HOT CORROSION , A DETAILED ANALYSIS

Hot corrosion mechanism

Study of  mechanism involved in  hot corrosion and erosion requires a good knowledge regarding the  identification of  the particle formation mechanisms. In large-scale diesel engines these mechanisms vary in magnitude and proportion basically due to environmental variables as compared to  light-duty or heavy-duty engines, due to different fuel specifications initially to begin with.

The fuel in large-scale diesel engines usually contains a high fraction of aromatics and ash (approx. 0.1 wt. %), which is composed of vanadium, sodium, calcium, nickel, magnesium, aluminium, and silicon. In addition, the sulphur content of the fuel oil is usually high, approx. 3 wt. %. Therefore, low-melting vanadylvanadates are formed. The high calcium content of the lubricating oil may also have a profound effect on the corrosion of some materials/coatings.

The residence time to prevent the nucleation of hydrocarbon (HC) species and, in the case of high S-content fuel oils, the nucleation/condensation of sulphuric acid. Nucleation is a competing process with adsorption and condensation on pre-existing soot particles. Thus the dynamic behavior of this vapor-particle system in the dilution process will determine the number size distribution after dilution and cooling. Particle and deposit characteristics play an important role in studying corrosion and erosion phenomena in combustion systems. Furthermore, the corrosion and erosion of different materials nearly always involve a growing deposit layer. To determine the effects and mechanisms of corrosion and erosion, the morphology structure, and composition of the depositing particles and the deposit are important. Additionally, the chemical reactions taking place in the deposit layer need to be recognized in order to find out how the depositing particles start to react/transform into the actual deposit. The formation of the deposit may have crucial effects on the corrosion/erosion mechanisms; these mechanisms can be different if the deposit is absent from the surfaces under corrosive/erosive attack.

Attempt is made to simplify the mechanism , as well as to explain in depth , so that the process can be understood and appreciated.

The  in the process of blending fuel , no method is specified other than the end result as ISO 8217/2015 - used Lubricating oil blending does take place. The effects sometimes detrimental due to burning characteristics and trace elements present.Some aid in reduction of corrosion - but the overall effect is to be studied basis condition checks and a thorough understanding of the process will aid in prevention or preventive steps , considering no punacea in the ever changing scenarios .

Effects of fuel additives in particulates and corrosive formations

Water : emulsifying, gave results that depended on the quality of the fuel oil; the addition of small amounts of water increased the particle mass concentrations for fuel oils with a high aromatic content (46 wt. %) and decreased them for fuel oils with a lower aromatic content

Mg-based compounds  prevent the formation of highly corrosive vanadylvanadate compounds that contain mainly vanadium, sulphur, and sodium of different compositions and have low melting points  the particle mass concentrations decreased clearly and SO3(g) concentrations increased. The main mechanisms for reduced particle emissions were: easily ionisable elements that reduce soot formation; elements that enhance OH-radical formation and increase soot oxidation, and elements that get absorbed by soot and increase their combustion rate . Mg can also enhance the burnout of heavy aromatics, soot, carbon, etc. in the particles acting as a carbon oxidation accelerator

The fuel oil additives used in burning heavy fuel oils are usually Mg-based vanadylvanadates that have melting points that are clearly higher than those of their sodium counterparts. Thus the deposits are no longer molten/highly The advantage of using these additives is that magnesium reacts with vanadium to
 form magnesium sintered and the detrimental fluxing of the elements is hindered. Moreover, the structure of the deposit is transformed into a more porous and less adhesive form .

To attain these favourable effects, the magnesium-vanadium weight ratio should be approx. 1.5:1 (3:1 in molar ratio ). Even higher Mg-V weight ratios are beneficial and silicon can also be advantageous . However, especially in oil-fired furnaces, the use of Mg-based additives causes other problems. The deposits can build bridges between heat exchanger tubes because of increased ash loading and thus hinder effective heat transfer . Increased ash loadings may also cause problems.

Ferrocene (Fe(C5H5)2  can enhance soot oxidation, thus reducing emissions.

 Cerium has also been studied as a fuel oil additive. It has been proposed that a Ce additive mainly reduces the concentration of organic carbon (OC), but not so much the concentration of elemental (EC) or black carbon (BC) in the exhaust particles

Mechanisms for deposition build-up

The mechanisms for deposition build-up on various components depends upon the conditions existing at the time of combustion and post combustion with reference to gas flow and constrictions. mechanisms for particles present in the gas phase are direct impaction, turbulent impaction, thermophoresis, Brownian motion, and gravitation. Vapours can deposit by direct condensation or by chemisorption, i.e. by adsorption on to the surface followed by chemical reaction

Basic mechanisms of deposition  build-up can be considered as follows:

1.       Direct and Turbulent impaction : Main deposition mechanism due to impaction at bends or direct incidental surfaces with a heavier particle sizes of higher velocities  with lower gas velocities The main issues to consider in particle deposition are the turbulence of the flow and the boundary layer of the particle concentration, consisting of the buffer layer and laminar sublayer. The turbulence of the flow will have an impact on all of the deposition mechanisms because it controls how great an amount of the particles will be able to penetrate the buffer layer. In turbulent impaction the turbulent eddies transport the particles through the buffer layer into the laminar sublayer. To be able to deposit on to the system surfaces the particles will have to penetrate through the laminar sublayer; this can occur if the particles have enough momentum from the turbulent flow. The mechanism is then called turbulent impaction. one example is soot along the pipelines , surfaces and fins on economizers(usually accounts to 2/3 rds of deposits).

1.         2. Thermophoresis, Brawnian movement Gravitation and Chemisorption and Maragoniconvection : Deposition mechanism due to smaller particles , Thermophoresis is defined as the movement of particles caused by the hot, more energetic gaseous molecules towards the gaseous molecules near the colder surface(s). where the impact is due to absorption/ adsorption on the surface due to surface roughness and peripheral layer being the reactive layer.This chemical effects aided by the rate of change of surface tension with temperature (Maragoni convection) in molten phase can also yield or emphasize reactivities leading to oxidation/detachment.

Deposition morphology and flake-off

Corrosion build up advances  from the bottom of the pit towards the surface of the deposit, the structure analyzed was initially crystalline due to various trace elements also sublimated under combustion and deposited forming complexes (such as Ca), but transformed near and at the surface of the pit into one that was more irregular and sintered/fused, increasing porosity.. This is caused by outward Cr lattice/grain boundary diffusion. From the interface between the pit and bottom reacted layer needles/pegs penetrating into the bottom reacted layer were seen the “keying effect” The pegs tend to  serve as “anchors” for the deposit/oxide layer . Al was found at the vertexes of these pegs , due to reverse convections on account of variable surface gradients of metal/oxide/ liquid sinters , resulting in not enough Al to form a continuous aluminum oxide layer.

A zone of “black islands”, an indicative of internal corrosion form in the bottom layer under the pit with varied sizes.

  Cr, Al, Ti, and Fe diffused from the base material (Nimonic 80 A) towards the surface of the pit. The highest Ti and Cr concentrations were observed at, or in the vicinity of, the bottom of the pit. The Cr concentration decreased almost steadily until it was almost zero near the deposit base, except for the case at 750°C, where Cr had propagated further in the deposit than in the other cases. Similar behaviour at 750°C was also observed for Ti. However, oxygen behaved differently in the high-temperature. At 700°C experiments the oxygen concentration increased as the concentration of Ni and V decreased and vice versa. Vanadium, on the other hand, diffused strongly towards the base material, as it was found throughout the corrosion pit, although the relative concentration of V was much lower in the high-temperature 750°C case than in the 700°C case. S was not able to penetrate deeply into the pit or the bottom layer. On the other hand, distinct “black islands” rich in S were detected in the bottom layer

Deposition flake-off and particle re-entrainment are the key causes of irregular shaped particles, thus ensuring . peeling of the deposits, mainly from the exhaust valve surfaces (also turbine blades. This could be explained by the cleavage of the deposit material and/or re-entrained particles from the surfaces of the combustion system.

There are several factors that may affect the cleavage of the deposit layer or the stickiness of the deposit, viz. fuel composition (the amount of ash), fuel additives, the composition of the lubricating oil, the chemical environment of the elements, the materials (valve materials, hardfacings, turbine blade materials, etc.), system temperatures, and flow velocities. Another important factor is the carbon (C) present in various nascent  forms in the particles; because these particles deposit on the surfaces, and nascent carbon present in the particles is burned off, heat is released. The released localized  heat can also have an effect on the deposit stickiness because of  instantaneous local spot  temperature increase, which facilitates melting/sintering phenomena. The chemical reaction(s) taking place in the deposit can also vary significantly, because the gas atmosphere surrounding the deposit may first be reducing and then convert to oxidizing one due to convective currents.

While all of these factors have an impact on the deposit stickiness, the contributing factor to all these can be  the composition of the fuel and lubricating oil, different materials, and process and material temperatures  determining the melting/sintering tendency of the deposited particles.

The effect of fuel oil composition on the composition, structure, and stickiness of the deposit altered significantly when the high ash content fuel oil was doped with a Mg-Si-based fuel oil additive; the thickness of the deposit layers was about 1/10 of that obtained with HFO only. The structure of the deposit was also more porous and clinker-like structures (cobble-stone) were more uncommon. The porous and brittle structure of the deposit contributed to the small amount of the deposit and the ease of cleavage.

The most critical variable in using fuel oil additives with high ash HFO fuel oils is the proportion of Mg-V and Si-Mg . The Mg in the additive mainly serves as a replacement for the Na present in the fuel oils, converting the sodium vanadylvanadates that may possibly form into less sticky magnesium vanadates with a higher melting point, thus reducing the amount and the corrosiveness of the deposition

.Corrosion propagation

The deposits that take place during transient conditions cannot be termed as totally melted ones , as a molten pool will have solid deposit due to flow of gases/ transport phenomena of other elements. One such phenomena is Sulphur and other elements , before the detachment and further action takes place , which can be explained as under for further propagation.

Sulphur transport mode (atomic or molecular SO2/SO3 diffusion) into the deposit/base material . For Nimonic 80 A under these conditions the only mode for sulphur transport was molecular SO2/SO3 diffusion. However, from the bottom of the deposit into the base material sulphur transport occurs by atomic sulphur diffusion via grain boundaries and/or by lattice diffusion.

As the corrosion at 700°C was strongest with both SO2(g) and Vanadium feeds, the mutual effect of SO2/SO3 and vanadium is critical. The Cr-based oxide layer in the pit area transformed into a more permeable form so that the diffusion over that layer with a  more rapid and  sulphur can increase the activity of vanadium compounds (mainly V2O5) and therefore enhance corrosion by solid/gas transport promoted by a trace of V in the oxide lattice, and no liquid phase is present at the scale/base material interface (no fluxing of the elements). This would explain the faster corrosion found with both SO2 and Vanadium feeds, even though no severe indications of sintering can be noticed.

The deposit and in the pit underneath the first particles formed by nucleation and were composed of NiO followed by sodium vanadates (Na2O·V2O5). Finally, vanadium oxide (VO2) and Na2SO4 condensedon the surfaces of the nucleated particles. However, VO2  oxidised further into V2O5. The composition of the particles indicated that the deposit contained, at least at the beginning, sodium sulphate.The sulphur (as SO2/SO3) diffused through the porous deposit layer by molecular diffusion. When S, as SO2/SO3, reached the interface of the bottom layer lower  in Cr and closer to base material, it reacted with Cr, which is the most stable oxide, and also with Ti and Ni to form the internal sulphidation , forming precipitates seen as “black islands” at the bottom layer initially due to low oxygen and distributed via the grain boundaries.

Availability of oxygen in transient exhaust further results in diffusion of molecular Oxygen through porous layers and displace sulphurin internal sulphidation by way of catalyzed V2O5 and furthering oxidation of Sulphur  to So2/SO3 – thus making a Sulphite layer into oxide layer and then the process continues to have a further front of Black islands of internal sulphidation reaction – under further favourable transient conditions of availability of Sulphur and Vanadium fronts.

Vanadium is known to (V2O5) catalyse the SO2(g)+O2(g) oxidation reaction, the yield being heavily dependent on the gas atmosphere the conversions can be as high as 80% in the presence of vanadium, but only a few per cent without vanadium. Because of the high amount of V present in the system as V2O5 (highly oxidising environment), SO2 can further react to SO3. SO3 is even more reactive than SO2. Therefore, with both SO2(g) and synthetic ash particles (SAP, including vanadium), more SO3  was formed and the corrosion rate can accelerate.

 The  most stable oxides at the pit are Cr2O3 and V2O5, but nickel is in the form of NiSO4 or (NiO)(Cr2O3). The most stable oxide forms are highly dependent on partial pressures of sulphur dioxide and oxygen., that the formation of the nickel chromates and sulphates leading to  type II hot corrosion, also called “low temperature hot corrosion” .