Atmospheric Corrosion Mechanism

Atmospheric corrosion

Atmospheric Corrosion Mechanism

Atmospheric corrosion is an electrochemical process, requiring the presence of an electrolyte. Thin film "invisible" electrolytes tend to form on metallic surfaces under atmospheric corrosion conditions, when a certain critical humidity level is reached. For Fe, this level is around 60%, in unpolluted atmospheres. The critical humidity level is not a constant - it depends on the corroding material, the hygroscopic nature of corrosion products and surface deposits and the presence of atmospheric pollutants.

In the presence of thin film electrolytes, atmospheric corrosion proceeds by balancing anodic and cathodic reactions. The anodic oxidation reaction involves the dissolution of the metal in the electrolyte, while the cathodic reaction is often assumed to be the oxygen reduction reaction. Oxygen from the atmosphere is readily supplied to the electrolyte, under thin film corrosion conditions.

Corrosivity Map of North AmericaThe corrosion severity in various regions of America can vary greatly depending on a multitude of factors. While accelerated laboratory testing can be satisfactory for evaluating the corrosion resistance of new materials and coatings, the automobile proving grounds are definitively the primary means for testing completed systems. Proving grounds are, in effect, large laboratories. But the proving ground test contents and procedures can differ sharply between manufacturers. Because each test is expressly different, each brings different results, and in this type of test, proper interpretation of the test results is the key to successful testing.For many years, bare steel coupons were attached to different vehicles in the northeastern U.S. and Canada. Periodically removed and measured for metal loss, the data from these coupons was used to target the corrosion test objectives to metal loss and to determine the most severe corrosion localities for captive fleet testing and future survey evaluations. The following map is a composite picture of the corrosivity differences observed in that study.

This map shows macroscopic differences between areas, with coastal regions (especially in humid tropical conditions) and regions under the influence of corrosive de-icing salts being the most severe. The high corrosion rates in the Gulf Coast and Florida can be attributed to the corrosive marine environment. In the North Eastern regions, de-icing salts applied to road surfaces in winter are primarily responsible for the high corrosion rates. Direct exposure testing rack

Atmospheric Corrosion Monitoring:Atmospheric corrosion accounts for the highest overall cost and metal loss of all the fundamental corrosive environments. A defining feature of atmospheric corrosion is the thin aqueous layer between the surface of the corroding material and the atmosphere. Three phases (solid [corroding substrate], liquid [thin aqueous layer] and gaseous [atmosphere]) and the interfaces between these phases are therefore important and can be used in corrosion monitoring principles.

Corrosion monitoring in outdoor and indoor atmospheres poses specific challenges related to characterizing corrosion damage (generally taking place at a low rate) in a short (practical) time frame. Three basic approaches to corrosion monitoring are available:

1. Direct measurement/monitoring of corrosion damage.(Examples include exposing actual components or coated coupons to corrosive atmospheres and evaluating these for corrosion damage periodically)

2. Indirect measurement/monitoring of corrosion damage with corrosion sensors.(Examples include thin film electrochemical sensors embedded under paint coatings and smart coatings)3. Classification of atmospheric corrosivity by categories and correlating such classifications to actual in-service performance and corrosion rates.

The main drawbacks of the direct measurement approach are the lengthy exposure time period usually required and that only a "snapshot" of cumulative damage is obtained when detailed analysis of corrosion damage is performed periodically.

The approach of atmospheric corrosivity classification is generally one of a simple, low-cost measurement in a short time frame. Ultimately, such simplistic measurements require correlation to actual long term service performance and therefore need to be linked to the other measurement methodologies.

Atmospheric corrosion monitoring is generally employed to quantify this type of damage and corrosion risk, rather than merely resorting to broadly descriptive atmospheric classifications such as "industrial", "rural", etc.

Smart Coatings:The use of "smart materials" for corrosion sensing purposes relies on a material undergoing a transformation through its interaction with the corrosive environment. It is such transformations that can potentially be used for indicating and detecting corrosion damage. Ideally, in principle, the sensing function could be integrated with additional actuation and control functions, designed to control corrosion damage.

Some smart coating principles relevant to corrosion sensing, or potentially relevant, include the following:

Paint systems with color-changing compounds, responding to pH changes that result from corrosion processes.

Changes of coating compounds from non-fluorescent to fluorescent states, upon oxidation or complexing with metal cations.

Release of color dyes, on coating damage, from incorporated dye-filled micro-capsules.

Piezo-electric thin film applications.

Fiber optics

Atmospheric Corrosion Tests:Traditional Salt Fog Tests:ASTM B117: Standard Practice for Operating Salt Spray (Fog) Apparatus

ASTM B368: Standard Test Method for Copper-Accelerated Acetic Acid-Salt Spray (Fog) Testing (CASS Test)

ISO 7253: Paints and varnishes - Determination of resistance to neutral salt spray (fog)

Cyclic Exposure Tests:ASTM D5894: Standard Practice for Cyclic Salt Fog/UV Exposure of Painted Metal, (Alternating Exposures in a Fog/DryCabinet and a UV/Condensation Cabinet)

ASTM G85: Standard Practice for Modified Salt Spray (Fog) Testing - Annex A5, dilute electrolyte cyclic fog dry test

GM 9540P: Accelerated Corrosion Test

SAE J2334: Cosmetic Corrosion Lab Test

Introduction to High Temperature CorrosionHigh temperature corrosion is a form of corrosion that does not require the presence of a liquid electrolyte. Sometimes, this type of damage is called "dry corrosion" or "scaling". The term oxidation is ambivalent since it can either refer to the formation of oxides or to the mechanism of oxidation of a metal, i.e. its change to a higher valence than the metallic state. Strictly speaking, high temperature oxidation is only one type of high temperature corrosion. In fact, oxidation is the most important high temperature corrosion reaction.

In most corrosive high temperature environments, oxidation often participates in the high temperature corrosion reactions, regardless of the predominant mode of corrosion. Alloys often rely upon the oxidation reaction to develop a protective scale to resist corrosion attack such as sulfidation, carburization and other forms of high temperature attack. In general, the names of the corrosion mechanisms are determined by the most abundant dominant corrosion products. For example:

Oxidation implies oxides,

Sulfidation implies sulfides,

Sulfidation/oxidation implies sulfides plus oxides, and

Carburization implies carbides.

High temperature corrosion is a widespread problem in various industries such as:

power generation (nuclear and fossil fuel)

aerospace and gas turbine

heat treating

mineral and metallurgical processing

chemical processing

refining and petrochemical

automotive

pulp and paper

waste incineration

Thermodynamic Principles

The stability of materials at high temperature has been traditionally introduced through plots of the standard free energy of reaction (G0) as a function of temperature, commonly called Ellingham diagrams. Such diagrams can help to visualize the relative stability of metals and their oxidized products. The values of G0 on an Ellingham diagram are expressed as kJ per mole O2 to normalize the scale and be able to compare the stability of these oxides directly, i.e. the lower the position of the line on the diagram the more stable is the oxide.

For a given reaction (M + O2 = MO2) and assuming that the activities of M and MO2 are taken as unity, the following equations can be used to express the oxygen partial pressure at which the metal and oxide coexist, i.e. the dissociation pressure of the oxide: or its logarithm form

The table visible here lists the coexistence equations, temperature ranges and standard energy changes that can be used to construct such diagrams. Ellingham diagrams may, of course, be constructed for any class of compounds. Vapor species that form in any given high temperature corrosion situation often have a strong influence on the rate of attack, the rate generally being accelerated when volatile corrosion products form. Gulbransen and Jansson have shown that metal and volatile oxide species are important in the kinetics of high temperature oxidation of carbon, silicon, molybdenum, and chromium. Six types of oxidation phenomena were identified: At low temperature, diffusion of oxygen and metal species through a compact oxide film

At moderate and high temperatures, a combination of oxide film formation and oxide volatility

At moderate and high temperatures, the formation of volatile metal and oxide species at the metal-oxide interface and transport through the oxide lattice and mechanically formed cracks in the oxide layer

At moderate and high temperatures, the direct formation of volatile oxide gases

At high temperature, the gaseous diffusion of oxygen through a barrier layer of volatilized oxides

At high temperature, spalling of metal and oxide particles.

Ellingham Diagram (Richardson Diagram)It is possible to use plots of the free energy of formation of metal oxides vs. temperature to predict the temperatures at which a metal is stable and the temperatures at which it will spontaneously oxidize. For temperatures at which the free energy of formation of the oxide is positive, the reverse reaction is favored and the oxide will spontaneously decompose to the metal.

From evaluation of the thermodynamic data presented in this figure, it can be seen that at 1100oC, Al will oxidize in an environment that has an oxygen partial pressure of 10-32 atm or greater, while chromium will oxidize in an oxygen partial pressure of 10-19 atm or higher. In general, a vacuum environment will be oxidizing to these elements unless a reducing species such as hydrogen is present. If inadequate oxygen is present, a non-protective oxide film may be formed which could promote alloy depletion and loss of strength.

Thermodynamic data for the oxidation of metalsRange Coexistence Equation Standard Free Energy Change

(K) (Oxidation Reaction) (J)

900-1154 Pd + 0.5 O2 = PdO -114,200 + 100 T (K)

884-1126 2 Mn304 + 0.5 O2 = 3 Mn2O3 -113,360 + 92.0 T

298-1300 3 CoO + 0.5 O2 = Co3O4 -183,200 + 148 T

892-1302 Cu2O + 0.5 O2 = 2 CuO -130,930 + 94.5 T

1396-1723 1.5 UO2 + 0.5 O2 = 0.5 U3O8 -166,900 + 84 T

878-1393 U4O9 + 0.5 O2 = 4/3 U3O8(-2) -164,400 + 82 T

967-1373 2 Fe3O4 + 0.5 O2 = 3 Fe2O3 -246,800 + 141.8 T

1489-1593 2 Cu + 0.5 O2 = Cu2O -166,900 + 43.5T

1356-1489 2 Cu + 0.5 O2 = Cu2O -190,300 + 89.5 T

924-1328 2 Cu + 0.5 O2 = Cu2O -166,900 + 71.1 T

992-1393 3 MnO + 0.5 O2 = Mn3O4 -222,540 + 111 T

1160-1371 Pb + 0.5 O2 = PbO -190,580 + 74.9 T

772-1160 Pb + 0.5 O2 = PbO -215,000 + 96.0 T

911-1376 Ni + 0.5 O2 = NiO -233,580 + 84.9 T

1173-1373 Co + 0.5 O2 = CoO -235,900 + 71.5 T

973-1273 10 WO2.90 + O2 = 10 WO3 -279,400 + 112 T

973-1273 10 WO2.72 + O2 = 10 WO2.90 -284,000 + 101 T

973-1273 1.39 WO2+0.5 O2 = 1.30 W O2.72 -249,310 + 62.7 T

973-1273 0.5 W + 0.5 O2 = 0.5 WO2 -287,400 + 84.9 T

949-1273 3 FeO + 0.5 O2 = Fe3O4 -311,600 + 123 T

770-980 Sn + 0.5 O2 = SnO2 -293,230 + 108 T

903-1540 Fe + 0.5 O2 = FeO -263,300 + 64.8 T

1025-1325 0.5 Mo + 0.5 O2 = 0.5 MoO2 -287,600 + 83.7 T

1050-1300 2 NbO2 + 0.5 O2 = Nb2O5 -313,520 + 78.2 T

693-1181 Zn + 0.5 O2 = ZnO -355,890 + 107.5 T

1300-1600 0.66 Cr + 0.5 O2 = 0.33 Cr2O3 -371,870 + 83.7 T

1050-1300 NbO + 0.5 O2 = NbO2 -360,160 + 72.4 T

923-1273 Mn + 0.5 O2 = MnO -388,770 + 76.3 T

1539-1823 Mn + 0.5 O2 = MnO -409,500 + 89.5 T

1073-1273 0.4 Ta + 0.5 O2 = 0.2 Ta2O5 -402,400 + 82.4 T

1050-1300 Nb + 0.5 O2 = NbO -420,000 + 89.5 T

298-1400 0.5 U + 0.5 O2 = 0.5 UO2 -539,600 + 83.7 T

1380-2500 Mg(v) + 0.5 O2 = MgO -759,600 - 30.83 T log T + 317 T

923-1380 Mg(l) + 0.5 O2 = MgO -608,200 - 1.00 T log T + 105 T

1124-1760 Ca + 0.5 O2 = CaO -642,500 + 107 T

1760-2500 Ca(v) + 0.5 O2 = CaO -795,200 + 195 T

Pilling-Bedworth RatioPhysical metallurgy relies on a set of guidelines to identify protective oxide coatings for corrosion protection. Such coatings should be pinhole-free, exactly as the IC industry requires for high-k metal oxides. Accordingly, guidelines developed for protective oxide coatings in physical metallurgy can be applied to the protective metal oxides in integrated circuits. In their 1923 paper "The oxidation of metals in high temperature" presented to the Institute of Metals, N. B. Pilling and R. E. Bedworth first correlated the porosity of a metal oxide with the specific density1. The Pilling-Bedworth ratio, (P-B ratio) R, of a metal oxide is defined as the ratio of the volume of the metal oxide, which is produced by the reaction of metal and oxygen, to the consumed metal volume:

M and D are the molecular weight and density of the metal oxide whose composition is (Metal)a(oxygen)b; m, and d are the atomic weight and density of the metal. Pilling and Bedworth realized that, when R is less than 1, a metal oxide tends to be porous and non-protective because it cannot cover the whole metal surface. Later researchers found that, for excessively large R, large compressive stresses are likely to exist in metal oxide, leading to buckling and spalling. In addition to R, factors such as the relative coefficients of thermal expansion and the adherence between metal oxide and metal should also be favorable in order to produce a protective oxide. Using the P-B ratio, Bruce Chalmers, Gordon McKay professor at Harvard University (Cambridge, MA), separated "protective" metal oxides from "non-protective" metal oxides. The table lists "protective" and "non-protective" metal oxides and their P-B ratios. Protective oxidesNon protective oxides

Be 1.59K 0.45

Cu 1.68Ag 1.59

Al 1.28Cd 1.21

Cr 1.99Ti 1.95

Mn 1.79Mo 3.40

Fe 1.77Hf 2.61

Co 1.99Sb 2.35

Ni 1.52W 3.40

Pd 1.60Ta 2.33

Pb 1.40U 3.05

Ce 1.16V 3.18

The list can be readily applied to the protective metal oxides used in integrated circuits. The intrinsic protective metal oxides, including the oxides of Be, Cu, Al, Cr, Mn, Fe, Co, Ni, Pd, Pb, and Ce, may be able to replace silicon oxide. On the other hand, a few popular metal oxides, e.g. Ti oxide and Ta oxide, are non-protective, suggesting a possible reason why these oxides have not been successfully used in commercial products after years of research. Besides oxides of elemental metal, the P-B ratio can be applied to oxides of metal alloy, metal nitrides and other metal ceramic systems. Protective metal oxides can be produced by two classes of methods: growth and deposition. Growth methods include thermal oxidation, plasma oxidation, anodization, and implantation. Deposition methods include direct sputtering, reactive sputtering, and CVD. The IC industry does not have much experience with production of protective metal oxides. Still, manufacturing experience with sub-10 nm silicon oxide suggests that it is better to form the protective metal oxide using a growth method, or to form at least a portion of the protective metal oxide using a growth method and the remaining portion using a depositing method. Improved oxide thickness uniformity is one added advantage of such a composite layer. High Temperature Corrosion KineticsThree basic kinetic laws have been used to characterize the oxidation rates of pure metals. It is important to bear in mind that these laws are based on relatively simple oxidation models. Practical oxidation problems usually involve alloys and considerably more complicated oxidation mechanisms and scale properties than considered in these simple analyses.

The parabolic rate law assumes that the diffusion of metal cations or oxygen anions is the rate controlling step and is derived from Fick's first law of diffusion. The concentrations of diffusing species at the oxide-metal and oxide-gas interfaces are assumed to be constant. The diffusivity of the oxide layer is also assumed to be invariant. This assumption implies that the oxide layer has to be uniform, continuous and of the single phase type. Strictly speaking, even for pure metals, this assumption is rarely valid. The rate constant, kp, changes with temperature according to an Arrhenius type relationship:

where:

x is the oxide film thickness (or the mass gain due to oxidation, which is proportional to the oxide film thickness)

t is time

kp is the rate constant, directly proportional to the diffusivity of the ionic species that is rate controlling

x0 is a constant

The logarithmic rate law is an empirical relationship with no fundamental underlying mechanism. This law is mainly applicable to thin oxide layers formed at relatively low temperatures, and therefore rarely applicable to high temperature engineering problems:

where:

ke is the rate constant

c and b are constants

The linear rate law is also an empirical relationship that is applicable to the formation and build-up of a non-protective oxide layer.

where:

kL is the rate constant

It is usually to be expected that the oxidation rate will decrease with time (parabolic behavior), due to an increasing oxide thickness acting as a stronger diffusion barrier with time. In the linear rate law, this effect is not applicable, due to the formation of a highly porous, poorly adherent or cracked non-protective oxide layers. Clearly, the linear rate law is highly undesirable.

Metals with linear oxidation kinetics at a certain temperature have a tendency to undergo so-called catastrophic oxidation (also referred to as breakaway corrosion) at higher temperatures. In this case, a rapid exothermic reaction occurs on the surface, which increases the surface temperature and the reaction rate even further. Metals that may undergo extremely rapid catastrophic oxidation include molybdenum, tungsten, osmium, rhenium and vanadium, associated with volatile oxide formation. In the case of magnesium, ignition of the metal may even occur. The formation of low-melting point oxidation products (eutectics) on the surface has also been associated with catastrophic oxidation. The presence of vanadium and lead oxide contamination in gases deserves special mention, as they pose a risk to inducing extremely high oxidation rates.

Practical High Temperature Corrosion ProblemsThe oxidation rate laws described above are simple models derived on the behavior of pure metals. In contrast, practical high temperature corrosion problems are much more complex and involve the use of alloys. For practical problems, both the corrosive environment and the high temperature corrosion mechanism(s) have to be understood. In the introduction, it was pointed out that several high temperature corrosion mechanisms exist. While considerable data is available from the literature for high temperature corrosion in air and low sulfur flue gases and for some other common refinery and petrochemical environments, small variations in the composition of a process stream or in operating conditions can cause markedly different corrosion rates. Therefore, the most reliable basis for material selection is operating experience from similar plants and environments or from pilot plant evaluation.

There are several ways of measuring the extent of high temperature corrosion attack. Measurement of weight change per unit area in a given time has been a popular procedure. However the weight change/area information is not directly related to the thickness (penetration) of corroded metal, which is often needed in assessing the strength of equipment components.

High Temperature Intergranular Attack

Metallographic method of measuring hot corrosion attack

D = original diameter

D1 = diameter of apparently useful metal

D2 = diameter of sound metal

The parameters shown in this figure relate to cylindrical specimens and provide information about the load bearing section (metal loss) and on the extent of grain boundary attack that can also affect structural integrity.Hot CorrosionAn accelerated corrosion of metal surfaces that results from the combined effect of oxidation and reactions with sulfur compounds and other contaminants, such us chlorides, to form a molten salt on a metal surface that fluxes, destroys, or disrupts the normal protective oxide.

Hot corrosion is an attack via a salt melt. A representative member of the hot corrosion agents is Na2SO4. Its generation is explained in combustion environments by

2 NaCl + SO2 + 0.5 O2 + H2O = Na2SO4 + 2 HCl

However, the effective corrosion action is generally believed not to be by the sulfate itself. It stems from the activity of Na2O arising from the decomposition

Na2SO4 = Na2O + SO3 because only the Na2O is attacking the protective scale from SiO2 by

x SiO2 + Na2O = Na2O*(SiO2)x This is the reason why other compounds releasing alkali oxide on decomposition, e.g. Na2CO3, have similar hot corrosion effects. It should be noted that complex salts, in particular those containing vanadium, can have an even stronger hot corrosion effect, as is well known from metals and oxides.HOT CORROSION by Vanadium Pentoxide (V2O5)

Ash forming contaminants in fuel: Vanadium, sodium and lead

Vanadium and sodium are the most harmful elements, respectively forming vanadium pentoxide (V2O5) and sodium sulfate (Na2SO4).

Vanadium appears in fuel in the form of oil-soluble porphyrins. These organic vanadium compounds decompose in the gas stream to give mainly V2O5. Vanadium pentoxide is most damaging since due to its low melting point (690 C) it is in its liquid state at normal combustion temperatures.

The mechanism of formation of vanadium compounds can be explained as follows:

In high-temperature (approximately 1,730 C), low-oxygen zones, the solid, non-volatile vanadium tetroxide (V2O4) is formed. Vanadium pentoxide is formed from the tetroxide in low-temperature (approximately 800 C), high-oxygen zones. In fact, deposits are observed to comprise both oxides, although the tetroxide oxidises to the pentoxide in excess air.

The extent of the vanadium attack on metal is determined by two factors :

The amount of corrosive vanadium compounds at the metal/oxide interface and the diffusion rate of oxygen to the metal oxide interface. In that respect, the nature of the oxide layer on the metal surface is very important as it may hinder oxygen diffusion towards the metal surface. For instance, Ni3(VO4)2 is formed on nickel-based alloys which forms a stagnant phase on its surface which stops oxidation

Sodium is normally present in the fuel as NaCl collected by the fuel either from underground water or transportation facilities. It can be removed by combined water-washing and subsequent centrifugation . Sulfidation attack (also known as "hot corrosion") is corrosion caused by sulfates, mainly Na2SO4, on nickel and aluminum alloys, by dissolving the carbide network of the metal. These alloys are commonly used in high metal temperature applications, in place of stainless steel and cobalt-based alloys. Gaseous sodium sulfate is almost harmless, unlike its solid or liquid forms, which are particularly harmful when they exceed the theoretical Na "dew point", over 60 ppm. This threshold is lowered by the presence of Mg, which could be introduced as an anti-vanadium reagent, and also by other alkaline metals such as Ca and K, which form eutectic mixtures with sodium (eg, Na2Mg(SO4)24H2O).

As a result, sodium can work to decrease the efficiency of Mg additives on vanadium corrosion, by forming eutectic mixtures of low melting point (below 590 C) with liquid vanadium pentoxide. Sodium vanadyl vanadate (Na2OV2O4V2O5), very corrosive above 647 C, is of particular importance . However, there seems to exist a threshold of Na2SO4 concentration beyond which the addition of more sodium sulphate decreases the corrosion activity of vanadium pentoxide.

Solid carbon deposits increase hot corrosion by reducing Na2SO4 to the very corrosive Na2S.

The harmful effects of Na are enhanced when Pb is present in the fuel. However, lead corrosion can also be tackled by Mg by means of the formation of compounds such as PbO and PbSO4 of high melting point. However, in the more realistic case of Pb, V and Na mixtures acting at high temperatures, Mg alone does not suffice against corrosion, and combinations of magnesium and silica are efficiently used if the concentration of sodium is reduced below 0.5 ppm by water-washing. Silica does not inhibit corrosion on its own, but enhances the efficiency of Mg additives and increases the friability (capacity of deposits to break into small pieces) of the ash deposits . Finally, greater amounts of Mg are to be used if the metal surface is exposed to higher temperatures.

Preventing or reducing ash deposition and corrosion

Several different methods have been devised in order to either diminish, avoid or remove corrosive deposits:

1. Coatings : Oxide scales build up a protective layer on the metal surface, separating the substrate from the corrosion environment. Coatings provide active elements for building-up this protective oxide scale .

2. Cooling of metal surfaces , intended to solidify deposits before their attack is initiated. Corrosion is increased exponentially by the metal surface temperature.

3. Additives: Additives combine with fuel constituents and combustion products to form solid, innocuous products that pass harmlessly through the combustion equipment. Additives may contain metals such as Mg, Al, Si, Mn or Ba , or combinations like Mg-Si, Mg-Al-Si, Al-Si. Data indicate that at certain temperatures and with sufficient additive the corrosion rates can be reduced to zero. Additives also help to decrease corrosion by preventing the catalytic formation of SO3). However, large concentrations of additives may increase particulate output. 4. Surface cleaning procedures in gas turbines: These can be split in three categories:

a. On-line techniques: Injection of an abrasive material such as crushed nutshells or coke. These techniques are effective on low temperature (under 970 C) deposits.

b. Off-line techniques: Usually a sequence of successive water-wash, soak and re-start is used to soften the MgSO4 deposits, then followed by the breakage of other deposits. Off-line techniques are efficient on deposits formed at any temperature, although at the cost of efficiency loss as the engine must be switched off.

c. Thermal excursions: Deposits suffer spallation if a sudden momentary increase of the turbine inlet gas temperature is caused.

Hot corrosion of ceramics

Hot corrosion is an attack via a salt melt. A representative member of the hot corrosion agents is Na2SO4. Its generation is explained in combustion environments by

2 NaCl + SO2 + 0.5 O2 + H2O = Na2SO4 + 2 HCl {1}

Since the action in the liquid state is regarded most effective, the first condition for the window of hot corrosion is given by melting and dewpoint of Na2SO4. The melting point of 884C does not vary much, the dewpoint depends on pressure, S-content of the fuel and NaCl-concentration in the atmosphere. For conditions with NaCl > 1ppm and combustion pressures of 100 bar it is calculated to run to 1100-1200C from eq. {1}.

However, the effective corrosion action is generally believed not to be by the sulfate itself. It stems from the activity of Na2O arising from the decomposition

Na2SO4 = Na2O + SO3 {2}

because only the Na2O is attacking the protective scale from SiO2 by

x SiO2 + Na2O = Na2O*(SiO2)x {3}

This is the reason why other compounds releasing alkali oxide on decomposition, e.g. Na2CO3, have similar hot corrosion effects. It should be noted that complex salts, in particular those containing vanadium, can have an even stronger hot corrosion effect, as is well known from metals and oxides

From eq. {2} it can be calculated that only for conditions with a low P(SO3) we will have a dissociation sufficient to drive reaction {3} to the right side. Consequently the window of hot corrosion has a third axis, the P(SO3) of the combustion, and it has been computed that the window should only open for low-S fuels ( 1000C).

In reality the attack starts already at the melting point of Na2SO4. This is attributed to internal controlling factors. Many SiC-based materials contain free carbon, which promotes the dissociation of Na2SO4 and makes the silicate melt more basic. The dramatic attack of Na2SO4 on SiC with free carbon has experimentally been confirmed.

As a result of new investigations of the hot corrosion of SiO2, Si3N4, and Si by Na2SO4-melts at temperatures up to 1150C, a new model for the hot corrosion attack of Na-sulfate on Si-containing materials was developed. In current models it is proposed that the Na2O-activity of the sulfate melt, and, hence, the corrosive attack is controlled by the partial pressures of gas species like O2 and SO3 in the ambient. In contrast to that it could be shown, that the oxygen partial pressure at the interface between substrate and sulfate melt is the controlling factor. Due to an extremely low oxygen partial pressure at the interface between Si-containing non-oxides and the sulfate melt, these materials act as reducing agents and promote the decomposition of the sulfate and the formation of Na2O. As a result a rather violent corrosion reaction is observed. In the case of SiO2 a much less violent reaction takes place which is related to a higher oxygen partial pressure at the interface.

During the hot corrosion of Si-containing materials by Na2SO4, SiO2-rich Na-silicate melts are formed which are immiscible with the overlying sulfate melt. Different surface tensions and dynamic processes at the interfaces between both melts lead to the formation of isolated melt areas at least in the beginning of the hot corrosion attack. Between these melt areas direct contact between substrate and sulfate melt exists allowing a local corrosive attack which may be responsible for pit formation.

To protect non-oxides against hot corrosion by Na2SO4 a direct contact between the substrate and the sulfate melt must be avoided. Preoxidation of the non-oxide materials to form thick and dense oxide scales or for example the deposition of mullite scales could be a suited means.