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DAVID N. FRENCH, INC.,METALLURGISTSONE LANCASTER
ROADNORTHBOROUGH, MASSACHUSElTS 01532TEL: (508)393-36351996 VO L .
X I I I , No.
A VIEW FROM THE PENTHOUSE: USE FUL INFORMATIO N FOR THE WORLD OF
BO IL ER S- A G A I N
Hydrogen damage and the associatedare a continuing
problemutility boilers. Theonal wisdom on the damagemmarized as:in
high-sure boilers, that is, boilersh pressures greater than
aboutpsi, under thick, usually quitelized deposits, on the water
sidethe boiler tube. The heavyact as a site for amechanism thates
the pH condition of theer. Under acidic conditions asater within
the sponge-likeH dropsvery acid levels. The hydrogenreacts with
steel to form ironand hydrogen, as shown in1.
Under basic conditions, the samemechanism raises the pHery high
levels, and thes with iron to
Methane is a large molecule andcannot diffuse from the steel
andtherefore collects at the ferritegrain boundaries. When
sufficientmethane collects and the pressurebecomes great enough,
grain-boundarcracks develop that weaken the steelThe microstructure
contains both thmethane-induced cracks anddecarburization that are
character-istic of hydrogen-damaged steel. Tultimate proof of
hydrogen damage ia metallographic examination thatfinds these
microstructural feature
m the ferroate ion and hydrogen,2. -- --- -- -- - Figure 1.--
-.- . -- - - -oth cases, one of the corrosionis atomic hydrogen.
Theogen is generated at the
teel interface and isle. Some of the hydrogen thenuses into the
steel where itwith iron carbide to formon, as shown in
3.
2 Hydrogen damage usually occursunder localized and thick
water-sidedeposits. Figure 1 is such anexample.Hydrogen-damaged
microstructuresalways contain intergranular crackinand often
substantial decarburizatioto the point where there may be
nopearlite remaining within thestructure, see Figure 2. Dependingon
the age of the damage, theintergranular cracking that is
almosalways connected to the ID surface
3 will contain (and be filled with)iron oxide as a result of
the
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f the crack surface with One possible explanation for theer
water. This is the same initiation of scab-like, localizedthat
forms the protective deposits is the incomplete removal scale
throughout the both copper and oxide deposits duriboiler. chemical
cleaning. Figure 3 shows
Figure 2.There are two questions that need
but exceedingly thickwhen the vast majoritytube surface is quite
clean,may occur long afteraction has been taken ton damage.
Figure 3.WHO MIGHT LIKE TO RECEIVE OUR
ID deposit with severai alternatinglayers of copper and oxide.
In ordto remove both oxides and copper, atwo-stage chemical
cleaning isrequired, one stage to remove theiron oxide and a second
stage toremove the copper. However, if theare multiple layers of
copper, thenthis two-stage cleaning cycle needsto be repeated. Once
a deposit hasformed, and the chemical cleaning hnot totally removed
the entiredeposit, the scab-like final deposihas a start.
Subsequent chemicalcleanings do not have time to remova still
thicker deposit.There are examples of depositanalyses associated
with hydrogen-damage failures that include bothcopper and zinc long
(more than 10years) after brass alloys have beenremoved from both
the condenser andlow-pressure feedwater heaters. Ththe copper-zinc
corrosion debris habeen incompletely removed duringsubsequent
chemical cleanings andremained in the deposit. In onecase, the unit
had- een chemicallycleaned twice since the removal ofall copper
alloys.In light of these delayed failurea more thorough chemical
cleaning mbe necessary to completely remove aof the localized
water-side depositsTo assure complete cleaning, aborescopic
examination or sampleremoval for visual inspection may bnecessary.
In the final analysis,hydrogen-damaged tubes may have to completely
replaced; as there is noknown method for repair of hydrogendamaged
tubes exce~t o replace
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