CORROSION ATLAS

32

CORROSION ATLAS CASE HISTORY

01.0 I.I 8.02

MATERIAL

SYSTEM

PART

PHENOMENON

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

Carbon steel (35 .8).

Steam system (low pressure).

Steam pipe section.

Intergranular stress-corrosion cracki ng (caustic stress-corrosion cracking).

Crack at weld.

Unknown.

Low-pressure steam (0.35 MPa).

Because of a leak in the production process, 50% caustic soda entered the low-pressure steam system via thc inj ection condensate for desuperheating. Undcr these circumstances , normall y harmless tensile stresses near welds led to caustic stresscorrosion cracking (scc also Case Histories 01.0 I.I 8.0S and 01. I 1.18.0 I).

• Improve the quality control of the condensate by means of conductivity monitoring; or

• Inject demineralized water.

CASE HISTORY

CORROSION ATLAS 01.01.01.09

MATERIAL Carbon steel.

SYSTEM Steam system.

PART Siphon of a condensate drain (left) and a steam sieve (right).

PHENOMENON Oxygen corrosion.

8\ 9 1 0 11' 1/2 , 3 '/4 1 11I1111 I111I1111 III1I1111 1111I1111 1111I1111 1111I1111 11I1I1III 1I11I1I11

81 91 110 1/1 112 113 114 1111 11111 1111 1111111 1111111111111111111111111111111111111111111111

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

Blockage due to magnetite deposition.

Unknown.

Oxygen-containing steam.

The de aerator performed inefficiently due to the supply of high-pressure condensate which expanded in the deaerator. As a result the deaerator did not receive any fresh sleam, and therefore the feed-water was insufficiently deaerated. The steam produced by the boiler consequently contained oxygen, which gave rise to excessive magnetite fonnation. Some of the magnetite was entrained with the steam, thereby causing blockages (see also Case History 01.01.01.07).

The high-pressure condensate should first be expanded or cooled before being fed to the deaerator. Any expansion steam should be routed to a low-pressure steam system.

9

10


01.01.03.0 1

MATERIAL

SYSTEM

PART

PHENOMENON

APPEARANCE

TIM E IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

Carbon steel.

Vertical waste-heat boiler of a catalytic cracker.

Quench cooler pipe section.

Cau stic corrosion .

I, II jill I I

.9 51 52 53 54 55 56 57

Uniform attack with cracking.

Unknown.

Alkaline boiler water around the pipes. Cracked gases, 400- 900°C, through the pipes.

As a result of defects in the water treatment, deposits had form ed on the pipe plate, under which the boiler water concentrated strongly, leading to high local caustic concentrations. Caustic attacks the protecting magnetite layer as well as the steel itself according to: 4NaHO + FeJO. -> 2NaFe02 + Na 2 Fe02 + H2 0 , and: Fe + 2NaOH-> Na, FeO, + H, (see also Case Histories 01.01.03.02 and 01.11.03 .01).

Prevent deposits by improvement of the boiler water treatment and its testing.

CASE HISTORY

CORROSION ATLAS 0 1.0 1.03.02

MATERIAL

SYSTEM

PART

PHENOMENON

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

Carbon steel (ASME SA-21 0, similar to AISI 1020).

Water-tube boiler (pressure 14.5 MPa).

Wall tube.

Caustic corrosion (high pH gouging).

Deep gouging beneath insulating internal deposits.

13 years.

Boiler water. Water treatment: congruent control and sodium polyphosphate.

Deposits shield the metal from the bulk water. Steam that forms under these thermally insulating deposits escapes and leaves behind a corrosive alkaline residue that can deeply gouge the metal surface. Sodium hydroxide will first dissolve magnetite: 4NaOH + Fe30. -> 2NaFeO, + Na,Fe02 + 2H 2 0. When magneti te is removed, the sodium hydroxide may react directly with the iron: Fe + 2NaOH -> Na 2 Fe02 + H 2 •

The formed hydrogen can lead to further corrosion: hydrogen damage (sec Case History 01.01.20.05) .

• Reduce the amount of available free sodium hydroxide by a water-treatment programme based on co-ordinated phosphate .

• Prevent excessive water side deposition by annual inspection and cleaning of the boiler if necessary.

II

' 2


01.01.04.01

MATERIAL Carbon steel (35 .8).

SYSTEM Shell-type fire-tube boi ler ( 1.2 MPa).

PART Fire tube.

PHENOMENON Acid corros ion.

1111 f 1111/1111/1111 JlI r I J lil'I' r I r /' r I r [II r J I r I r ' /11 r 11111'] J r 111112.:111 , 11 / 111\111111111 \ 1\ I 1\ \ II '\ 111\ \ \ \I \ \' 1'1\ \ \ \I 3 44 45 46 47 48 49 50 51 52 53 54 ~ 1I111111 J I" ,1/, J' I, ",1,1111,11' I n II J I' /11111/1, r 111,1" IIII r /11111/1 r I 1I 1I I1 1I IIII1I1 11 1111 111 I \ 111\ \ I1 II \ \ 11 1 \ II \

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

- -----Grooved attack over .he entire surface.

Corrosion took place in 3 hours.

Cleaning acid (inhibited hydrochloric acid 9.5 %, 95°C). Chemical cleaning was necessary following deposition of iron oxide and hardness sediments.

The uncontrolled performance of a chemical cleaning treatment to the boiler, with failure to notice inhibitor breakdown. Combined with an excessive pipe wall temperature, this resulted in 400 kg of steel being dissolved on a heated surface of 100 m2

. The damage was the result of poor workmanship (see also Case History 01.01.04.04).

The fire tubes were replaced at the cleaning firm' s expense.


01.01.04.02

MATERIAL Carbon steel (35.8).

SYSTEM High pressure water-tube boiler, fired with heavy oil.

PART Furnace evaporator tube.

PHENOMENON Acid corrosion (dew-point corrosion).

nn •• maa K •• n •• 40 ~JU"q.nYY ~ ~uu.~~~~

APPEARANCE Uniform attack.

TIME IN SERVICE Unknown.

ENVrRONMENT Deposits with acidic sulphur components at the flue gas side.

CAUSE During shutdown, the sulphur compounds reacted with water to form sulphuric acid, which attacked the steel. This acid corrosion is also defined as dew-point corrosion during idle periods (see also Case History 01.01.04.09).

REMEDY • After taking the boiler out of service, remove fire-side deposition from metal surfaces immediately by using high-pressure water sprays. This should be followed by a lime wash to neutralize remaining acidic substances .

• Alternative: change to firing wi th low sulphur oil or natural gas.

13

14


01.01.04.03

MATERIAL

SYSTEM

PART

PHENOMENON

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

Carbon steel.

Refuse incinerator.

ERW (electric resistance welded) furn ace tube.

Aci d corrosion (dew-point corrosion).

-c

-

-

Severe attack to the longitudinal scam weld.

Unknown.

Flue gases contaminated with HCI originati ng from the combustion of plastics (PVC etc.).

As a result of a difference in structure between the weld and the base material , partly caused by welding at low rrequency , the weld scam is selectively attacked by the HCI (at temperatures below the dew point). This dew-pOint corros ion during service is also called cold-end corrosion (ror selective attack of the weld, see also Case History 01.11.04.01).

• Use a seamless tube; or • Higher frequency welding; or • Application of post-weld heat treatment (PWHT) for normali zing the welds.


010104.04

MATERIAL

SYSTEM

PART

PHENOMENON

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

Carbon steel.

Shell-type fire-tube boiler (1.6 MPa).

Fire tube.

Acid corrosion.

Jo/ _I

." (

'{ . , "- • ,L~

Grooved allack, tube imploded under pressure-testing at II bar after cleani ng.

About 40 hours (duration of the cleaning operation).

.,.,

Cleaning acid (inhibited HCI 6%); average temperature (calculated subsequently) about 90°C. The boiler was chemically cleaned in view of the presence of organic contamination entrained with the condensate.

Errors made during the cleaning operation: • Firing the boiler filled with cleaning acid, allowing the wall temperature of the flame

pipes to rise to excessive levels. • Using an inhibi tor unsuitable for industrial cleaning purposes. • The lack of any monitoring during cleaning, as a result of which among other things

the formation of FeCI 3 was not detected in time and addi tional allack took place. This case also is the result of poor workmanship (see also Case History 01.01.04.0 I).

The boiler had to be renewed ; total damage $300,000. Note: chemical cleaning should be performed by specialized firms.

15

L

t6

CASE HISTORY


MATERIAL

SYSTEM

PART

PHENOMENON

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

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

Water-tube boiler (pressure 1.1 MPa).

Horizontal economizer tube (diameter: 6.0 em).

Acid corrosion (low pH gougi ng). ,

A large elliptically shaped gouge covered with a thick, irregular layer of hard dark iron oxides with a laminated structure.

7 years.

Boiler water conditioned with chelant. Make-up water treatment: water softener and reverse osmosis. Boi ler water analysis: conductivi ty 10 mS/ m, hardness 0 ppm.

Leakage of sodium chloride from {he generation of a water softener coupled with non-nucleate boi ling in the

hot end of the economizer, caused by excess ively high heat fluxes. The heat fluxes in th is area were 40%

higher than design values. Thi s phenomenon may also be regarded as heat-flux corrosion. The concentration

of acid-producing salls may induce hydrolysis to produce localized low pH conditions, while the bulk water

remains alkaline: M +CI - + H20~ MOH! + H +CI-. The hydrogen fanned in the reaction of Hel with

steel can lead to further corrosion: hydrogen damage (see Case Histories 0 1.01.20.02, 01.01.20.04 and

01.01.20.07).

• Prevent inadvertent release of regeneration chemicals from make-up water equipment.

• Prevent excessively high heat fluxes by shortening the economizer.

CASE HISTORY


MATERIAL

SYSTEM

PART

PHENOMENON

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

Carbon steel (ASME 210, similar to AISI 1020).

Water-tube boi ler (pressure 13.5 MPa).

Vertical wall tube (OD: 6.3 cm).

Acid corrosion.

Etched, bright metallic. The photo, taken at 7.5 X magnification, reveals irregul ar islands of uncorroded metal standing 0.12 mm above the surroundi ng corroded surface.

15 years. Corrosion took place in 24 hours.

Citric acid during cleaning.

It is suspected that a combination of thermal decomposition of the inhibitor and lack of proper monitoring were responsible for the corrosion, i.e. poor workmanship (see also Case Histories 01.01.04.01 and 01.01.04.04).

During acid cleaning, give careful attention to cleaning solut ion temperatures, exposure time, and frequent monitoring of solution chemistry.

1-

18


01.01.04.07

MATERIAL

SYSTEM

PART

PHENOMENON

APPEARANCE

TIME IN SERVICE

ENVIRONMENT

CAUSE

REMEDY

Carbon steel (ASME 210, simil ar to AISI 1020).

Water-tube boi ler (pressure 10.3 MPa).

Vertical wall tube (outer diameter: 7.6 cm).

Acid corrosion.

Strong grooved attack of the internal surfaces of wall tubes opposite external circumferential welds.

30 years. Corrosion took pl ace over numerous, successive acid cleanings.

Mineral acid.

Residual stresses remaining from the weld make the heat-affected zones subject to preferential corrosion when exposed to strong acid (sec also Case Hi story 01.01.04.03).

Damage of this type may be minimi zed by careful attention to cleaning solution temperature, exposure time, and frequent monitoring of solution chemistry during acid cleaning.

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CORROSION ATLAS