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D A V I D N . F R E N C H , I N C . , M E T A L L U R G I S T SO N E L A N C A S T E R R O A D

N O R T H B O R O U G H . M A S S A C H U S E T T S 01532 VUL. VII. ND.

A VIEW FROM THE PENTHOUSE: USEFUL INFORMATION FOR THE WORLD OF BO ILERS

The morphology of th e cor ro si on damage thatrs on the water. or steam si de of a b oi le rca l metal temperature, microst ructured s t a t e of stress, deposit thickness, theygen conten t of th e b oi le r water, and t he pHca ll y between th e deposit and t he tubetal . This l a t t e r fac tor can bestan tial ly dif fe ren t from the bulk pH.sits a r e porous, and bo il er water turn s t oam with in the dep osi t. The hydroxide that

in the water, that i s now a steam bubble.l e f t behind within the deposit . Graduallythatca ll y th e pH can be s tro ng ly ba sic , pH 12 o rWaterside deposits impede the heatan sf er between t he t ube and t he steam/waterThe thermal conductivity of thesei s much less than that of the s tee l ;i s much l e s s eff ec ti ve cooling of t hee by th e flu id. The ne t ef fe ct i s t o r a i se

A s the tube-metalperature inc reases, so does th e corrosiont e (gre ater wastage o r wall thinning) and s otructur e. These a r e competingin the sense that fa i lu res can

G r a i n boundaries are zones of weaknessa short-range atomic scale, thems a r e not as neat ly arranged as they ar eh in a c rys ta l l a t t i ce of an indiv idualThis short-range disorder leads t o aig ht ly higher int ern al energy, and corrosionAtoms a r e more ea s il y diss olvedom regions of high energy, l e s s energy i suired f or solut ion. A t elevateddeformation occurs more re ad il yth e gra in boundaries. Thus creep damagepears i n t he microstructure a s voids and

What follows i s a catalogue of s ever althat result from

there a re s imi la r i t i es t o the microst ructurein that they a l l contain grain-boundarycorrosion or cracks.Figure 1 shows a characteristicmicros truct ure of hydrogen damage. mdrogendamage occurs i n waterwall t ubes under thic kwaterside corrosion deposits. One of t heproducts of corrosion i s atomic hydrogenwhich, being a small atom, diff uses i nt o thesteel. mdrogen reac ts with i ro n carbide t oform fe r r i t e and methane. The methanemolecule i s lar ge and cannot di ff us e throughthe steel ; s o c o l l e c t s a t t h e f e r r i t e gr ai nboundaries. When the methane pr es su re i ssuff ic ient ly great , in tergranular cracks foralong the ferrite grain boundaries.

Figure 1. 200xThere are two character is t ic features tohydrogen damage. 1 ) inte rgra nula r cracks, a2) los s of iron carbides (decarburization).Figure 1 shows both of these characteristicsThe cracks ar e qui te long, extending wer magrains and some are oxide cwer ed withcorrosion products.

o r steamside corrosi on. I n each case Figure 2. 200x

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Figure 2, from the same boiler, shows thecond ef fe ct of th ick waterside dep osit s,is, elevated temperatures and creepmage. The microst ructure showspr creep voids,Here th e voids appear t oo longer than a si ng le cry st al boundary

d a re more hole than extended crack. TheI D surface of thet t h is location shows no interg ranularackirig to the f e r ri te grains. For waterwallbes, creep fai lur es usual ly s ta r t a t the ODis 75-1000Fan a t the I D . Thus, this par t i cu la ri s an elevated-temperature s t r e s sThat these two microstructures came from

lu x (metal temperature), pH (whichas it affects tube-metal

3 . 500xFigure 3 shows intergranular corrosion of abon s t e e l waterwall tube. Here th e

ong the fe r r i t e grain boundaries. That thiss not hydrogen damage may be seen from thetha t pear l i t e co lonies a re s t i l l in tac tn of inte rgra nul ar corrosionte enc ircl ed zone). Since t h i s damage wasted a t the bottom of a f ai rl y large p i tis by oxygen attack,th er than by a pH excursion.

Figure 4. lOGxFigure 4 shows grain-boundary corrosion a 304 stainless superheater tube. A ttemperatures abwe about 1050F, chromiumcarbides form along the austenite grainboundaries. The formation of these carb ide

depletes the austenite grains of chromiumadjacent to the grain boundaries. As thechromium content decreases below about 12%.the corrosion resistance i s severely impairThis s t ruc ture is said t o be llsensi.tized.lleff ect , the aus teni te grain boundaries ar e longer stainless steel, and corrosion procerapidly. The net ef fe ct is the attack alonthe a ust en ite g rain boundaries, a s shown inFigure 4.In summary, t he l oc al ized cor ros ion andmetallographic circumstances dictate thecor ros ion morphology. Grain boundaries beina zone of higher in te rna l energy may be morread ily corroded, referred t o asintergranular at tack ( IGA) . A t elevatedtemperatures th e gra in boundaries s l i p pas tone another, a process known a s anthe damage collects along grain boundaries,seen in the microstructure as "creep damageThe details of the corrosion vary fromlocation t o location within the boi le rdepending on the local temperature andcorrosion conditions.(508) 393-3635 fo r metallurgical help!!!!!!!!

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