Corrosion reactions condensate

Depending on the degree of oxygen infiltration, the temperature of the condensate and the presence of other gases such as carbon dioxide, various corrosion reactions may take place in the steam distribution and CR systems. The most basic reaction associated with oxygen infiltration results in oxygen corrosion, which can produce deep pitting in the pipework and is described later in this chapter. 

Thus, oxygen infiltration gives rise to a range of localized steam and condensate system corrosion reactions and products. These reactions may, in turn, lead to further downstream problems of corrosion debris transport when the condensate returns to the FW system. 

Ferrous bicarbonate may be transported to a point where little or no amine is available, which then provides the source for various secondary corrosion reactions. Corrosion mechanisms and deposition formation often take place at points in the system well downstream of the original points of steam condensation and initiation of corrosion. 

As the nature of the electrified interface dominates the kinetics of corrosive reactions, it is most desirable to measure, e.g., the drop in electrical potential across the interface, even where the interface is buried beneath a polymer layer and is therefore not accessible for conventional electrochemical techniques. The scanning Kelvin probe (SKP), which measures in principle the Volta potential difference (or contact potential difference) between the sample and a sensing probe (which may consist of a sharp wire composed of a conducting, stable phase such as graphite or gold) by the vibrating condenser method, is the only technique which allows the measurement of such data and therefore aU modern models which deal with electrochemical de-adhesion reactions are based on such techniques [1-8]. Recently, it has been apphed mainly for the measurement of electrode potentials at polymer/metal interfaces, especially polymer-coated metals such as iron, zinc, and aluminum alloys [9-15]. The principal features of a scanning Kelvin probe for corrosion studies are shown in Fig. 31.1. 

Sherstobitova et al. studied the kinetics of the H2 evolution reaction at iron in 4.5% H2SO4 containing n-butyiamine, pyridine, antipyrine, and condensation products of antipyrine with two aldehydes. They found that these compounds inhibit the corrosion reaction throughout the range of surface coverage in conformance with an adsorption isotherm, although some accelerate the H2 evolution reaction at iow and intermediate coverage, which are not in conformance with the isotherm. 

The basic construction material of condensate systems are iron and copper. These materials must not, therefore, be subjected to corrosion. The majority of corrosion products that are deposited in the boiler originate in the condensing system. High levels of iron oxide are formed in the condensate system. The following are the major corrosion reactions ... 

The time of wetness is obviously strongly dependent on the critical relative humidity. Apart from the primary critical humidity, associated with clean surfaces, secondary and even tertiary critical humidity levels may be created by hygroscopic corrosion products and capillary condensation of moisture in corrosion products, respectively. A capillary condensation mechanism may also account for electrolyte formation in microscopic surface cracks and the metal surface-dust particle interface. Other sources of surface electrolyte include chemical condensation (by chlorides, sulfates, and carbonates), adsorbed molecular water layers, and direct moisture precipitation (ocean spray, dew, rain). The effects of rain on atmospheric corrosion damage are somewhat ambiguous. While providing electrolyte for corrosion reactions, rain can act in a beneficial manner by washing away or diluting harmful corrosive surface species. 

Effect of Water. Except in cases of high-tempa-a-ture oxidation, gas-metal reactions, fretting, and certain hot, anhydrous organic chemicals such as phenol and methanol, alurrrinum does not corrode unless water is present on the surface. The water can appear in the form of isolated droplets, as a thin film of moisture condensed on an aluminum surface below the dew point, or as an aqueous solution. Water in contact with air contains dissolved oxygen, which must be present for corrosion of aluminum to occur. Deaeration usually stops the corrosion reaction. The protective surface filrn on aluminum thickens on exposure to water. This reaction is more rapid in the absence of oxygen. 

Carbon dioxide (CO2) is a very common contaminant in hydrocarbon fluids, especially in gases and gas condensate, and is a source of corrosion problems. CO2 in the gas phase dissolves in any water present to form carbonic acid (H2CO3) which is highly corrosive. Its reaction with iron creates iron carbonate (FeCOg) ... 

Acetylene is condensed with carbonyl compounds to give a wide variety of products, some of which are the substrates for the preparation of families of derivatives. The most commercially significant reaction is the condensation of acetylene with formaldehyde. The reaction does not proceed well with base catalysis which works well with other carbonyl compounds and it was discovered by Reppe (33) that acetylene under pressure (304 kPa (3 atm), or above) reacts smoothly with formaldehyde at 100°C in the presence of a copper acetyUde complex catalyst. The reaction can be controlled to give either propargyl alcohol or butynediol (see Acetylene-DERIVED chemicals). 2-Butyne-l,4-diol, its hydroxyethyl ethers, and propargyl alcohol are used as corrosion inhibitors. 2,3-Dibromo-2-butene-l,4-diol is used as a flame retardant in polyurethane and other polymer systems (see Bromine compounds Elame retardants). 

The monohalide vapors are conveyed to a slightly cooler zone (700—800°C) where the reaction reverses, resulting in the condensation of pure aluminum. The monochloride process was carried to the demonstration plant stage but was abandoned because of corrosion problems (24). 

In many cases, cold spots on the reactor shell will result in condensation and high corrosion rates. Sufficient insulation to maintain the shell and appurtenances above the dew point of the reaction gases is necessary. Hot spots can occur where refractory cracks allow heat to permeate to the shell. These can sometimes be repaired by pumping castable refractoiy into the hot area from the outside. 

In particular, corrosion of boiler surfaces may result from the direct reaction with ferric oxide particles entering the boiler. The usual source of ferric oxide is from corroded circulation loops in HW heating boilers and condensate systems, or in pre-boiler sections in steamraising plants. 

To consider these reactions and the problems caused, it may be useful to refer back to the basic corrosion mechanisms of iron and water as shown in section 4.5.1. It can be seen from the second anodic half reaction that ferrous hydroxide is formed from ferrous ions and hydroxide ion where water (condensate) is present. 

Where pressure drops and oxygen infiltration can occur, the presence of both gases leads to a reaction that produces enhanced condensate line corrosion, which generates more carbon dioxide and becomes self-perpetuating ... 

Carbohydrazide itself is of very low volatility, but it decomposes at relatively low temperatures to produce volatile carbon dioxide and ammonia. In theory, the combined corrosive effects of these two materials should be negated in the condensate system, but in practice, this is not always so and both steel and copper corrosion transport problems may develop, primarily as the result of corrosion-enhancement reactions resulting from oxygen in-leakage. It is presumed, therefore, that (similar to hydrazine) some deliberate after-desuperheating line addition of CHZ is necessary if post-boiler section corrosion is to be avoided. 

Corrosion of steel by carbonic acid is probably the most common problem in the post-boiler section, producing pipe grooving and general metal wastage, especially in threaded joints. This form of corrosion is not self-regulating and the reaction products can produce more carbon dioxide, thus perpetuating the corrosion problem. Typically, the condensate pH level is depressed to around 5.0 to 5.5. 

Corrosion inhibiting compositions for metals subjected to highly acidic environments may be produced by reacting in a condensation reaction a styrene/ maleic anhydride copolymer with a polyamine to produce a polyimidoamine inhibitor [1568]. These inhibitors exhibit film-forming and film-persistency characteristics. Some relevant polyamines are listed in Table 6-2. 

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