CORROSION ATLAS CASE HISTORY

01.01.04.0S

MATERIAL

SYSTEM

PART

PHENOMENON

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

Carbon steel (ASME SA-106-B, similar to AISI 1030).

Oil-fired water-tube boiler.

Horizontal economizer tube (00: 4.S em).

Acid corrosion (cold-end corrosion).

The external surface, including fin surfaces, exhibited smooth, general metal loss.

Unknown.

S02 and S03-containing flue-gases. Temperature: entry: 300°C; exit: ISSoC.

Condensation of sulphuric acid at the cold end of the boiler, the economizer, where the temperature of the metal drops below the sulphuric acid dew-point of the flue-gas . The sulphuric acid attacks the steel (dew-point corrosion). The oil contained 1.9% S. The pH of a I % slurry of the corrosion products was measured at 2.3 (see also Case History 01.01.04.03).

• Specify fuel with low sulphur and low moisture content. • Operating the boiler at or below 5% excess air will keep the SO, percentage as low

as possible. • Substantial design changes are often required to eliminate metals with surface

temperatures below the sulphuric acid dew-point. • Feed fue l additi ves as a chemical solution of the problem.

t9

CASE HISTORY

CORROSION ATLAS 01.01.04.10

MATERIAL

SYSTEM

PART

PHENOMENON

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

Carbon steel (35.8).

Was(e-incineration boiler.

Header of the vaporizer section of the rotating drum furnace.

Dew-point corrosion.

Pilling attack to the welds, which ronn the connection between the header and the vaporizer tubes. More

specifically in the weld zone with a characteristic cast structure (stalk-shaped crystal s).

8 years (during which leakages occured regularly).

Acid off-gases condensing in boiler [eedwater (demineralized make-up water + recycled condensate). Acid deposits consisting of si licates, iron oxide and chlorides.

Because of leaking economizer tubes, part of the vaporizer section was regularly hosed with water, resu lting in local dew-point corrosion due to contact with the acid off-gases. This caused pitting attack to the welds as these are partly less corrosion-resistant, because as weld metal hardens a coarse cast structure is formed with segregation phenomena (see microphoto), which makes these parts less homogeneous and hence more sensitive to corrosion.

Partial replacement of the economizer.

2 1

20

CORROSION ATLAS CASE HISTORY

0 1.01.04.09

MATERIAL

SYSTEM

PART

PHENOMENON

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

Carbon steel (ASME SA-I92, similar to AISI 1012).

Black liquor-fired recovery water-tube boiler (pressure 4.1 MP.).

Vertical screen tube (00: 5.1 cm).

Acid corrosion (dew-point corrosion).

General metal loss with an irregular surface contour.

!O years.

Sulphur-containing deposits.

Combustion of black liquor gives sulphurous ash depos its covering the tubes. As the boiler cools during idle periods, the temperature of its external surface may drop below the dew-point, allowing moi sture to form on the tube surface. The acidic solution formed attacks the steel (see also Case History 01.01.04.02).

• Remove the fire-side depOSition from metal surfaces immediately after boiler shutdown by using high-pressure water sprays, followed by a lime wash to neutralize remaining acidic substances.

• Keep the boiler firc side dry , by circulation of warm boiler water (by means of a heat exchanger and another boiler), or by circulation of dried air.

• In general, specify fuels with lower sulphur content to prevent dew-point corrosion.

22

CORROSION ATLAS CAS E HISTORY

01.01.04.1 I

MATERIAL

SYSTEM

PART

PHENOMENON

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

Carbon steel (35.8).

Shell-type boiler; 0.15 MPa.

Fire tubes.

Acid corrosion.

Severe localized uniform corrosion with a yellow and red corrosion scale.

3 months (upper tube) to 2 years (Jower tube).

Boiler water (P-alkalinity: 10-20 meq/I; chloride: 100 to 400 ppm CI; phosphate: 25- 50 ppm P,O,; sulphite: 50- 100 ppm Na,S03; sulphate: 5,000- 10,000 ppm Na,SO •.

This was suspected to be a form of acid corrosion by the sulphite (sulphurous acid). Investigation by Kema

(see next page) led 10 the following conclusions:

In view of the uniform attack, the presence of the sulphur-rich compound on the oxide-metal interface, and

the local pits underneath the oxide layer where the sulphur-nch compound was found , the cOlTos ion is

believed very probably to be a consequence of an acid sulphur compound.

It may be that the heat load coupled with high sulphite dosages enables an acid to form. such as sulphuric

acid, sulphurolls acid or poly thionic acid.

• Replace the sulphite by another oxygen scavenger. • Change to thennal de aeration of the feedwater. • Usc a calmed steel (with higher Si-content) which is morc resistant to acids.

Case History 01.01.04.11 Microscopic research by Kema Nederland B.V.

21 nn Rer: iii <on %-OI . P'IP' B4 I'r . Nr: . 51310.1l1lll LtI9fI042:t -Fig. 1 Close-up of one of the pipes

Fig. 3 Photograph taken with polarized light

Discussion

Fig. 2 Photograph taken with incident light

Fig. 4 DICOM image

Various photomicrography techniques were applied: normal incident light, polarized light, and the DIeOM imaging technique. In the polarized light photograph (Fig. 3) , a yellowish-orange corrosion product is found. Electron microscopy and EDS analyses were used to analyse the phases at various locations. In all the specimens , a sulphur-rich compound is found on the oxide-metal interface. The phase which reflects orange in polarized light is also found to contain much sulphur. Note the skeleton structure in the oxide and the pits on the oxide-metal interface, where a clear sulphur enrichment is found. In the electron microscopy image (not reproduced here), even small intergranu­lar sites of attack are visible.

23

24

CORROSION ATLAS CASE HISTORY

0 1.01.06.0 I

MATERIAL Steel (13 CrMo 9 10).

SYSTEM Oil-fired water-tube boiler (6.1 MPa).

PART Superheating tube.

PHENOMENON High-temperature corrosion (oxidation).

" " J" " I " "~"' I'~RWJ\'~~ " I" " J" " I""J"" I ""JIIIIII IIIJIIIIIIIIIJIIIIIII IIJ IIII I I IIIJIIIIIII II JlII I ' 11 ' J" "I " "J "" I ""J""I"' IJ" " I o 11 ~2 13 14 15 16 17 18 19 20 21 2 23 24 25 21 1IllIllldllllllll llllllJllllhllllllld ll lllllldlll lllllll'lllllllllllllllIld llll ll ,IlIIlIlIl';;I II IIJ II III IIII 111 111 11 11 11 1111111111111111111,1,111111111

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

External uniform attack leading to cracking by superheating.

5 years.

Deposit containing over 70% sodium sulphate and 4% vanadium oxide.

The presence of oxides of heavy metals (V, W, Nb) catalyses the oxidation at hi gh temperature. Heavy oi l contains sulphur, vanadium and sodium. At wall temperatures above 600°C vanadium oxide and sodium sulphate melts occur on the superheater tubes, leading to corrosion of the steel. This phenomenon is also known as hot corrosion or in oil-fired boilers oil-ash corrosion.

Initially, dosing of various oil additives proved ineffecti ve; subseq uently, for other reasons as well, switched to firing sulphur-free natural gas; the entire superheater was replaced.

J

gh ,;

m

r

CASE HISTORY

CORROSION ATLAS 01.01.07.01

MATERIAL Carbon steel.

SYSTEM Forced circulation water-tube boiler (pressure 10 MPa).

PART Evaporator tube .

PHENOMENON Heat-nux corrosion (chloride induced).

~;\'~ ,..>' \, 't"'/'~, '" "" ~./ % :""'<"'" " '" 'T~~~~ ,:1. VI'nldI'lIJeI"PIJP '/ :', 5 I1t "/I) '" / k~" '/4-'" ' ," ~ w: "", "1"-~'! ' ;:;c 1

A ~ 'F WI}. ,£ '" '" ' ,~A d';:",

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

The weld and the surrounding pipe surface was corroded.

5 years.

Inside: ammonia-conditioned boiler water.

Condenser in leakages resulted in chloride-containing boiler water. The pre' laminated oxide layers are typical of Cl-induced corrosion under heat-tlux conditions. The corrosion started from the weld and expanded over the pipe surface. The weld material has too Iowa corrosion resistance for application in evaporators. The critica1 FeCI, value was only 15, whereas the recommended selection criterion is 35. (Ref. Huijbregts, W.M.M.: "Corrosion in Power Stations", paper presented at VGB conference: Chemistry at Power Stations, Essen, 1994.

• More attention was given to boiler water quality, and the condenser inleakages were prevented.

• In this case the tube was replaced without paying attention to the composition of the steel and weJdm1

material.

• To prevent chloride-induced heat-flux corrosion: specify the acid-corrosion resistance of carbon Sled

(tube and welding material) for evaporators. A critical FeCI :!: value of at least more than 35 mmol i!

recommended.

26

CASE HISTORY

CORROSION ATLAS 01.01.07.02

MATERIAL

SYSTEM

PART

PHENOMENON

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

Carbon steel.

Experimental water-tube boiler.

Evaporator tube.

Heat-flux corrosion (alkali induced).

'~.'

KEMA~

A thick porous oxide layer wi th pitting underneath the oxide scale . The lopolactically grown oxide is very

porous. The cpitactica i oxide layer on the outside consists of coarse crystalline magnetite octahedra.

2 weeks.

Boiler water with NaOH water treatment.

Water conditioning with non-volatile NaOH and presence of a fouling layer of iron oxides. Because of high heat flux under the deposit layer, the NaOH concentrates to high values, resul ting in corrosion of the carbon steer .

• Chemical cleaning of the tubes to remove the fouling layer of iron oxides. • Changing the water chemistry method to a volatile method (with prevention of

condenser cooling water in leakages).

[ CORROSION ATLAS CASE HISTORY

01.01.07.0 '

MATERIAL

SYSTEM

PART

PHENOMENON

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

Carbon steel.

Experimental water-tube boiler.

Evaporator tube.

Heat-flux corrosion (induccd by nucleate boiling).

•

Pock-shaped pustules (like doughnuts), with pitting underneath.

2 weeks.

Boi ler water with ammonia treatment.

Nucleate boiling causes depos it pustules to form, under which slight corrosion takl

place in the case of non-severe aggressive water. The original protective magnetite layer can often be found on the metal-deposit interface. In pure boiler water, this pitting is not a severe problem. In fo uled boiler tubes, this nucleate boiling pitting can resul t in se,'ere damage (see also Case History 05 .01.07.0 I). The corrosion is frequentl y and erroneously ascribed to oxygen corrosion during shut-down periods .

• In thc case of fouled boiler tubes, lower heat flux by mounting clay mass on the tubes with highest heat fluxes .

• Avoid use of non-volatile water conditioning chemicals and prevent cooling water inlcakages.

28

CORROSION ATLAS CASE HISTORY

01.01.07.04

MATERIAL

SYSTEM

PART

PHENOMENON

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

Carbon steel.

Waste-heat boiler (pressure 4.0 MPa).

Evaporator tube.

Heat-flux corrosion (induced by nucleate boiling).

Thick corrosion scale at the heat-flux side. Only slight deposits on the non-heated side. Pitting corrosion on the boundary of the thick corrosion scale.

Unknown.

Boiler water with volatile water treatment.

Contamination of the boiler water with a sulphur-containing chemical (S was demonstrated at the pits). In the pits Ferrolls oxide was tangled and occasional intergranular corrosion was Found. At the inlet' of the evaporator there was very high heat-flux, so that severe nucleate boiling (columnar boiling) took place. The resultant steam bubbles caused the Formation of oxide-free tunnels, as can be seen on the microprint.

• Find the contamination source of the inleakage. • Lower the heat-flux by mounting clay mass on the tubes at the inlet of the

evaporator.

34

CASE HISTORY

CORROSION ATLAS 01.0 l.l 8.0-1

MATERIAL

SYSTEM

PART

PHENOMENON

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

Carbon steel (Cor-Ten A).

Gas-fired water-tube boiler.

Chimney.

Nitrate stress-corrosion cracking (intergranular).

{ I

~~. < . . ~

,t ,.

Multiple branched cracks near welds.

2.5 years.

Exhaust gases containing nitrogen oxides.

t

( -~ .

1;; ~:

Intermittent use of the boiler caused condensati on of the exhaust gas in the chi The formed solution caused intergranular cracking of the weld area, because presence of internal stresses (see also Case Hi story 01.1 1. 18.02).

Use AI-coated steel. That is fairly resistant to dilute concentrations of mineral a..: pH 4 (penetration s 0.1 mm/ year).