile amines have been used in commercial return- line corrosion inhibitors. These inhibitors are generally combinations of morpholine and cyclohexylamine blended so as to obtain the benefits of the differing distribution ratios. Amine requirements are approximately 3.6ppm morpholine (40%) or 3.0ppm cyclohexylamine (40%) per ppm of carbon dioxide to elevate the condensate pH to 7.0. 

Another approach to the prevention of steam condensate and return line corrosion is that of using film-forming chemicals to lay down a protective film on surfaces. This approach has come into widespread use with the development of suitable long-chain nitrogenous materials for this purpose. It is especially effective in systems where high concentrations of carbon dioxide make the use of neutralizing amines uneconomical. 

Oxygen O2 Corrosion of water lines, heat exchange equipment, boilers, return lines, etc. Deaeration, sodium sulflte, corrosion inhibitors, hydrazine or suitable substitutes... 

Condensate returns lines are often copper. Copper has good corrosion resistance to oxygen and carbon dioxide individually. When both gases are present in the condensate, copper is susceptible to corrosion. Copper picked up in the condensate system and returned to the boiler causes serious corrosion problems in the boiler and any steel feedwater and steam pipework. Boiler tubes should last for 25 years but can fail within one year in a mismanaged or ill-designed boiler system suffering from these faults. 

On plant there is significant transport of corrosion product oxide ( crud ) from the feed and return lines. This can be the dominant factor, and overall the accumulation rate is effectively linear. It should be emphasised that corrosion of the type found in operating plant can only be reproduced in the laboratory by employing far higher concentrations (1 000 to 100000 times) than those existing in bulk solution in practice. Interpretations therefore invoke the concept of concentration mechanisms. 

Where problems develop, there is always a cause-and-effect process. In this case, as oxygen infiltrates the CR system, enhanced condensate line corrosion results (i.e., corrosion over and above the level that may be caused by the carbonic acid formed during steam condensation). This enhanced corrosion, in turn, creates the potential for further downstream corrosion debris pickup by the returning condensate and transporting this material back to the FW system. 

The pickup, transport, and redeposition of corrosion debris and deposits can happen anywhere in steam distribution and condensate return systems and are not confined to any particular boiler plant size or pressure rating. For example, deposit pickup may occur in a superheater with redeposition taking place perhaps in a pressure reducing station, steam trap, or condensate line. The starting point for transport mechanisms is often a combination of BW carryover and condensate line corrosion. 

Hydrazine can be fed to the condensate system during operation to help promote the formation of a magnetite film in the the return lines. This surface is more corrosion-... 

Since iron is not commonly used in strongly acid environments, the factors governing its corrosion in media of low pH are less important than those in natural waters and soils. Nevertheless, there are certain applications where such factors must be considered—for example, in steam-return lines containing carbonic acid, as well as in food cans where fruit and vegetable acids corrode the container with accompanying hydrogen evolution. 

Corrosion of water lines, heat-exchange equipment, boilers, return lines, etc. 

Steam condensate and return systems. Corrosion of steam condensate and return systems presents a twofold problem to power-generating and steam-heating plants. Equipment damage and frequent replacement of lines, valves, and traps result in a serious maintenance problem. In addition, corrosion products frequently formed are carried back into the steam-generating equipment and deposited there. The result is plugging of lines, localized overheating, and promotion of corrosion in the boiler system itself. 

One approach involves fihn-forming materials, such as sodium silicate, oils, or polyphosphates. Sodium sihcate reduces corrosion but cannot prevent it entirely. A very successful approach is the use of long-chain nitrogenous compounds as film formers for condensate and return lines. They do not normally accumulate in the boiler because they are either eliminated at the vent of the deaerating heater or steam distil from the boiler water. 

As discussed previously, serious fouling, deposition, and corrosion problems may occur in the FW line as a result of the entry of carryover, after-precipitation, corrosion debris pickup, or oxygen and other contaminants, from either the MU or returning condensate. 

Corrosion and mineral debris can form in condensate lines from a variety of means, and it is not uncommon for the resultant debris accumulated over many years to slough off and return to the boiler when operating conditions change. The result is often a thick boiler-bottom sludge that settles out in the water space or baked-on sludge, which mars efficient combustion and water tube heat transfer. 

Reverse Flow Fall in line press, (compressor fails) high pressure at reactor NH3 in compressor — explosion hazard fit non-return valve (NRV1) hot wet acid gas-corrosion fit second valve (NRV4)... 

Figure 24 Schematic Evans diagram and polarization curve illustrating the origin of the negative hysteresis observed upon cyclic polarization for materials that do not pit. Line a represents the (unchanging) cathodic Evans line. Line b represents the anodic Evans line during the anodically directed polarization, while line c represents the anodic Evans line for the material after its passive film has thickened because of the anodic polarization. The higher corrosion potential observed for the return scan (E (back)) is due to the slowing of the anodic dissolution kinetics.

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