Corrosion for Engineers Chapter 11: HYDROGEN EFFECTSDr. Derek H. Lister page 11 - 1

HYDROGEN EFFECTS Hydrogen can degrade metals by:- hydrogen blistering;- hydrogen embrittlement;- decarburization;- hydrogen attack.

Blistering

Hydrogen enters the Lattice of a metal diffuses to voids, creates high internalstresses ~ blisters . ..

Blistering may occur duringexposure to:- hydrocarbon;- electroplating solutions;- chemical process streams;

pickling solutions;- H-containing contaminants

during welding;- general corrosive environments.

Cross section of a carbon steel plate removedfrom a petroleum process stream showing alarge hydrogen blister. Exposure time: 2 years.

Universify of New Brunswick, Canada Chulalongkom Universify, Thailand

Corrosion for EngineersDr. Derek H. Lister

Chapter 11: HYDROGEN EFFECTSpage 11 - 2

Embrittlement

Similar to blistering . .. hydrogen enters metal lattice . ..

BUT . .. interaction with metal lattice different.

High-strength (and more brittle) steels are susceptible.

H-embrittlement different from see in nature of cracks . ..• stress-corrosion cracks usually propagate anodically;

hydrogen-embrittled cracks occur under cathodic conditions . ..

~ Oirection of Odvoncin; croekinQ into metal t

Reqion ofhydrogen embfittlement

Cathodic current2s+2H+-2H

oAnodic currentM-M+++2s

ReQ ion of anodicstre,s corrosion crocking

_v

."+" ,+J

I~AnOdic stress

corrosion C(OCkinQ

v_

Hydrogenembrilliement

Schematic differentiation of anodic cracking and cathodically sensitive embrittlement.University of New Brunswick, Canada Chulalongkorn University, Thailand

Corrosion for EngineersDr. Derek H. Lister

Chapter 11: HYDROGEN EFFECTSpage11-3

Hydride-forming metal are susceptible to H- embrittlement ...

e.g., Zr-alloy pressur tubes (in CANOUs) and fuel sheathing (in all water­cooled reactors) pick up hydrogen (or deuterium in heavy water) bygeneral corrosion. The hydrogen (0) migrates through the metal latticeto cool regions and to regions of high tensile stress - can precipitate asseparate phase - zirconium hydride.

These hydrides are themselves brittle, and crack, and the crack can propagatethrough the material, with more hydride progressively precipitating at the crack tip.-N.B. Enoughhydride canprecipitate toform a "hydride"blister ...c.f. "hydrog,gn"blister.

University of New Brunswick, Canada

Representation of tbe Growtb of a Crac~ !b~ougbFailure of Zirconium Hydride tbat PreCIpitatesat Crack Tips

Chulalongkorn University, Thailand

Corrosion for EngineersDr. Derek H. Lister

Chapter 11: HYDROGEN EFFECTSpage 11-4

N.B. The mechanism of hydrogen uptake by metals must involve

Electrolyte

H+ H+

t +H .... H2.-H H H

e H~ ~

H_e ./ + + Schematic illustration showing

d~H26bthe mechanism of hydrogen

H H blistering.

+ + Void

H-H2-H

Air

BUT ... remember that molecular hydrogen may absorb and dissociate onmetal surfaces.

University of New Brunswick, Canada Chulalongkom University, Thailand

Corrosion for EngineersDr. Derek H. Lister

Chapter 11: HYDROGEN EFFECTSpage 11 - 5

Decarburization and Hydrogen Attakk

High temperature process - C or carbide in steels can react with gaseoushydrogen ...

C + 2H2 -+ CH4

Note that the reaction con occur with atomic H in the metal lattice ...

C + 4H -+ CH4

May crack the steel from high internal pressure.

May cause loss of strength as C disappears.

University of New Brunswick. Canada Chulalongkorn University, Thailand

Corrosion for EngineersDr. Derek H. Lister

Chapter 11: HYDROGEN EFFECTSpage 11- 6

PREVENTION

Blistering• Use steels with few or no voids;• Use coatings;• Use inhibitors;• Remove impurities that can promote hydrogen evoution ...

S-2 (particularly bad), As, CN", ect.• Use different materials (Ni-base alloys have low diffusion rates for

hydrogen).

EmbrittJement• Reduce corrosion rate (inhibitors, coatings, ect.);• Change electroplating process to minimize H effects (voltage, current

dens.ity, bath composition, ect.);• Bake material to remove H;• Minimize residual stresses;.• Use less susceptible material;• Maintain clean conditions during welding.

University of New Brunswick, Canada Chufafongkorn University, Thailand