Waterside Scaling Deposition and Corrosion in Steam Generators

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TV Industrie Service GmbHTV SD GroupWestendstrasse 19980686 MnchenGermany

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Waterside Scaling, Deposition and Corrosion in

Steam GeneratorsLudwig Hoehenberger, TV SD, IS-ATW1, Munich

E-Mail: [email protected]

Frequently waterside deposition and corrosion were seen as two different andindependent incidents, but deposits at heated surfaces in steam generators particularly incombination with non-volatile constituents of the boiler water may lead to severe corrosionproblems, chiefly if the boiler feedwater and boiler water doesnt meet their requirements.Critical are mostly spots with the highest heat transfer or heat flux respectively.

Generals and Basics

A well-designed steam generator achieves satisfying lifetime onlyif the boiler steel is ableto develop and to maintain a thin protective layer of iron oxides (exactly composed ofthree different layers of oxides), with its main constituent magnetite (Fe3O4), see Figs. 1 a+2. A real protective magnetite layer must be compact, nearly free of pores and adherentto the metal surface.

The thickness of a protective magnetite layer depends on the material, operation tempera-ture and operation time and results for 100.000 hrs (12 years) - calculated for the wateror steam side - on low alloyed steel (2 Cr 1 Mo) at wall temperatures about 350 C(=125 bars) to 10 -15 m and for 500 C to about 350 m, see Fig. 3 and Photo 1.

Similar conditions for carbon steel (e. g. St. 35.8) lead to 10 - 20 times thicker layers. Acompact oxide layer of 300 m on the inner side of a evaporator tube increases the walltemperature for about 30-40 K.

Denseprotective layers (porosity < 5 %) doesnt affect the heat transfer significantly as faras their thickness do not exceed approx. 0.2 0.3 mm, depending on heat flux, but anyhigher porosity or stratified layer (scale) affects the wall temperature more and more, seeFigs. 4. A thin magnetite protection layer with low porosity is an urgent necessary scalebut must not be mixed up with deposit formation due to precipitates from the BW! Highlyporous layers of corrosion iron products can be seen in Photo 2, stratified layers ofmagnetite in Photo 3.

In practice, the best boiler feedwater (BFW) and boiler water (BW) contains always tracesof not avoidable impurities like non-volatile water constituents and particularly circulatingmetal compounds (corrosion products) of iron and copper (and zinc) in form ofundissolved (oxide particles) and/or dissolved solids.During start-up impurities shows much higher concentration, especially iron compounds.

Normal specifications for boiler feedwater for high-pressure boiler beside others are:

Acid-Conductivity at 25 C < 0.2 S/cm(=Cation Conductivity at 25 C < 0.2 micro mho/cm)Silica (silicon oxide) < 0.020 mg/l SiO2Iron total (dissolved + undissolved) < 0.020 mg/l Fe

optimal operation value < 0.005 mg/l FeCopper total (dissolved + undissolved) < 0.003 mg/l Cu

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Conductivity

The theoretic lowest conductivity of pure water at 25 C is about 0.055 S/cm, that meansgood quality BFW with an acid conductivity at 25 C of 0.15 S/cm contains still impuritiesof about 0.1 S/cm. These might be:

Volatile carbon dioxide from: Air (equilibrium up to 0.6 S/cm)Decomposed oxygen scavenger (e. g.

Carbohydrazide)Degraded organics

Steam-volatile organic acids from: Hydrolysed and oxidised organic aminesDegraded organics

Steam-volatile amines: Not 100 % hold back from the cation exchangerNon-volatile mineral acids from: Salt-like matter (slipped through the make up-water

demineralisation and/or through the condensatepolishing plant) exchanged in the cation exchangerinto acids.

Fig. 5shows the specific conductivity of some chemicals at low concentrations. Silica inppm-concentrations doesnt affect the conductivity.

Non-volatile impurities of the BFW were concentrated in the BW depending on theconcentration cycles, commonly in HP boilers around 50 to 200 times. This results tosignificant concentrations of impurities in the BW e.g. 20 g/l (ppb) chloride in the BFWwere concentrated to 1000 or 4000 g/l or 1 or 4 mg/l respectively. In case of AVTconditioning of the steam-water-cycle it may lead to a too low pH in BW and to corrosion.

Silica / Silicon oxide (SiO2)

Silica is commonly present in BFW up to the limits but in the BW mostly below the limitsand happens mostly from slippage through the demin plant and/or condensate polisher.Although silica is a solid mineral matter, it is steam soluble in considerable concentrations,see Fig. 6.

The steam solubility depends on pressure and temperature and results to a steam-side orsteam born blow down of the boiler, that means the number of concentration-cyclescannot be calculated from the silica concentrations of the BFW and BW. Consequentlysteam contains silica depending on pressure, temperature and concentration in the BW,see Fig. 7.

Frequently is thought that silica limits in BFW and BW were defined only for turbineoperation to avoid silica deposition in steam turbines, which is true on one hand but silicacan also form problematic silicate scale together with hardness or aluminium with low heattransfer (silica scale shows only 10 % of the heat transfer of phosphate scale!) and can beremoved with hydrofluoric acid only. Silicate scale may appear underneath oxide layers,see Photos 4 a-d.

Iron (Fe) and Copper (Cu)



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Iron compounds in alkaline BFW may be dissolved (e.g. in case of corrosion or erosioncorrosion [FAC] in the BFW-line) and undissolved in form of fine oxide particles.

In alkaline hot BW iron compounds were mostly present in form of undissolved oxideparticles because of the water circulation (agitation) and pH and temperature above BFWconditions.Contrary to iron, copper compounds are mostly soluble in water, particularly in presenceof ammonia and amines.

Limitations for both are expressed for BFW only because it is very difficult to take reallyrepresentative water samples including undissolved particles. Results for BW fluctuatingstrongly, which depends particularly on boiler load or pressure changes and samplingconditions (e.g. material for pipe-work, flow rate, vibrations, water hammer etc.)

Evaporation conditions and steam production

Steam generation in boilers and evaporation of water seems to be very simple and peoplemainly think they are wasting time if they try to understand the proceedings of boilingwater at a heated surface in a steam generator.

The literature offers catch-words like under-cooled boiling, nucleate boiling, filmboiling, steam blanketing etc. and suddenly boiling of water becomes a very complexprocess if one will understand the conditions directly at the heated surface.This paper will not explain all these effects but focuses on the deposition and corrosionrelated effects during steam production.

If one wants to understand these localised conditions regarding deposition and corrosionit is necessary to realise that exactly the interface between metal surface and boiler wateris the most important one because it is the location:

Where heat from the furnace heats the fireside of the evaporator surface andcreates there a metal temperature T1

Where the energy from the furnace passes through the evaporator wall to the BW andcreates on the waterside a metal temperature T2.

T1 is always higher than T2, see example Photo 5. Waterside scale increases T2!

Where the protective magnetite and other layer on the BW side are built

Where the wall material must be cooled to avoid over-heating

Where BW evaporates, that means- BW must be transported to the surface (mass flow),- pure water evaporates to pure steam and- water with dissolved and undissolved impurities becomes locally concentrated to

conditions far away from that of the circulating BW

Cooling effects and heat extraction from evaporator surfaces

Very important is the fact, that the best cooling effect to evaporator surfaces shows liquidwater under boiling (saturated) conditions in equilibrium between pressure andtemperature because the (latent) evaporation energy extracts most energy from thesurface. For example the evaporation energy at 120 bars (325 C) is about 1.200 kJ/kg.



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Water below saturated conditions must be heated first to mentioned conditions and duringheating of hot water to boiling conditions liquid water extracts for a temperature difference

of e.g. 20 K at 120 bar only around 130 kJ/kg from the surface, see enthalpy tables.These conditions may lead also to under-cooled boiling.

Even superheating of steam shows low cooling effect because superheating to about 20C or 120 C above saturated conditions at 120 bar extracts only about 150 kJ/kg or 520kJ/kg from the surface.

Erosion Corrosion / Flow assisted Corrosion (FAC)

Erosion corrosion or flow assisted (accelerated) corrosion (FAC) results if an aqueousmedium or gas (e.g. air, flue gas, steam) with high flow rate removes an existing or freshlybuilt protection layer faster than it can be newly built. Media with two phases e.g. gas &

ash, water & sand, steam & water droplets, water & steam bubbles are more dangerous.The latter both may also cause additional impingement or cavitation attack.

Steel may be affected at a flow rate > 8 - 10 m/s in weak alkaline medium (pH < 8.5 9),particularly at temperatures around 150 C in absence of oxygen. An increasing pH andlittle of oxygen (about 15-30 ppb) are reducing the risk of FAC, see Fig. 8. Also materialwith increased Cr-content reduces the risk of FAC, see Fig. 9.

Particularly susceptible for erosion corrosion are metals with relatively soft protectivelayers, like copper and aluminium. For copper and aluminium the local maximal flowvelocity of a non-corrosive medium should not exceed 1.6 - 1.8 m/s.

Eroded surfaces are mostly metallic shiny without corrosion product and appear a smoothsurface (structure like sand dunes).

Hot Water Oxidation (Steam Side Burning)

This corrosion phenomenon was formerly called inexactly Steam Splitting Corrosion anddescribes an excessive reaction of carbon steel or low alloyed steel and hot water orsteam forming oxide layers and hydrogen.

At temperatures below 570 C iron and hot water or steam react slowly to little wustite(FeO), major magnetite (Fe3O4) and again little hematite (Fe2O3), see Fig. 1 a. Thereaction rises with increasing temperature, but is acceptable low below 450 C!

[1] Simplified: 3 Fe + 4 H2O Fe3O4 + 4 H2At temperatures > 570 C (see Fig. 1b) iron and hot water or steam react very fastprimarily to FeO (wustite), little Fe3O4(magnetite) and again little Fe2O3(hematite), seePhoto 6 a+b.

[2] Simplified: Fe + H2O FeO + H2

The hydrogen produced exists first in atomic status and recombines later into molecularhydrogen. Atomic hydrogen is able to migrate into the metal lattice and may causehydrogen damage on carbon steel and low-alloyed steel, see Photo 6 c.



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The hot water oxidation rises exclusively in heated boiler tubes as a consequence of toohigh temperature at the tube walls due to unsatisfactory cooling of the tubes. This process

can already be initiatedat wall temperatures from ca. 500 C upwards.

On-load Corrosion / Under-deposit Corrosion

Localised corrosion at boiler evaporator areas with high heat transfer due to deposition ofcorrosion products - first of inert iron oxides precipitated from the BW - subsequentlyformed on site, in combination with boiler water notin accordance with the specification.At areas with high heat flux, first porous (sponge like) deposits of undissolved solidsbecame formed. Soluble impurities of the BW penetrate into these porous deposits,accumulate and concentrate and remain there for longer time frequently in presence ofan insulating steam phase within the pores. The result is a material attack either due to alow pH (by acidic acting boiler water remains, e.g. from cooling water ingress) or high pH

(by caustic acting remains, caustic gouging).

Hydrogen Damage

Hydrogen damage covers different material defects due to hydrogen influence, likehydrogen embrittlement, decarburisation, fissure and crack formation on carbon and low-alloyed steels. Affecting is atomic hydrogen only, developed by corrosion effects as wellas by thermalformation from molecular hydrogen depending of its partial pressure.

In boilers, significant amount of hydrogen may be produced by an excessive reaction ofiron with water or steam to iron oxide and hydrogen, particularly at temperatures > 570 C:

[3a] Fe + H2O FeO + 2 {H} atomic[3b] 2 {H} H2 molecular.

Atomic hydrogen migrates into the lattice of the mentioned steels and leads to hydrogenembrittlement and after molecule formation at voids or inter-metallic phases (e.g. manga-nese sulfide) to inter-granular fissures, some times in the shape of a fish eye and to brittleruptures (see Photo 6 c).Atomic hydrogen reacts additionally with carbon of the steel by developing methane thatproduces localised fissures and cracks too. Because of the reduction of the carboncontent in the steel, in parallel its strength will be reduced.

Statement

Considering the very important formation of protective layers and the mentionedevaporation and operation conditions of boilers, scaling and deposition is a logicalconsequence of longer boiler operation and may lead to local corrosion, as far as scale istoo thick, too porous or contains aggressive media.

Whilst scale formation of protective layers in both boilers and super-heaters isessentially, in boilers additionally deposition of omnipresent iron corrosion product existingundissolved in the BW cant be avoided. Preferable locations for deposition areevaporator areas with the highest heat transfer (heat flux) and/or low mass flow (lack inBW flow due to boiler design and operational conditions like heavily fluctuating load and

too fast start-up as well as deposition).



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Super-heaters facing different conditions because the medium steam is purer than BWand the temperatures are much higher.

First the porosity of the protective magnetite layer but also from other deposits isimportant. As higher the porosity as lesser the heat transfer due to stagnant steam phasewithin the pores and as higher the wall temperature depending of the heat flux.

In evaporators made of carbon steel with operational temperatures < 400 C the thicknessof the real protective layer is very thin (


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Contrary to boilers, super-heaters and re-heaters show rarely porous deposits of ironoxide but mostly compact thicker protecting oxide layers due to the elevated

temperatures. Temperatures above design (considering steam temperature and material)lead to too thick oxide layers, which chips away due to the increasing thermal expansionrate of oxide in comparison of that of steel, see Fig. 10 and Photo 1 & 7.

Contamination of super-heaters or re-heaters is possible if the steam/water separation inthe drum is insufficient or if particular operational conditions (like heavily load, pressure,and/or drum level fluctuations) lead to carry-over of BW. To the extent that the BWcontains non-volatile substances, deposition of these substances at the inlet of theheaters is possible.Another reason for deposition is contaminated spray water for steam attemperation, whichmay also contain iron and copper compounds.

As far as the contamination of the steam is not severe, deposition in super-heaters or re-heaters must not be the logical consequence because many solids are more or less reallysoluble in steam. It is well known, that silica is highly steam soluble, see Fig. 6, evencaustic soda (NaOH, see Figs. 11 & 12) shows a significant solubility in steam. Thesteam solubility for phosphates, chlorides, sulfates and metal oxides is much lesser.

The steam solubility of solids is strongly depended of pressure and temperature, whichconveys problems into turbines or steam lines for pack-pressure or extracting steam.However turbines are mostly affected by deposits and subsequent lower efficiency, insteam lines caustic stress corrosion cracking is not rare, particularly if superheated steamcontains more than 10 ppb sodium hydroxide!

Photographs: 1-7Figures: 1-12

Photographs:

Photo 1



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Photo 2



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Photo 3

Photo 4 a



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Photo 4 b

Photo 4c

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Photo 4d

Photo 5



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Photo 6 a

Photo 6 b



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Photo 6 c

Photo 7



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Figures

Fig. 1

Fig. 2



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Fig. 3



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Figs. 4

Fig. 5

Fig. 6



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Fig. 7

Fig. 8



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Fig. 9

Fig. 10



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Fig. 11

Fig. 12

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Waterside Scaling Deposition and Corrosion in Steam Generators