1 E ffe c t o f le a d o n SCC susc e ptib ility o f ste a m g e - - PowerPoint PPT Presentation

E ffe c t o f le a d o n SCC susc e ptib ility o f ste a m g e ne ra to r b o dy ma te ria l unde r PWR se c o nda ry side c re vic e c o nditio ns

T iina I käläine n, E ssi Jäppine n, T imo Saario , Ko nsta Sipilä & Aki T

• ivo ne n

VT T T e c hnic al Re se arc h Ce ntre o f F inland L td

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SCC>60%

SAF IR 2018 –pr

• gr

amme final se minar Mar c h 21-22, 2019, Hanasaari, He lsinki

Wo rldwide c a use s o f PWR ste a m g e ne ra to r (SG) tub e de g ra datio n [E PRI, 2006]. Ove r 60% o f SG tub e fa ilure s a re c a use d b y SCC.

Ste a m g e ne ra to rc re vic e e nviro nme nt

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Ma g ne tite (F e 3O 4) o xide partic le s fo rm due to c o rro sio n o f fe e d wa te r line , e nte r the SG a nd de po sit the re .

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Wa te r filling the de po sits e va po ra te s a s a lmo st pure ste a m due to b o iling , le a ving impuritie s b e hind a fte r so me time o f o pe ra tio n, a n ag re ssive c re vic e c he mistry is fo rme d (e ithe r a c idic o r a lka line ).

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L e a d (fro m 100 up to 10000 ppm) ha s b e e n fo und in almo st a ll ste am g e ne ra to r de po sit sa mple s e xtra c te d fro m o pe ra ting pla nts.

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In so me VVE R –pla nts (Russia a nd Cze c h Re pub lic ) wa ll-thro ug h SCC c ra c ks ha ve be e n fo und ne a r the bi-me ta llic we lds o f c a rbo n ste e l to stainle ss ste e l c o nc e rn o f the po ssible de trime nta l e ffe c t o f Pb.

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SG b o dy ma te ria l (c a rb o n ste e l 22K ) Slo w stra in ra te te sting - 1

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Ca rbo n ste e l 22K is susc e ptible to SCC in b o th a c idic a nd a lka line c re vic e c o nditio ns

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SCC susc e ptibility is stro ng e st in a c idic c o nditio ns witho ut le a d (Pb )

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• to minimize SCC in c a rb o n ste e l,

pla nts sho uld run the se c o nda ry side c he mistry so that the c re vic e c he mistry will b e ne utra l o r slig htly a lka line

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• Pb is be ne fic ial

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SG b o dy ma te ria l (c a rb o n ste e l 22K ) Slo w stra in ra te te sting - 2

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Oxyg e n in-le a ka g e into the SG wa s simula te d b y inc re a sing the po te ntia l by ab o ut 0.1 to 0.2 V

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T he fra c ture stra in de c re a se d ra dic a lly to a b o ut 10% o r le ss, indic a ting a ve ry stro ng susc e ptib ility to SCC, irre spe c tive o f Pb

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• xyg e n in-le a ka g e is

de trime nta l a nd sho uld b e avo ide d

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SG b o dy ma te ria l (c a rb o n ste e l 22K ) F ra c ture surfa c e a ppe a ra nc e

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I n a ir, the frac ture surfa c e is 100% duc tile .

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I n a lka line c re vic e c o nd itio ns, the frac ture surfac e is 73% duc tile witho ut Pb a nd 97% d uc tile with Pb .

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I n a c idic c re vic e c o nd itio ns, the frac ture surfac e is 43% duc tile witho ut Pb a nd 86% d uc tile with Pb .

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I n a c id ic c re vic e c o nd itio ns a t e le va te d po te ntial (simula ting o xyg e n in-le akag e ), the frac ture surfac e is

• nly 12% duc tile , indic a ting ve ry hig h

susc e ptibility to SCC

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SG b o dy ma te ria l (c a rb o n ste e l 22K ) T he e ffe c t o f le a d (Pb )

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Witho ut Pb , the c urre nt de nsity inc re ase s as a func tio n o f po te ntial.

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Abo ve -0.35 VSHEthe ra te o f inc re ase b e c o me s hig he r, b e c ause o f mag ne tite starts to transfo rm to he a ma tite .

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I n pre se nc e o f Pb, an ac tive pe ak appe ars at abo ut -0.55 VSHE, indic a ting disso lutio n o f Pb a s PbCl-, fo llo we d b y passiva tio n (de c re a se o f c urre nt de nsity), a nd furthe rra pid inc re ase abo ve -0.35 VSHE.

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T he e ffe c t o f Pb is to partially we ake n the passive mag ne tite laye rso that c o rro sio n lo c a lisa tio n ne c e ssa ry fo r SCC be c o me s mo re diffic ult.

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SG b o dy ma te ria l (c a rb o n ste e l 22K ) T he e ffe c t o f le a d (Pb )

Coc lusions

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Bo dy ma te ria l o f ste a m g e ne ra to r (c a rb o n ste e l 22K ) is susc e ptib le to SCC in typic a l c re vic e c o nditio ns, muc h mo re so unde r a c idic tha n a lka line c o nditio ns to minimize SCC in c a rb o n ste e l, plants sho uld run the se c o ndary side c he mistry so that the c re vic e c he mistry will b e ne utral o r slig htly alkaline .

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Additio n o f le a d (Pb ) impro ve s the re sista nc e o f c a rb o n ste e l 22K to SCC in b o th a c idic a nd a lka line c o nditio ns no c o nc e rn with Pb in c ase o f c arbo n ste e l.

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T he e ffe c t o f Pb is to pa rtia lly we a ke n the pa ssive ma g ne tite la ye r so tha t c o rro sio n lo c a lisa tio n ne c e ssa ry fo r SCC b e c o me s mo re diffic ult.

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Inc re a se o f po te ntia l dra ma tic a lly inc re a se s the susc e ptib ility to SCC plants ne e d to pre ve nt o xyg e n in-le akag e into the ste am g e ne rato r.

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