Engineering Development – MaterialsIntelligent material solutions for catalyst technologies

Principal Engineer María José Landeira Østergård1

Hydrogen attack

Stress Corrosion Cracking

Hydrogen Sulphide & CO2

Oxidation

Carburization

Metal Dusting

Synthesis gas: Hydrogen plants and front end of ammonia and methanol plants

Corrosion in HTAS’s related technologies

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As in previous slide and:

Nitridation

Ammonia induced stress corrosion cracking (carbon steel storage tanks)

Ammonia plants

As in slide about synthesis gas and:

Carbonyl corrosion

Methanol corrosion

Methanol plants

WSA-plants

Dew point corrosion

High temperature sulphidation

High temperature H2 corrosion

High temperature H2S corrosion

Polythionic acid corrosion

Ammonium bisulphide

Salt corrosion: pitting, crevice, Cl SCC

Sulfide stress corrosion

Acid corrosion

Amine corrosion

Naphthenic acid corrosion

Refinery

Corrosion in HTAS’s related technologies

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After about 5 years of operation severe leakage occurredat the inlet tube sheet of the boiler.

The inlet tube sheet is thermally protected by a layer ofrefractory.

Moreover, several cracks were found in the tube to tube sheet welds, mainly at the centre portion. Severecorrosion was found at the outside of the tubes in thevicinity of the tube sheet.

Case 1: Tubes and Inlet Tube Sheet Waste Heat BoilerEl Segundo Refinery, California.

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5

7

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6Tube sheet

Longitudinal Transversal

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7Tube sheet

Longitudinal

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The microstructure of the tubes and the tube sheet is pearlite+ferrite with the exception of specimen 7 whichshows spheroidite.

This is a strong indication that only specimen 7 has beenexposed to temperatures above 650 °C

This can only be explained if the refractory has beenmissing.

Tubes and Inlet Tube Sheet Waste Heat BoilerEl Segundo Refinery, California.

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Case 2: Cracks of Ammonia converter weld, India

• A circumferential weld of one ammonia converter in India was found cracked after 6 months of service by through thickness passing cracks in a circumferential segment of ca 1.5 m, through which hydrogen leakage from the reactor was caused.

• After removal of the external insulation, cracks were found on the external reactor surface, transverse oriented with respect to the circumferential weld and completely contained within the weld metal.

• 20 distinct cracks on the external reactor surface were detected on the external reactor surface in the segment of the leaking weld.

• The following inspection of the inner reactor surface showed that the damage was localised and included a weld segment of ca 1.6 meters.

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HTAS S-50 Ammonia Converter

20250 mm20250 mm3000 mm169 mm (3x42 + 43 mm multiwall)2.25Cr-1Mo225 bar430 °C182 bar90 bar370 °C420 °C

Straight lenghtTotal LengthInternal diameterWall thicknessShell materialDesign pressureDesign temperatureNormal operation pressureNormal H2 partial pressure Normal operation shell temperatureMax temp. during catalyst reduction

3H2 + N2 ↔2NH3Iron catalyst

Temperature 400 – 500 °CPressures > 100 bar

Recirculation of unconverted synthesis gas

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Hydrogen uptakeHumid flux

Too large weld beadsToo low weld rate

Large residual stressesToo low temperature at PWHT

Reheat crackingRepear welds

Brittle microstructureToo rapid cooling

Cold crackingToo low interpass temperature

Large grain sizes, reheat crackingToo high heat input

Cold crackingToo low preheat temperature

Risk ofIssue

Fabrication - Welding issues

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Service IssuesHigh Temperature Hydrogen Attack

Nitriding

Low Temperature Hydrogen Attack (at shut downs)

H adsorption, H absorption, H2 formation

Cracks along grain boundariesH - H H - HH H

H-H

H H

Low Temperature Hydrogen Attack

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– CH4 formation and decarburization

– Cracks parallel to the surface

H - H H - HH H

CH4

H H

C (from the steel) + 4 H atoms → CH4

High Temperature Hydrogen Attack (HTHA)

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HTHA – Nelson Diagram

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Nitriding

Diffusion controlled process. Parabolic reaction rate

Formation of nitrides and methane

Hardness increase

Cracks parallel (perpendicular?) to the surface

Cr23C6 + 11.5N2(g) + 12H2(g) = 23CrN + 6CH4(g)

V2C + N2(g) + 2H2(g) = 2VN + CH4(g)

N - N N - NN NCrN, VN

Sound metal

Nitrided region

2.25Cr – 1 Mo

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Depending of the stress level in the welds, nitriding is acceleratedby a factor of 3 – 5

If sufficiently high load and/or residual stresses apply in the axial or tangential direction to the vessel wall, cracks have to be expected propagating slowly across the wall, as crack growth is controlled by diffusion of nitrogen and transformation of carbides to nitrides.

Schematic of crack formation in low stress regions (base metal) and high stressed region (e.g. coarse grained welds, welds with insufficient PWHT, etc)

InnerOuter

Nitriding

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Internal reactor surface

Internal reactor surface

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Before Stress Relief heat treatment at 690 °C for 1.25 h After Stress Relief heat treatment at 690 °C for 1.25 h

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Conclusion

• Too elevated hardness values measured in the weld and at the Heat Affected Zone

• Harndess values considered originated by an erroneous or not performed Post Weld Heat Treatment.

• Taking into account the service conditions (presence of gaseous hydrogen and atomic hydrogen) it can be stated, that the part of the circumferential weld of high hardness was subjected to Hydrogen embrittlement.

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Thanks for your attention

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