A commercial chemical under the title of CWT-NC is used as corrosion inhibitor in the process water system which acts as cooling medium for different systems. Nitrite plays a vital role in inhibition of corrosion. This analyte is not only monitored but also determined at regular interval to check the chemistry of PW system. At present there is no existing method to find out concentration of nitrite at trace level.
1.1 Corrosion Inhibitor
A corrosion inhibitor is a substance when added in a small concentration to an environment decreases the corrosion rate of a metal exposed to that environment. Inhibitors often play an essential role in the oil extraction and processing industries where they have always been considered to be the first line of defense against corrosion. [1]
1.2 Classification of corrosion inhibitors
Corrosion inhibitors are classified as follow
1.2.1 Anodic inhibitors
Anodic inhibitors usually act by making a protective oxide film on the surface of the metal causing a large anodic shift of the corrosion potential. This shift forces the metallic surface into the passivation region. They are also sometimes referred to as passivators. Chromates, Nitrates, Tungstate, Molybdates are some examples of anodic inhibitors. [2] [3]
1.2.2 Cathodic inhibitors
Cathodic inhibitors act by either slowing the cathodic reaction itself or selectively precipitating on cathodic areas to limit the diffusion of reducing species to the surface.
The rates of the cathodic reactions can be reduced by the use of cathodic poisons. However, cathodic poisons can also increase the vulnerability of a metal to hydrogen induced cracking since hydrogen can also be absorbed by the metal during aqueous corrosion or cathodic charging. [2] [3]
The corrosion rates can also be reduced by the use of oxygen scavengers that react with dissolved oxygen. Sulfite and bisulfite ions are examples of oxygen scavengers that can combine with oxygen to form sulfate.
1.2.3 Mixed inhibitors
Mixed inhibitors work by reducing both the cathodic and anodic reactions. They are typically film forming compounds that cause the formation of precipitates on the surface blocking both anodic and cathodic sites indirectly. Hard water that is high in calcium and magnesium is less corrosive than soft water because of the tendency of the salts in the hard water to precipitate on the surface of the metal forming a protective film.
The most common inhibitors of this category are the silicates and the phosphates. Sodium silicate, for example, is used in many domestic water softeners to prevent the occurrence of rust water. In aerated hot water systems, sodium silicate protects steel, copper and brass. However, protection is not always reliable and depends heavily on pH.
Phosphates also require oxygen for effective inhibition. Silicates and phosphates do not afford the degree of protection provided by chromates and nitrites; however, they are very useful in situations where non-toxic additives are required. [2]
1.2.4 Vapor phase inhibitors
Vapor Phase Inhibitors (VPI), are compounds transported in a closed environment to the site of corrosion by volatilization from a source. In boilers, volatile basic compounds, such as morpholine or hydrazine, are transported with steam to prevent corrosion in the condenser tubes by neutralizing acidic carbon dioxide or by shifting surface pH towards less acidic and corrosive values.
When these inhibitors come in contact with the metal surface, the vapor of these salts condenses and is hydrolyzed by any moisture to liberate protective ions. It is desirable, for an efficient VCI, to provide inhibition rapidly while lasting for long periods. Both qualities depend on the volatility of these compounds; fast action wanting high volatility while enduring protection requires low volatility. [2]
1.3 Evaluation of Corrosion Inhibitors
Electrochemical methods are used routinely for the evaluation of the efficiency of corrosion inhibitors. The advantages of electrochemical methods are short measurement time and mechanistic information that they provide which help not only in the design of corrosion protection strategies but also in the design of new inhibitors. [2]
1.4 Properties of Sodium Nitrite
Sodium Nirtite is a white crystalline salt as shown in Figure 1. Anodic Corrosion inhibitor for Mild Steel, Cast Iron, Galvanized Steel, Aluminum, Tin & used for Closed hot and chilled water loops and glycol systems. Chemical Formula: NaNO2
Molecular Weight: 69 [4]
Figure 1 1: A glass bottle containing Sample of Sodium Nitrite salt
Functionality: Forms a resistant oxide film to protect the metal from further attack. This film is formed by the combined action of nitrite and dissolved oxygen and then involved in curing by the nitrite alone. Nitrite is not consumed to any practical extent since little is needed to form this thin film. (300 to 2000 ppm as NO2) The concentration required increases with temperature. [5]
Synergies: Sodium nitrate, sodium molybdate, sodium silicate. Nitrite polyphosphate mixtures have been reported to give longer lasting films than obtainable alone.
Concentration Effects:
Too Low: Nitrite can sometimes be considered a “dangerous†corrosion inhibitor because too low concentrations (<300 ppm) can result in more corrosion than no nitrite at all. This occurs because the nitrite concentration is too low to repair breaks in the passivized film on the metal surface. In a galvanic reaction, metal loss occurs at the anode. Since nitrite is an anodic inhibitor, metal loss becomes concentrated in these film breaks and can cause pitting. [5]
Too High: Very high concentrations of nitrite can damage mechanical seals, gaskets, and hosing. Nitrite is a mild oxidant. High levels of nitrite can oxidize and degrade gaskets and hosing. High levels of nitrite (>4,000 ppm NO2) also add to the dissolved solids content of the water. Water that makes it past the mechanical seal on a pump will evaporate and leave the dissolved solids behind. These accumulated dissolved solids can eventually scratch the mechanical seal away. [5]
Microbiological Effects: Nitrifying and de-nitrifying bacteria are nourished by nitrites and can convert them to nitrates, ammonia, and nitrogen. Non-oxidizing biocides should be used. If there has been no water loss and the conductivity and other tested parameters have remained the same, but the system nitrite level drops, this could be an indication of microbiological activity. Test for aerobic and anaerobic microbiological activity. [5]
Temperature Effects: Decomposes above 608°F.
pH Effects: Nitrites perform best in a pH range of 8-10 and should never be allowed to drop below 7. Borate buffers are often used in nitrite formulations to maintain a safe pH.
1.5 Nitrite based Corrosion inhibitor for Closed Circuit Water
Nitrite based Corrosion Inhibitor for chilled water systems is a multi-metal corrosion inhibitor based on nitrite and other organic compounds with special yellow metal corrosion inhibitor and anti-scaling agent that help to prevent scale deposition or also removes adhering loose scale deposits from the heat transfer fluid system.
The corrosions caused by chiller water, Mono Ethylene Glycol or Brine can cause system leakage in PHE (Plate Heat Exchanger) and AHU (Air Handling Unit). Corrosion can also cause plugging and clogging of tubes and thus reduce flow rate. This will affect the heat transfer rate and will force the system to reduce heat transfer efficiency. This way the system needs regular descaling that incurs shutdowns, leads to additional costing for labor and wastage of time. [6]
1.5.1 Scaling
Scale is a hard deposit of inorganic material on heat transfer surfaces. This is caused by gradual precipitation of mineral particles in water on the surface. During the evaporation of water in an evaporative condenser, pure water vaporizes out and the dissolved solid materials concentrate in the remaining water on the surface. The solids will then settle in pipelines or on heat exchange surfaces, where it frequently solidifies into a relatively soft, amorphous scale. Next to scale, the next most important concern is the prevention of “corrosionâ€. [7]
1.5.2 Corrosion
Corrosion is defined as the destruction or loss of metal through chemical or electrochemical reaction with its surrounding environment or material. Mild steel is a most commonly used metal in the cooling water system. But this metal is most susceptible to corrosion. However in some waters and in presence of dissolved gases, such as H2S or NH3, the corrosion to these metals is more severe.
Common problems arising from corrosion are reduction in heat transfer and water flow resulting from a partial or complete blockage of pipes, valves, strainers, etc. Also, excessive wear of moving parts, such as pump, shaft, impeller and mechanical seal, etc. may resist the movement of the equipment. Hence, thermal and energy performance of heat exchange may degrade.
The water treatment in closed system is thus not very critical. Once the initial volume is chemically treated, the quality of the circulating fluid needs to be monitored on a regular basis and additional chemicals added as required to maintain recommended residual concentrations of treatment chemicals. Most closed-loop water systems use a sodium nitrite based inhibitor for corrosion control and a biocide to prevent biological contamination. These inhibitors will protect both the ferrous and nonferrous materials in your piping system. [7]
1.6 Process Water System in KANUPP
1.6.1 System Purpose
The purpose of the process water system is to meet the cooling demand of various plant systems by transporting heat from the systems to the process water system via process water heat exchangers.
Closed loop type of Process water system containing de-mineralized water has been provided to avoid expensive corrosion damage resulting from use of seawater.
1.6.2 Brief description
The process water system is a closed loop containing de-mineralized water, with an overhead pressurizing tank riding on it to uniquely determine the pressure at one point in the system. This water loop is used as a heat transport system and serves all equipment which could suffer expensive or irreparable corrosion damage if seawater or impure water were to be used. The system is subdivided into a high pressure and a low pressure system each being served by two 100% pumps. [8]