Heating systems, corrosion inhibitors

The control of corrosion in cooling and heating systems by inhibitors is an established technology [3]. In these systems, corrosion occurs under near-neutral conditions, with the reduction of oxygen as the cathodic reaction. In addition, corrosion may be promoted by... 

Empirical C7H10N2O Formula (H2N)2C6H30CH3 Properties Needles m.w. 138.16 m.p. 58-63 C Toxicology LD50 (oral, rat) 460 mg/kg, (IP, rat) 116 mg/kg mod. toxic by ing. poison by IP route skin irritant confirmed carcinogen tumorigen human mutagenic data experimental reproductive effects TSCA listed Hazardous Decomp. Prods. Heated to decomp., emits toxic fumes of NOx Uses Colorant in hair dyes intermediate in prep, of oxidation color systems corrosion inhibitor for steel... 

Use antifouling agents and corrosion inhibitors in heat transfer systems... 

Anodic polarization also may occur. Typically, this begins with the formation of a thin, impervious oxide film, chemisorbed at the anode (as on the surface of stainless steels). However, for most metals used in boiler plant systems this chemisorption process must be aided by anodic corrosion inhibitors to reduce corrosion rates to tolerable levels. An example is the application of nitrite-based inhibitors, widely used in HW heating systems. 

In all space heating boiler systems there is a tendency to keep water treatment programs as simple as possible. Ideally, chemical inhibitors should be added in proportion to MU demands, metered water consumption, oxygen content, or other preemptive measurement. More typically, the standard process is to periodically (weekly to monthly) analyze the BW for a few basic control parameters, including measuring the multimetal corrosion inhibitor reserve, and then to merely top-up the inhibitor when the reserve is below the minimum specification. Chemical treatment often is added directly to the BW by hand-pump via a hose cock (bib cock) connection. 

Where the systems are tight and almost no losses occur, treatment programs based on these inhibitors can work very effectively (as long as the systems have been properly cleaned and passivated beforehand). For HW heating systems where significant water losses occur, however, (as when MU requirements exceeds, say, 5-10% of the system volume per month), the loss of treatment permits significant oxygen corrosion to occur. 

For larger, more complex LPHW systems and for LP steam heating systems, silicates are seldom used alone but are formulated with nitrite or molybdate inhibitors to provide synergistic corrosion protection. 

To increase equipment reliability and plant efficiency, corrosion inhibitors are used in boiler and cooling water programs to control fouling and deposition on critical heat-transfer surfaces. In cooling systems, corrosion inhibition is commonly achieved through the use of passivators, which encourage the formation of a protective metal oxide film on the metal surface ( 1). ... 

Although chromate is the best aqueous corrosion inhibitor available, its use has been severely curtailed due to toxicity and environmental concerns ( ). One of the more successful non-chromate treatments involves the use of phosphate/phosphonate combinations. This treatment employs high levels of orthophosphate to promote passivation of the metal surfaces. Therefore, it is important to control calcium phosphate crystallization so that high levels of orthophosphate may be maintained in the system without fouling or impeding heat-transfer functions. 

There are many corrosion mechanisms which can take place in today s engines. The complexity has increased with the use of aluminum alloys in the head and block. Aluminum used in areas such as the head, where large quantities of heat are liberated to the coolant, is subject to a unique heat rejection corrosion. To protect against the heat rejection corrosion of aluminum, a coolant having special corrosion inhibitor systems must be used. 

Compositions and functions of typical commercial products in the 2-alkyl-l-(2-hydroxyethyl)-2-imidazolines series are given in Table 29. 2-Alkyl-l-(2-hydroxyethyl)-2-imidazolines are used in hydrocarbon and aqueous systems as antistatic agents, corrosion inhibitors, detergents, emulsifiers, softeners, and viscosity builders. They are prepared by heating the salt of a carboxylic acid with (2-hydroxyethyl)ethylenediamine at 150—160°C to form a substituted amide 1 mol water is eliminated to form the substituted imidazoline with further heating at 180—200°C. Substituted imidazolines yield three series of cationic surfactants by ethoxylation to form more hydrophilic products quatemization with benzyl chloride, dimethyl sulfate, and other alkyl halides and oxidation with hydrogen peroxide to amine oxides. 

Utility systems for equipment and space heating and cooling frequently use heavy metal corrosion inhibitors in their heat transfer fluids. Chromate compounds are among the best corrosion inhibitors available. Nonchromate inhibitors that have proved to be feasible substitutes include polyphosphates, organophosphates, zinc, molybdates, and aromatic azoles. Some of these compounds have their own environmental impacts, however. Azoles, for instance, can be quite dangerous to human health. 

Benzotriazole and its derivatives have proved to be powerful corrosion inhibitors for many metals and alloys, including copper, solder, brass, steel, cast iron and aluminum, in e.g. heat exchangers, heating systems or automatic radiators. For some recent literature, see <79MI41102, 81JAP(K)81108882, 81JAP(K)81122884). 

Corrosion Inhibitor CS [Nalco]. TMfor a synergistic combination of sodium nitrate, borax, and organic inhibitors, used to prevent corrosion of ferrous and nonferrous metal and alloy surfaces in low-makeup closed cooling and heating systems. 

Shell side corrosion is caused by decomposition of the bath. (The decomposition products of amines and glycols are corrosive.) Some decomposition and corrosion are inevitable however, excessive decomposition is usually due to overheating near the firetube. Corrosion inhibitors are commonly added. There are numerous reasons for overheating the bath localized ineffective heat transfer caused by fouling, excessive flame impingement, etc. An improper flame can sometimes be modified without system shutdown. Fouling, however, requires removal of the firetube. 

Hot carbonates are well suited for the removal of C02 at moderate or high levels in the presence of little or no H2S. The process acquired its name from the use of elevated temperatures in both the absorber and the regenerator (110—115°C). Hot carbonates such as the Benfield and the Koppers Vacuum Carbonate utilize K2C03 to remove H2S, COS, and C02 from gas streams [35]. Their heat requirements and high solvent circulation make hot carbonates more expensive than other acid gas removal processes. Other hot carbonate processes, including the Catacarb and the Giammarco-Vetrocoke processes, use catalysts, corrosion inhibitors, and/or activators to enhance the removal of the acid gases. Hot carbonate-promoted systems are able to decrease the C02 level from 1% to 0.1%. Promoters include DEA, amine borates, and hindered amines [36]. 

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