9.1 general aspects of corrosion - ?· general aspects of corrosion iron metallurgy processes...
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485VOLUME V / INSTRUMENTS
In the hydrocarbon extraction and treatment industry, allclasses of materials are used: metallic, polymeric, ceramic,composite and cement materials. In terms of quantity andtype of use, metallic materials represent the maincomponent. Evidence of this are, for example, oil wells,refining columns, transportation pipelines, storage tanks andmany other important parts of facilities. Given theirimportance and strategic impact, this chapter will dealmainly with metallic materials and the main forms ofcorrosion affecting them.
Generally speaking, materials in contact with aggressiveenvironments undergo a form of chemical and physical decaywhich, as far as metallic materials in particular are concerned,is known as corrosion. Corrosion can be defined as an attackby atmospheric agents or other aggressive means onmaterials, especially metals, which leads to the slow butprogressive alteration of the characteristics often not onlyon the surface of the material. It can also be defined as thedestruction or deterioration of a material by reaction with theenvironment or as the tendency of a metallic material toreturn to its original state, as it is found in nature (Fontana,1986); as a consequence, it is also known as metallurgy inreverse. Corrosion processes tend to bring metallic materials
spontaneously to their most stable thermodynamic state, inwhich they combine with other elements, especially oxygenand sulphur. Starting from this state, metallic materials areobtained (extracted) with metallurgy processes that entailthe supply of large amounts of energy (Fig. 1).
The economic impact of corrosionAn idea of the importance of corrosion in industrial
activities can be obtained from its economic impact. Inindustrialized countries, the cost of corrosion is around 3-4%of gross domestic product, calculated as the sum of the costsof direct damage (such as the cost of damaged materialswhich must be replaced, the cost of replacement operations)and of indirect damage (such as the cost of lost production,plant inactivity, the costs of pollution, poor image in thecommunity, the cost of environmental cleanup, damage topersons and things; Hoar, 1971; Eni-Agip, 1994). Indirectcosts, which are difficult to evaluate, generally exceed directcosts. It has been estimated (Hoar, 1971) that the cost ofcorrosion can be reduced by 15-20% by applying techniquesthat make use of a basic understanding of corrosion; thebest-known of these include: cathodic protection, selectingthe most resistant material, the use of corrosion inhibitors,improvements in design (for example, eliminating stagnantconditions).
General aspects of corrosion
aggressive environment(gas, process fluids,
aggressive environment(soil, sea water,
Fig. 1. The corrosion process as metallurgy in reverse (Fontana, 1986).
Morphology of corrosion processesCorrosion phenomena may occur at the surface of
metallic materials in a generalized or localized way(Pedeferri, 2007). Generalized corrosion occurs when theattack affects the entire surface of the material exposed tothe environment, and uniform corrosion when thegeneralized attack takes place in a uniform way. Localizedcorrosion, by contrast, occurs when the attack affects onlysome parts of the surface of the material exposed to theenvironment, with a specific morphology, for example in theform of fissures or cracks, cavities, craters, ulcers. We speakof selective corrosion when specific constituents of thematerial are attacked, such as some phases present as grainsor around the grains.
Corrosion rateAny corrosion process, regardless of the morphology of
attack, entails a loss of mass. The corrosion rate can beexpressed as the mass loss per unit of surface area per unit oftime. Generally speaking, however, it is preferable toconsider the penetration rate of corrosion; in this case, thecorrosion rate is expressed as the decrease in thickness perunit of time. For some forms of corrosion, such as stresscorrosion cracking or corrosion fatigue which lead to theformation of cracks, mass loss and the decrease in thicknessare less important than the time to failure or the growth orpropagation rate of the cracks.
Uniform attackIn conditions of uniform corrosion, in other words an
attack distributed uniformly over the surface of the material,the rate of mass loss per unit of surface area exposed to theaggressive environment expresses the extent of damagecaused by the attack over time and can be calculated with theequation:
where Dm is the mass loss during time interval t, and A isthe exposed surface area. The most commonly used unit ofmeasurement for the corrosion rate in terms of mass loss isthe mg/dm2day (mdd). Mass loss becomes important whenwe wish to know the quantity of dissolved metal, forexample to evaluate the pollution produced. An example istin poisoning caused by the tins used to preserve tomatoes.
Generally speaking, thinning is more important thanmass loss, so the uniform corrosion rate is expressed as theloss of thickness, given by:
where g is the density of the metal. In this case, the mostfrequently used unit of measurement for the corrosion rate ismmyr. For the most widely used metals (iron, copper andzinc), which have densities between 7 and 8 t/m3, thefollowing rough equivalents can be obtained 1 mdd5mmyr; 1 mmyr220 mdd. The rate of generalizedcorrosion is usually classified according to the values shownin Table 1.
Localized attackIn conditions of localized corrosion, it is necessary to
distinguish between the rate of mass loss (which expresses a
mean velocity over the whole exposed surface) and the rateof penetration into the area attacked. In the presence oflocalized attack, the loss of efficiency is given, for example,by the perforation of the metal wall (as in the case of a tankor pipe) and not by the metals mass loss.
Types and mechanisms of corrosionThe corrosion of metallic materials takes two forms:
high temperature corrosion (or dry corrosion), typical ofmetallic materials operating at high temperatures in thepresence of hot gases, such as the fume side of boilers andgas turbines; wet corrosion, characteristic of materialsexposed to an electrolytic solution such as sea water, soil,concrete polluted by chlorides or carbonated concrete,process fluids. The distinction between wet and drycorrosion derives from the two different mechanismsgoverning the phenomenon: in the first case, anelectrochemical mechanism; in the second, a chemicalmechanism, typical of heterogeneous reactions.
9.1.2 High temperature corrosion
The corrosion of metals in contact with air at temperaturesabove 400C and up to 1,300C is known as high temperaturecorrosion. The presence of oxygen leads to the formation ofan oxide scale on the surface of the metal, whilst the presencein the hot gases of some chemical species such as sulphur,sodium and vanadium leads to the formation of salts with alow melting point that react with the metal.
The oxidation of metals and alloys at high temperature isknown and well-documented (ASM, 1987; Revie, 2000). Inorder to predict the formation of the scale and its growth, thethermodynamic conditions and the kinetics of the reactionsinvolved must be considered. The thermodynamic conditionsdetermine if the oxidation reaction proceeds spontaneouslyat the operating temperature, whilst the kinetics determinethe velocity at which the scale growth reaction takes place.
Decay processes at high temperature include: a) thinningdue to the formation of a non-protective scale; b) corrosionby molten salts with evaporation of the corrosion products;c) erosion-corrosion caused by solid particles in suspension;d ) localized attacks at the grain boundaries; e) embrittlementof the material.
Given the fairly extreme operating conditions, whichoften present the risk of catastrophic consequences should afailure occur, the choice of materials generally requiresgreater care than for low temperature applications.
corr m= =
V mAtcorr m,
486 ENCYCLOPAEDIA OF HYDROCARBONS
Table 1. Classification of the corrosion rate
Uniform corrosion rateCorrosion rate
Very severe 1,000
Thermodynamic conditionsWhereas wet corrosion processes are electrochemical in
nature, high temperature corrosion works in accordance withthe kinetics of chemical reactions in the gas phase;thermodynamic conditions and solid-phase diffusionprocesses in the products or corrosion scales are thereforeimportant.
Thermodynamic studies show that all metals oxidizespontaneously in the presence of oxygen or in air with theexception of gold and platinum. However, numerous metalscan be used for extremely long periods even at hightemperatures because the oxide growth kinetics aresufficiently slow. This fact lies behind the development ofalloys resistant to oxidation due to the formation of a scalethat acts as a barrier between the metal and the environment,characterized by a low growth rate.
In the absence of aqueous solutions, in other words indry oxygen or air, metals at ambient temperature form aprotective scale 1-10 nm thick which prevents furtheroxidation of the metal. As the temperature rises, thethickness of the film increases, leading to mechanicaldetachment on many metals due to the excessive volume ofthe oxide film.
From a thermodynamic point of view, the oxidation of ametal occurs only if the partial pressure of oxygen at