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n mn v bo v kim loi. NXB i hc quc gia H Ni 2006.

T kho: n mn v bo v kim loi, n mn in hc hc, n mn.

Ti liu trong Th vin in t H Khoa hc T nhin c th c s dng cho mc

ch hc tp v nghin cu c nhn. Nghim cm mi hnh thc sao chp, in n phc

v cc mc ch khc nu khng c s chp thun ca nh xut bn v tc gi.

Mc lc

Chng 6 Cc dng n mn...............................................................................................2

6.1 n mn u .............................................................................................................2 6.2 n mn cc b.........................................................................................................2

6.2.1 n mn tip xc (cn gi l n mn Ganvanic) ....................................................2 6.2.2 S n mn kim loi do s chnh lch kh..............................................................7

6.3 n mn l hay cn gi l n mn im (pitting corrosion)......................................10 6.4 Mt s dng n mn khc.......................................................................................13

6.5.1 n mn ranh gii ...............................................................................................13 6.5.1 n mn nt do ng sut .....................................................................................13 6.5.1 n mn mi .......................................................................................................14 6.5.1 n mn chn lc ................................................................................................15 6.5.1 n mn mi mn................................................................................................15 6.5.1 n mn do ma st ..............................................................................................15

6.5 nh hng ca mi trng n qu trnh n mn kim loi.....................................16 6.5.1 n mn trong mi trng kh quyn...................................................................16 6.5.2 n mn kh quyn vng thnh ph v khu cng nghip ......................................18 6.5.3 n mn trong kh quyn bin .............................................................................18 6.5.4 n mn trong mi trng t .............................................................................18 6.5.5 n mn trong btng ct thp ............................................................................19 6.5.6 n mn vi sinh...................................................................................................21

Chng 6. Cc dng n mn Trnh Xun Sn

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Chng 6

Cc dng n mn

Hin tng n mn kim loi do mi trng gy ra rt a dng v phc tp, c th tm phn thnh cc loi sau y.

6.1 n mn u

Dng n mn ny rt ph bin (xem hnh 6.1) vi nhng c im sau: tc n mn mi ch trn b mt gn bng nhau. V d thp t trong mi trng H2SO4. nh gi tc n mn ny ngi ta thng s dng phng php trng lng (g/cm2.gi) hoc da vo gim chiu dy ca mu th nghim Pmm/ nm hoc mA/cm2.

Hnh 6.1

Dng n mn u

6.2 n mn cc b

Dng n mn ny xy ra u tin ti mt s phn din tch b mt kim loi tip xc vi mi trng n mn. Hin tng n mn cc b ny cng rt ph bin v rt a dng, c th chia thnh cc loi sau:

6.2.1 n mn tip xc (cn gi l n mn Ganvanic)

Khi c hai kim loi khc nhau tip xc vi nhau hoc hp kim c thnh phn khc nhau tip xc vi mi trng cht in li sinh ra hin tng n mn tip xc. Qu trnh n mn din ra nh l s hot ng ca mt pin in khp kn mch. Tc n mn ph thuc vo hiu s in th n mn (xem bng 6.1) ca hai kim loi trong dung dch cht in li v ngoi ra n cn ph thuc vo mt s yu t khc nh in tr ca dung dch cht in li, pH, nhit mi trng

Bng 6.1 Gi tr th n mn Em ca mt s kim loi trong nc bin nhn to 25oC

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Kim loi Ni Cu Thp cacbon Al

99% Zn Mg

Em(NHE) 0,046 0,01 0,335

0,667

0,809

1,355

Mt s trng hp xy ra n mn ganvanic: + Cc inh c vt hoc cc tm thp m km trong mi trng cht in li, km c in

th m hn kim loi nn v s d dng b n mn theo c ch ganvanic. + Cc c vt bng thp s b n mn khi tip xc vi cc vt liu ng thau trong mi

trng nc bin. Trn c s ng cong phn cc c th gii thch hin tng n mn tip xc cho mt

s trng hp sau y: V d 1: H tip xc ca Fe v Zn trong mi trng axit H2SO4 long khng c oxi.

Hnh 6.2

Cc ng phn cc ca h n mn tip xc Fe-Zn trong mi trng axit H2SO4 long khng c oxi

1- ng phn cc catt thot H2 trn km; 1- ng phn cc ho tan km;

2- ng phn cc catt thot H2 trn st; 2- ng phn cc ho tan st; 3- ng phn cc thot H2 trn st v km

Khi Zn tip xc vi thp (Fe) trong mi trng axit H2SO4 long khng c oxi s tn ti cc pin in sau y:

+ Nu Zn v Fe khng tip xc ta c hai pin tch ri nhau: () Zn/H2SO4/(H2) Zn (+) (6.1)

() Fe/H2SO4/(H2) Fe (+) (6.2)

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i vi pin (6.1) khi hot ng ta c dng im ca km bng: Znmi = Znai = 2

Hc,Zni ng vi

th n mn ZnmE .

Tng t i vi pin (6.2), ti vng anot st b ha tan v cng gii phng hiro trn vng anot, vy ti FemE ta c

Femi =

Feai = 2

Hc,Fei .

Khi so snh s hot ng ca hai pin (6.1) v (6.2) cho thy dng n mn ca Zn Znmi ln hn dng n mn ca st Femi , iu ny l do trong mi trng axit km hot ng hn st.

Nu ta gi thit dng 2Ho,Zni = Feo,Fei th dng n mn ca km cn ln hn so vi st rt

nhiu.

+ Nu st v km tip xc vi nhau ta xem h cn li mt pin, khi vng catot trn c hai vt liu u thot kh hiro. Vy Em ti ch tip xc ca hai kim loi c k hiu l Zn FemE

+ ng vi dng n mn Zn Femi

+ v tng dng n mn ca hai kim loi km v st hoc bng tng dng n mn 2Hci trn hai kim loi (xem hnh 6.2). Gi tr ny ln hn dng n mn

Znmi hoc

Femi .

Zn Femi+ = Znmi +

Femi = 2

Hc,Zni + 2

Hc,Fei (6.3)

V d 2: H tip xc ca Fe v Ni trong mi trng H2SO4 long khng c oxi v gi thit rng tc phn ng gii phng hiro trn hai kim loi bng nhau.

Trn hnh 6.3 cho thy trong mi trng H2SO4 long tc n mn Ni nh hn tc n mn ca Fe. Trn hnh cng cho thy: Khi kim loi Ni tip xc vi Fe th th n mn

Ni FemE+ nm gia 2 gi tr th n mn ring bit ca st v niken, ngha l:

FemE < Ni FemE+ < NimE

v Ni Femi+ = 2Hc,Fei + 2

Hc,Nii =

Nimi +

Femi

v Ni Femi+ > Femi >

Nimi

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Hnh 6.3

Cc ng phn cc ca h n mn tip xc Ni+Fe trong dung dch H2SO4 long khng c oxi

1- ng phn cc thot H2 trn st v niken; 2a- ng phn cc ant ho tan niken. 2b- ng phn cc ho tan st; 3- ng phn cc ant ho tan ng thi niken v st;

3a- ng phn cc catt thot H2 ng thi trn niken v st V d 3: H tip xc gm mt kim loi b n mn c ghp vi mt in cc khng b

n mn (vt liu tr - v d Pt, cacbon, v.v). Xt h km tip xc vi Pt (Zn-Pt) c t trong mi trng H2SO4 long.

Pin n mn tng t pin in: () Zn / H2SO4 / Pt / Zn (+) (6.4)

Ti anot km b ho tan: Zn 2e Zn2+ ng thi ti vng catot ion H+ c kh thnh kh H2:

2H+ + 2e H2 trn c hai phn kim loi Pt v Zn. V th tc gii phng H2 tng ln dn n tng s

ho tan km. iu ny c th hin trn hnh (6.4). Trong mi trng axit, tc gii phng H2 trn nn Pt cao hn trn nn Zn, v vy khi

Zn tip xc vi Pt tc thot hiro tng ln ng k v th tng dng n mn cng nh th n mn Zn PtmE

+ ti ch tip xc dch chuyn dn v pha gn vi th cn bng thot hiro.

Vy: +2

cb2H / H

E > Zn PtmE+ > ZnmE >

cbZnE

v Zn Ptmi+ > Znmi

Hnh 6.4

ng phn cc ca h n mn tip xc Zn-Pt trong mi trng axit H2SO4 long khng c oxi

1- ng phn cc ant ho tan km; 2- ng phn cc catt thot H2 trn km;

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2a- ng phn cc catt thot H2 trn Pt; 2b- ng phn cc thot H2 trn h Zn+Pt

C rt nhiu yu t nh hng n qu trnh n mn Ganvanic, song c hai yu t quan trng:

dn in ca dung dch cht in li: Nu in tr dung dch ln, v d nc sng, ao h dn n s gim tc n mn ni chung, song c th dn n tng tc n mn cc b ti vng tip xc.

Khi hai kim loi tip xc vi nhau trong mi trng n mn, nu din tch ca kim loi c in th dng hn cng ln (vng catot), phn cc catot cng gim th tc n mn cng ln. V vy, chng n mn kim loi in ha, nu din tch ca vng anot so vi vng catot cng b th dng n mn cng gim (xem hnh 6.5).

Ngoi ra cn mt s yu t khc v d nhit , pH dung dch cng nh dng trao i io i vi qu trnh gii phng H2 trn cc nn kim loi khc nhau u c nh hng n tc n mn tip xc.

lm gim tc n mn tip xc ca mt thit b hoc ca mt cu kin no trong mi trng n mn cn phi trnh s tip xc ca hai kim loi c th in cc khc nhau; chn t l din tch vng catot v anot thch hp. C th bo v chng n mn bng phng php bo v catot bng cch gn mt kim loi hot ng hn vo ch tip xc, v d km hoc nhm hoc Mg, kim loi ny c in th m hn cc kim loi tip xc.

Hnh 6.5a

ng phn cc ca h n mn tip xc Zn-Pt trong mi trng axit (din tch Pt tng ln)

1. ng phn cc catot kh H+ trn nn Zn; 2. ng phn cc catot kh H+ trn nn Pt c din tch nh hn;

3. ng phn cc catot kh H+ trn nn Pt c din tch ln hn; 4. ng phn cc catot tng cng kh H+ trn nn Pt-Zn vi din tch ln

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6.2.2 S n mn kim loi do s chnh lch kh

V d kim loi st (Fe) t trong dung dch NaCl, song c s chnh lch nng kh O2 ho tan ti hai vng trn b mt kim loi. Ti nng oxi ca khng kh ho tan vo nc vng gn mt nc cao hn trong th tch.

Vy mi vng l mt pin n mn m s ho tan kim loi gn lin vi s kh oxi.

Mch pin ca h ny c th m phng nh sau: Fe (O2)2 / NaCl / FeFe / NaCl / (O2)1 Fe (6.5)

Trong p sut kh O2 (khng kh) ( )2O 2P > ( )2O 1P . S ch to c hnh dung theo hnh v 6.5b.

1 1

2

4

35 5

1. in cc st;

2.Tip xc ng, ngt mch;

3. Cu ni hai dung dch (NaCl);

4. Dung dch NaCl;

5. ng dn kh

Hnh 6.5b

S ch to pin n mn kim loi khng kh

Khi hai in cc 1 v 2 khng ni vo nhau (tip xc 2 ngt mch) ta c hai pin n mn, mi pin gm cc in cc l st v in cc oxi. Hai in cc oxi c nng khc nhau. Cc pin hot ng, dng n mn im v th n mn ca chng c xc nh da vo ng phn cc (xem hnh 6.6).

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Hnh 6.6

ng phn cc ca h n mn st trong dd NaCl c thng kh

ng CC2 - Phn cc catot vi p sut O2 ln (2

2OP )

ng CC1 - Phn cc catot vi p sut O2 nh (2

1OP )

ng CC1 - Phn cc catot vi p sut O2 rt nh ng AA1 - Phn cc anot ho tan st

Ta xt trng hp sau:

i vi cc bn tri c p sut oxi ln 2

2OP

(hnh 6.5b) st b n mn gn lin vi s tiu th oxi vng catot theo phn ng:

O2 + 4e + 2H2O 4OH (6.6) Dng n mn Fem2i vi p sut 2

2OP

bng dng gii hn ca qu trnh catot tiu th oxi

2di .

Fem2i = 2di ng vi th n mn Fem2E tnh theo im ct ca cc ng 1 v CC2.

Cc bn phi ca hnh (6.5b), p sut kh O2 rt nh (2

1OP ) st b n mn km theo s tiu

th O2 theo phn ng (6.6) vi dng n mn Fem1i = 1di (dng gii hn ca qu trnh tiu th O2) ng vi im ct ca 2 ng 1 v CC1.

Vy hai kim loi st t trong hai cc (hnh 6.5b) c kch thc hnh hc v dung dch NaCl hon ton ging nhau, nhng tc n mn ngn c nng oxi ln c dng n mn ln.

2

2OP > 2

1OP ng vi Fem2i >

Fem1i

Nu nng O2 rt nh, dng khuch tn oxi rt nh 1d

i , phn ng (6.6) xy ra vi tc rt nh, song nu phn cc m tip tc theo ng CKC3 s xy ra phn ng catot:

2H2O + 2e H2 + 2OH (6.7) on thng KC3 ct ng 1, im ct tng ng vi dng n mn Fem1'i > 1'di .

Nu ng kho tip xc 2 ni in cc 2 v 1, din tch anot b n mn gp 2 ln, dng anot tng gp 2 ln, dng catot b khng ch ch yu bi dng gii hn

2di . Khi th n

mn ca hai in cc 1 v 2 ghp li ghpmE nm gia hai gi tr Fem1E v

Fem2E ng vi im

ct ca hai ng tng cng anot P1P, catot QQ1.

Dng n mn ca h ghp ghpmi > 2di > 1di

hoc ghpmi > Fem2i >

Fem1i

Tm li, i vi pin thng kh, dng n mn tng v cn lu rng c trng hp xy ra ngc li nu khu vc c p sut

2OP ln, phn ng kh (6.6) xy ra lm cho pH dung dch

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tng ln dn n s th ng, cn li khu vc c 2O

P thp s ho tan st li lm gim pH theo phn ng:

Fe + H2O FeOH+ + H+ + 2e v km theo phn ng ho hc FeOH+ + H2O Fe(OH)2 + 2H+ Fe + 2H2O Fe(OH)2 + 2H+ (6.8) S to thnh ion H+ lm gim pH mi trng v thc y s ho tan st. Trong trng

hp ny ti khu vc c nng 2O

P thp li c tc n mn ln hn.

Hin tng n mn chnh lch kh thng xy ra trong cc trng hp sau:

1. n mn khe Ti nhng khe gia cc mt bch c lp m ca cc ch ni hai ng kim loi, hoc di

cc vt m tn c (tn riv) hoc c c xit c th sinh ra n mn cc b v trong cc khe nng oxi ca khng kh ngho hn pha ngoi.

2. n mn ti vng mn nc Ti nhng vng mn nc, v d vng mn nc ca chn cu st cm xung sng vng

nc l, mn nc ca thnh tu bin v.v ti c 3 pha tip xc vi nhau: khng kh (cha oxi), pha rn (thp), pha nc (dung dch mui hoc nc l) v c th tn ti mt pin hot ng th ng do s chnh lch v thong kh (xem hnh 6.7).

2

1

K

N

T

T- Pha rn thp ;

K- Pha kh - khng kh ;

N- Pha lng - dung dch nc bin ;

1- Vng b n mn ;

2- Lp g

Hnh 6.7

n mn mn nc

Vng mn nc ti lp g 2 c cu trc xp. S hnh thnh lp g din ra theo cc phn ng:

Ti vng anot ngho oxi st b ho tan:

Fe 2e Fe2+ Ti vng mn nc giu oxi hn v ng vai tr catot xy ra phn ng:

1

2O2 + 2e + H2O 2OH

v hnh thnh lp kt ta:

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Fe + 1

2O2 + H2O Fe(OH)2

ng thi xy ra phn ng ho hc:

2Fe(OH)2 + 1

2O2 + H2O 2Fe(OH)3

v 2Fe(OH)3 Fe2O3.3H2O Fe(OH)2 + Fe2O3 Fe3O4.H2O 2Fe3O4 + 3H2O +

1

2O2 3Fe2O3.3H2O

Lp g xp b kim ho v di lp g ny thp b th ng. Vng 1: ngho oxi nhng li b n mn mnh hn (ho tan hot ng) l do s hot

ng ca pin hot ng - th ng.

3. n mn do lng ng Dng n mn ny tng t n mn khe di cc lp ct, bn, sn phm n mn lng

ng. Ti nhng ch b che khut ngho oxi v b n mn (xem hnh 6.8).

Hnh 6.8

n mn lng ng

1. Kim loi; 2-2. Vng giu oxi; 3. Vng ngho oxi do che khut; 4. Vt lng ng

6.3 n mn l hay cn gi l n mn im (pitting corrosion) n mn l l mt dng n mn cc b to ra cc l c kch thc nh, su ca l c

th ln hn ng knh ca n. Dng n mn ny xy ra trn cc kim loi, hp kim c mng th ng (Al, Ni, Ti, Zn, thp khng g) hoc c cc lp ph bo v b xuyn thng.

Hin tng n mn ny thng xy ra khi mi trng n mn c cha cc cht oxi ho (NO3, NO2, CrO42) ng thi c mt cc cht hot ho Cl, Br, I (v d mi trng nc bin) ca cc thit b my mc

Hnh dng cc l do n mn gy ra ph thuc vo bn cht cc lp ph. Trc ht ta xt c ch n mn l i vi kim loi c lp ph l mng th ng.

1. Lp ph l mng th ng

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Mt s kim loi hay hp kim c bn chng n mn cao l nh c lp th ng bm trn b mt kim loi ngn cch kim loi vi mi trng n mn. V d: Fe, Al, Ni, Cr, Ti v cc hp kim Fe-Cr, FeCr-Ni song trong mi trng xm thc c cha cc ion halogen Cl, Br, I th chng s b n mn l bao gm cc giai on sau:

S hnh thnh l

Cho n nay vn cha c c quan nim r rng v s hnh thnh l. Song c th gi thit rng: ti mt s ch b mt mng th ng cha hon chnh c s hp th cc ion halogen, v d Cl, ti kim loi b ho tan vi tc ln to l pht sinh n mn im v to nn cc mui d tan. Nhng v tr c mng th ng cha hon chnh thng l bin gii gia kim loi v tp cht phi kim. Ti mng th ng khng bo v c kim loi v d hp th cc ion halogen. Gi tr th , ti lp th ng bt u b xuyn thng, n mn im bt u gi l th n mn l (th pitting - El) (xem hnh 6.9).

Trn hnh 6.9 ng cong OABCD l ng phn cc ho tan kim loi trong dung dch khng cha ion halogen.

OA- Vng ha tan hot ng ca kim loi Me ze Mez+ AB- Kim loi i vo trng thi th ng:

Me + zH2O Me2Oz + 2ze + 2zH+ (6.9) BC- Vng th ng - c mng th ng

CD- Qu th ng, trong dung dch nc xy ra phn ng gii phng oxi:

2H2O O2 + 4e + 4H+ (6.10) ng cong OABCDE - ng phn cc ho tan kim loi trong dung dch c cha

ion halogen X (Cl, Br, I).

Hnh 6.9

ng phn cc vng ng vi s c mt n mn im

1. ng phn cc anot (vng) trong dung dch khng c ion halogen;

2. ng phn cc anot (vng) trong dung dch c cha ion halogen

Ti im C ng vi in th bt u n mn l El, mng th ng bt u b chc thng, n mn l bt u (Z+= 3):

Me + H2O + X MeOHX+ + H+ + 3e (6.11)

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Cc gi tr th trong min th ng m hn th bo v Ebv th ti khng xy ra s n mn l. Gi tr th n mn l El cng dch chuyn v pha m th kh nng n mn l xy ra cng d dng. V d trong mi trng NaCl 0,1N, gi tr El ca Al bng ElAl = 0,43 V v ElNi = 0,28 V th Al b n mn l d dng hn Ni. Mt khc, khi tng nng ion Cl th gi tr th El s dch chuyn v pha m nhiu hn, ngha l s n mn l xy ra cng d dng.

i vi mng th ng ca Fe tn ti dng FeOOH v b ho tan rt chm:

FeOOH + H2O Fe3+ + 3OH Khi c mt ion Cl mng oxit st d dng b ha tan:

FeOOH + Cl FeOCl + OH FeOCl + H2O Fe3+ + Cl + 2OH FeOOH + H2O Fe3+ + Cl + 2OH y c th xem ion Cl ng vai tr l cht xc tc cho qu trnh ho tan mng th

ng.

S pht trin l Ti v tr n mn l, s n mn kim loi lm cho pH ca mi trng gim (theo phn ng

6.11) v vi s c mt ion halogen trong l hn ch hin tng ti th ng. Mt khc v cc l nh, s l t cho nn din tch ho tan anot ca kim loi rt b so

vi din tch mng th ng ng vai tr l catot cho nn mt dng ho tan ti cc l rt ln, khi xem s ho tan ca cc l nh l cc anot hi sinh. V vy m kch thc cc l to dn ln, ngha l c s pht trin l. i vi nhng kim loi c mng th ng dn in rt km, v d trn kim loi Al, Ti th s pht trin l din ra rt chm.

Ngoi cc anion halogen, mt s cu t c mt trong kim loi, v d cc sunfua trong thp khng g, hoc cc kim loi dng hn c mt trn b mt th ng (v d ng trn b mt Al) c tc dng kch thch s n mn l.

Hnh 6.10 m t dng n mn l ca thp th ng trong dung dch trung tnh c mt Cl.

Hnh 6.10

Dng n mn l ca thp th ng trong mi trng c mt Cl

C tc gi cho rng s pht trin n mn l tng t n mn khe. Di y gii thiu mt s bin php bo v kim loi th ng khi s n mn l.

La chn kim loi

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Thp khng g hp kim Fe-Ni-Cr c th xy ra n mn l trong nc bin. Hp kim Fe, Ni, Cr, Mo c kh nng chng n mn cao trong mi trng c ion Cl, hp kim cha Ti cng c kh nng chng n mn l do ion Cl gy ra...

Thay i mi trng: loi ion Cl, thm vo mi trng cc cht hn ch s hp ph ca ion Cl, v d OH hoc ion Cr2O7 hoc cc cht c ch khc c th l NaNO2, NO3...

Dch chuyn th vo vng th ng.

6.4 Mt s dng n mn khc 6.5.1 n mn ranh gii

Hin tng n mn ranh gii c lin quan n s tn ti ca cc pha d th ti bin gii cc ht trong hp kim a tinh th, v d thp FeNi8Cr18, khi lm ngui dn t 1100oC c dng tinh th cacbua crom (CrFe)23C6 s kt ta bin gii ht. Lng crom bin gii ht gim v mt kh nng th ng, vng th th ng b hp li, dng th ng ln ln v s ho tan trong min th ng ln ln. Vy mt in th th ng nht nh bn trong mt bin gii b th ng, cn bin gii ht th hot ng, trong trng hp ny c hin tng n mn gia cc tinh th.

trnh hin tng n mn gia cc tinh th i vi thp hp kim FeCr, trong qu trnh thp cn trnh hin tng kt ta hoc dng thp c cha mt s nguyn t nh Ti hoc Nb, chng kt hp vi cacbon.

6.5.1 n mn nt do ng sut n mn nt l do s nt gy bi ng sut ko v mi trng n mn gy ra. Hu ht cc hp kim kt cu dng trong k thut (v d cc kt cu thp ca cu, gin

khoan, tu thuyn ...) u c th b rn nt v sau b gy di tc dng ca mi trng n mn ng thi di tc dng ca lc ko. Trong qu trnh cc cu kin b n mn dng nt di ng sut phn ln b mt kim loi khng b xm hi r rng, nhng li c nhng vt nt nh xuyn qua chng, v th hin tng n mn nt rt nguy him, c bit xy ra trong cc bnh nn kh.

C th ni rng loi n mn ny rt phc tp, cho n nay ngi ta vn cha bit y v n.

Di y l mt s yu t nh hng n hin tng n mn ny:

a) Mt s mi trng c th gy ra s n mn nt do ng sut i vi mt s vt liu Thp khng g FeNi8Cr18 trong dung dch c cha ion Cl (BaCl2, NaCl-H2O2...);

Thp thng trong mi trng cha OH hoc NO3; Hp kim nhm trong dung dch cha ion Cl, trong nc bin, hi nc;

Hp kim ng trong mi trng kh NH3 hoc trong dung dch cha ion NH4+, tip xc vi cc hp cht amin, hi nc

Hp kim magie trong dung dch NaCl-K2CrO4; Hp kim titan trong HNO3 bc khi, nc bin

b) Cc giai on ca n mn nt Khi u ca s n mn nt

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S n mn nt do ng lc bao gm nhiu giai on: Pht sinh vt nt (t vi giy n nhiu ngy), s lan truyn vt nt (tc nt 1011 102 m/ngy), cui cng l giai on gy v ti trng c hc qu cao. Di tc dng ca ng sut ko trn b mt nhn ca kim loi thng xut hin cc ch hng cc b gi l bc trt, ti xy ra s khi u n mn.

Nu kim loi nhanh chng b th ng th khng nguy him, cn nu thi gian th ng ko di th s c s n mn im v nh vy l khi u s n mn nt. C trng hp s khi u n mn nt khng phi l bt u t n mn im m xut hin vt nt ti cc bin gii ht tinh th. Nu bin gii ht khng ng nht v thnh phn ho hc cng gy ra s n mn, khng nht thit l dng vt nt.

Pht trin n mn vt nt S lan truyn cc vt n mn nt c gii thch bng nhiu gi thuyt:

S lan truyn cc vt nt xy ra ch yu theo bin gii ht v hot ng tng t n mn ranh gii. Ti c s phn cc anot, kim loi b ho tan.

S lan truyn cc vt nt do bin dng ca mng bo v b nt gy, tc gy mng hoc cn gi l tc bin dng, tc lan truyn khi c ti trng tnh ph thuc vo cc yu t:

+ Kh nng tip xc ti u mt ca vt nt vi cc phn t xm thc, v d ion Cl hoc oxi.

+ Tc th ng, nu tc ti th ng din ra qu chm th tc ho tan kim loi s xy ra ng k ti cc u mt v vt nt bn cnh. Nu s ti th ng din ra nhanh th tc lan truyn vt nt s b chm li.

i vi kim loi th ng, khi dch chuyn th v pha qu m s c hin tng n mn nt do hiro thm trong kim loi, iu ny cng cnh bo v tc hi ca s bo v catot qu mc. Ngc li, khi phn cc anot vi th dng kh cao vt ngng s to ra oxi cng dn n s n mn nt.

Nu trn b mt kim loi c s hp ph c bit ca cc phn t hot ng t dung dch cng lm suy gim bn c hc ti u mt cc vt nt dn n ph hu lin kt gia kim loi-kim loi lm cho kim loi d dng b ph hu. V d s hp ph ca cc nguyn t hiro s lm yu lin kt kim loi- kim loi

c) Cc bin php chng n mn do ng lc C mt s bin php sau: Thay i iu kin ng lc. V d thay i thit k sao cho cng ng lc gim ti

a hoc x l nhit lm gim ni ng lc. Thay i mi trng n mn. V d loi cc cht gy n mn nt (ion Cl) hoc thm

vo mi trng cht gy th ng hoc c ch qu trnh n mn. Thay i th in cc a kim loi vo trng thi bn vng.

Thay i cu trc kim loi: lm gim khuyt tt, gim s kt ta ti bin gii ht bng cch nhit.

Sn ph b mt kim loi, ngn cch b mt kim loi vi mi trng n mn.

6.5.1 n mn mi

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Cc cu kin kim loi chu ti trng ng c th b gy do mt ti trng nh hn ti trng cc i m chng chu ng c trong iu kin tnh. Hin tng gi l s mi v dn n cc vt nt do mi.

Mi trng n mn lm tng tc giai on khi u v tc lan truyn cc vt nt do s mi gy ra v t hp hai yu t li gi l s n mn mi.

Hin tng n mn mi cng rt phc tp, cho n nay vn cha c c ch r rng v nh hng ca cc phn ng n mn i vi qu trnh mi.

Trong mt kt cu c th tn ti mt b phn c khuyt tt trn b mt do cc tp cht, hoc cc vt nt do hn, g hoc l n mn v xem chng nh cc khe nt ban u, vy tui th ca cu kin c quyt nh do tc lan truyn cc vt nt hoc cc khe khi c ti trng. ng nhin s c mt ca mi trng n mn c nh hng ng k n tc lan truyn.

trnh n mn mi c th s dng cc bin php sau: gim ti trng trn cc cu kin, phun cc lp ht hoc nitrat ho b mt kim loi.

Mt khc cn phi loi tr bt cc tc nhn gy n mn ca mi trng, thay i th bo v catot hoc sn ph.

6.5.1 n mn chn lc V d ho tan km trong hp kim ng thau bng dung dch axit H2SO4 long. Sau mt

thi gian ngm mu, km b ho tan v cn li mt khi ng xp. S n mn nh vy gi l s n mn chn lc. Dng n mn ny xy ra trong nhng iu kin nht nh - hp kim l dung dch rn ng th, gi tr th ca kim loi b ho tan m hn so vi kim loi bn trong mi trng n mn.

i vi hp kim Cu-Zn ngi ta gi thit rng cc ion km chuyn n b mt b n mn bng dng khuch tn t th tch

Mt khc c th gi thit cho rng c hai kim loi Cu v Zn u b ho tan do phn ng anot, sau ng b kh v kt ta tr li.

S n mn chn lc hp kim ng niken (Cu-Ni) hoc hp kim thp khng g FeNi8Cr18. trnh hin tng n mn chn lc nn s dng cc hp kim cha t kim loi c in

th m. V d gim hm lng km trong hp kim ng km. Thng vi lng km trong hp kim nh hn 15% th km khng b ho tan. Nu thm vo hp kim 1% Sn hoc mt hm lng nh asen (As) th tc n mn chn lc ng thau gim mt cch ng k.

6.5.1 n mn mi mn Khi kim loi b vt th rn tc ng ln b mt kim loi lm cho cc ht bi kim loi tch

ra khi b mt ca s mi mn. Mi mn c th gy ra do dng chy ca cht lng tc ng ln b mt vt rn.

Khi s mi mn lm mt mng th ng, trong mi trng xm thc, kim loi d dng b n mn. C th ni rng n mn mi mn rt a dng.

6.5.1 n mn do ma st n mn do ma st thng gp ti ch ghp ni ca vng bi trn mt trc. trnh loi n mn ma st ngi ta hn ch s chuyn ng lp li nhiu ln, ti ch

ghp cht cn c lp m bng kim loi mm, v d ng hoc nhm mt pha hoc c hai

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pha hoc photphat ho sau tm du, ph lp mng cht hu c polytetrafloruaetylen (teflon), lp mng MoS2.

Nu ma st do dng chy gy ra th phi hn ch dng chy ri lon hoc cho thm c ch vo hn ch s mi mn dng th ng.

6.5 nh hng ca mi trng n qu trnh n mn kim loi 6.5.1 n mn trong mi trng kh quyn

Hin tng n mn kim loi trong iu kin kh quyn m l n mn in ho. Do s ngng t hoc ngng ng ca nc trn b mt kim loi c mt mng mng nc, s ho tan ca cc kh CO2 hoc SO2 to ra dung dch c pH thp v kim loi b n mn mt cch d dng. Hn na, s ho tan ca oxi trong khng kh vo mng mng cht in li cng l mt yu t quan trng i vi qu trnh n mn in ho ho tan kim loi.

n mn kh quyn ph thuc vo nhiu yu t sau y:

a) m m l iu kin cn phi c xy ra hin tng n mn in ho, nhng cha phi l

iu kin . c th xy ra hin tng n mn in ho trn b mt phi tn ti mt lp mng mng cht in li gm mng mng nc v cc cht in li ho tan, v d Cl, CO32, SO32 Tu theo m trn b mt m ngi ta phn loi n mn kh quyn:

n mn kh quyn kh - kim loi tc dng trc tip vi oxi to ra mng oxit cc mng, c th xem l cc phn ng ho hc, v khng gy ra hin tng n mn kim loi ng k, nhng khi c tp cht n mn th tc n mn c th tng ln.

n mn kh quyn m - trong iu kin m vi m tng i H < 100%, thng ln hn 70%, trn b mt kim loi c lp mng mng nc v c tp cht th xy ra n mn theo c ch in ho. V d Cu, Ni, Zn b n mn trong kh quyn khi m tng i ln hn 60%.

ng nhin tc n mn ph thuc vo m tng i ca khng kh, m ca kh quyn v tnh ht m ca sn phm n mn.

Cn nhc li rng m tng i H c tnh bng cng thc sau:

o

PH .100%

P= (6.12)

trong : H - m tng i; P - p sut ring phn ca hi nc trong kh quyn ti nhit kho st;

Po - p sut bo ho ca hi nc ti nhit kho st. n mn kh quyn t - qu trnh n mn kim loi xy ra khi m tng i gn

100%, c git nc ngng t trn b mt kim loi, hoc c git ma, to ra lp cht in li trn b mt kim loi. Hin tng n mn kim loi ny xy ra theo c ch in ha.

b) nh hng ca cht nhim bn Cc mng m nc to ra trn b mt kim loi, v d khi m cao, sng m thng

cha tp cht: kh oxi hoc kh CO2 ho tan ( min nng thn, min ni), cc kh CO2, SO2,

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cc oxit nit (NxOy), kh H2S (vng cng nghip), ion Cl (vng bin, ven bin). Cc tp cht ny gp phn lm tng tc ph hu kim loi (xem hnh 6.11).

Hnh 6.11

Tn tht trng lng (g/m2) ca thp cacbon theo thi gian (ngy)

1 - n mn kh quyn c kh SO2; 2 - n mn kh quyn khng c kh SO2

i vi khng kh sch, s n mn xy ra vi tc rt nh (ng 2 hnh 6.11). Sau khi c mt kh SO2 trong khng kh m tc n mn tng ng k. Ngoi kh SO2, cc kh CO2, NO2, Cl u l tc nhn gy ra n mn kh quyn.

c) C ch n mn in ho trong mi trng kh quyn n mn trong mi trng kh quyn l n mn in ho vi qu trnh catot, trong nhiu

trng hp l s kh oxi.

Anot: Me ze Mez+ Catot:

z

2O2 + ze + zH2O 2zOH

ng hc ca qu trnh ho tan kim loi ph thuc vo chiu dy lp mng m trn b mt kim loi.

Vi mi trng khng kh kh, m thp, dy mng m khong 10 nm th c ch n mn xy ra ch yu theo c ch ho hc.

Vi dy lp mng m c 10 nm 1 m, trn b mt c lp cht in li, s n mn kim loi b khng ch bi qu trnh anot, v mng mng s khuch tn din ra nhanh, ngha l qu trnh catot din ra nhanh. Nu c sn phm n mn dng oxit bao ph ln b mt th qu trnh anot din ra phc tp hn.

Nu b mt lp mng mng m c dy t 1 m n 1 mm, oxi ca khng kh xm nhp vo b mt kim loi kh khn. V th qu trnh n mn kim loi b khng ch bi phn ng catot.

Khi lp mng m c dy ln hn 1 mm th s n mn kim loi xy ra tng t trng hp kim loi ho tan trong dung dch cht in li (Ct st li n tn ti 2500 nm, thnh phn tp cht ca ct thp: Si 0,05%, S 0,006%, P 0,114%, C 0,08%, Mn vt.

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Ct thp ny c bn chng n mn rt cao v thp sch v trn b mt kim loi lun lun kh ro).

6.5.2 n mn kh quyn vng thnh ph v khu cng nghip thnh ph v khu cng nghip c mt s kh thi v ch yu l SO2 t cc l t than,

t du. Khi kh thi ny ho tan vo nc s axit ho mi trng nc v lm tng tc n mn kim loi. Tc n mn kh quyn c tnh theo cng thc:

b 2P at (g / m )= trong : P l tc n mn;

a, b l cc h s; t l thi gian nm.

6.5.3 n mn trong kh quyn bin Trong kh quyn bin lun lun c ion Cl, v th tc n mn kim loi tng ng k so

vi min nng thn.

i vi n mn kh quyn, ngoi cc tp cht gy n mn th thi gian lu m (TOW) tc l khong thi gian khng kh c m tng i H > 80% v nhit ln hn 0oC ng mt vai tr rt quan trng quyt nh tc n mn kim loi.

Nhng mng mng do nc ma hoc do sng trn b mt kim loi thng c m gn bng 100%.

vng nng thn kh quyn t tp cht c hi cho nn tc n mn kim loi nh.

Vic nghin cu cc bin php chng n mn kh quyn rt c ngha thc t. C th nu ln mt s bin php chnh sau:

To ra cc lp bao ph, sn hu c, c lp ph v c, hoc xi m ngn cch kim loi vi mi trng gy ra n mn. V d sn ph bo v cu st, tu thuyn, cc cu kin khc: t, tu ho, m cc ph tng t, xe my

Chn cc kim loi c bn chng n mn cao: thp khng g, cc loi hp kim.

i vi cc cu kin nh v lu gi trong kho c mt cc loi cht c ch bay hi bo qun trong khng kh kh.

6.5.4 n mn trong mi trng t Trong lng t, c bit cc khu cng nghip c rt nhiu thit b chn ngm di t:

h thng ng dn nc, dn kh ga, cp in, ng ng dn nhin liu lng, xng du c th b n mn lm gim tui th ca chng v i khi cn lm gim cht lng sn phm nc ( nhim nc, gim cht lng xng du).

Tc ph hu cc cu kin do n mn ph thuc vo a hnh. nhng v tr cao, kh ro, khng c cc m mui th tc n mn kim loi rt thp.

Hin tng n mn kim loi c tc ng k thng xy ra nhng vng m, c tp cht gm cc mui v c, v d vng t m ven bin.

nh gi kh nng n mn kim loi ca mi trng t ngi ta dng ch s pH hoc dn in ca t.

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Vi pH = 5 8 v nn t m, mi trng axit c dn in tt, kim loi d dng b n mn. Trong cng mt iu kin, t ct gy ra n mn cao hn t st v d dng thng kh oxi. Cn lu rng trong mi trng t, s c mt ca vi sinh vt cng nh hng ng k n tc n mn.

S n mn kim loi trong mi trng t l n mn in ho vi catot kh oxi, i khi kh ion H+ nu mi trng n mn l axit.

Do cu trc ca t khng ng nht cho nn s thng kh khng u v thng gy ra n mn thng kh, khi ni c nhiu oxi ng vai tr l catot cn vng t oxi ng vai tr l anot, phn ng xy ra ph hu kim loi.

Dng n mn kim loi trong t rt a dng, thng l n mn cc b nn kim loi b ph hu dng n mn im, h su, dng r

hn ch kh nng n mn do mi trng t, cc cu kin phi c bo v (sn ph bc kim loi bng vt liu polyme, sn bitum) hoc cc loi sn v c (sn xi mng) Thng thng, ngi ta s dng phng php in ho bo v catot bo v cc ng ng dn nhin liu nm trong lng t.

6.5.5 n mn trong btng ct thp Ngy nay, vic nghin cu s n mn v bo v cc cng trnh btng ct thp l mt

ti kh hp dn, c nhiu nh khoa hc nc ngoi v trong nc quan tm v s lng cng trnh btng ct thp rt nhiu: cc cng trnh dn dng nh ven bin, cu cng ven bin, cu cng bin, cc cng trnh khai thc du kh v.v

Btng ct thp gm hai phn: phn v l vt liu nhn to gm cc thnh phn c t l hp l: cht kt dnh xi mng, nc, ct liu th (si ct) v cc ph gia khc, chng c trn vo nhau sau c thnh khun v phn th hai l ct li bng thp.

Trong qu trnh ng rn ca btng, cc thnh phn ca khong xi mng b thu ho v to mi trng kim (pH = 13 14). Trong mi trng ny ct thp b th ng.

Phn ng thu ho (hirat ho) ca cc khong silicat nh sau:

2(3CaO.SiO2) + 6H2O 3CaO.2SiO2.3H2O + 3Ca(OH)2 2(2CaO.SiO2) + 4H2O 3CaO.2SiO2.3H2O + Ca(OH)2 i vi khong C3A phn ng xy ra:

3CaO.Al2O3 + 6H2O 3CaO.Al2O3.6H2O 2(3CaO.Al2O3) + 21H2O 4CaO.Al2O3.13H2O + 2CaO.Al2O3.8H2O Khi c mt thch cao xy ra phn ng to thnh ettringit:

3CaO.Al2O3 + 3(CaSO4.2H2O) + 6H2O 3CaO.Al2O3.3CaSO4.12H2O Ettringit l cc tinh th hnh kim c hnh thnh nhanh trn khong C3A cha thu ho

to lp v bc khng cho nc thm qua. V vy thch cao c dng lm ph gia iu chnh thi gian ng kt xi mng.

Qu trnh thu ho din ra sut thi gian s dng btng.

Nu lp v cht xt c kt cu tt th ct thp lun lun trng thi th ng v khng b n mn.

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Nu lp v btng qu mng hoc c kt cu xp th oxi s xm nhp vo ct thp v gy ra n mn. ng nhin khi btng ct thp ngm trong nc, lng oxi thp, kh xm nhp ct thp v s ph hu ct thp cng b hn ch.

Trong nc bin cha nhiu ion Cl, s xm nhp ca n vo cu trc b tng s nh hng ln n tui th ca cc cng trnh.

Ion Cl xm nhp vo b tng theo c ch khuch tn v thng tn ti cc dng: hp ph vt l trn b mt rn ca l xp, tham gia lin kt ho hc to C3A.CaCl2.10H2O hoc C3A.3CaCl2.3H2O

Nu tn ti t do trong cc mao qun, l trng ion Cl s ph hu v n mn btng ct thp.

Qu trnh n mn ct thp btng l n mn in ho. Khi c ion Cl xm nhp vo lp th ng ca btng, lp th ng b ph v v thp b n mn, pH gim:

Fe 2e Fe2+ Fe2+ + 2H2O + Cl Fe(OH)2 + 2HCl Ngng ti hn ph hu mng th ng v gy n mn ct thp l [Cl]/[OH] = 0,6. Khi

lp th ng b ph v mt phn hoc hon ton th th in cc ca ct thp dch chuyn v pha m hn, ti ng vai tr l anot v thp b ho tan (xem hnh 6.13).

Hnh 6.12

S n mn ct thp btng

Ti anot: Fe 2e Fe2+ Tai catot:

1

2O2 + 2e + H2O 2OH

Qu trnh n mn ch xy ra khi vng catot c H2O v O2. Nu in tr ca btng rt ln th tc n mn nh (5 7.104 /cm).

Oxi l cht ng vai tr kh catot thc y s ho tan kim loi, song Cl ng vai tr rt quan trng.

Khi c mt Cl, in tr btng b gim d lm cho dng n mn tng ln v n ng vai tr khi mo cho phn ng thng qua phn ng ph v mng th ng vi s hnh thnh hp cht phc:

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Fe + 3Cl FeCl3 + 2e FeCl3 + 2OH Fe(OH)2 + 3Cl Phn ng catot to ra OH, pH tng ln cho nn s n mn su trong l b gim, s n

mn tip tc th nng Cl trong l phi ln thay th OH, nu thiu Cl s sinh ra ti th ng. Vi nng Cl = 0,5% so vi khi lng ximng th xy ra s lan truyn khi mo n mn. Mt khc, vi s c mt Cl, xy ra s n mn l lm cho t l in tch catot/anot ln, mt dng s tng cao.

nng cao cht lng cng trnh btng ct thp c th s dng cc bin php sau:

+ To mt lp v btng bm chc ct thp, cht xt, ph kn hn ch s thm nc v thm cc cht gy n mn ca mi trng.

+ Bo v bng phng php in ha - bo v catot. + Sn ph lp v btng bng cc loi sn v c hoc hu c.

6.5.6 n mn vi sinh Vi sinh vt thng tn ti trong lng t, y cc b nc ngm, cc b cha nc, y

cc bn cha xng du... S hot ng ca cc vi sinh nh hng n qu trnh n mn kim loi theo c ch in ho.

S hot ng ca vi sinh hoc sn phm hot ng ca n c tc ng n cc qu trnh catot hoc anot ca s n mn in ho. C hai loi vi khun nh hng n tc n mn: vi khun hiu kh v vi khun k kh.

n mn kim loi c vi khun hiu kh:

Loi vi khun hiu kh tiu th oxi, s hot ng ca vi khun ny sinh ra cht thi dng bn v bm trn b mt kim loi to ra pin thng kh gy ra s n mn in ho ph hu kim loi. Mt khc, loi vi khun hiu kh oxi ho sulfua, v d vi khun Thiobaccillus, c kh nng oxi ho nguyn t lu hunh hoc cc hp cht c lu hunh trong kim loi thnh axit sulfuric H2SO4 theo phn ng sau:

2S + 3O2 + 2H2O 2H2SO4 v to ra mi trng axit lm tng tc ho tan kim loi.

Vi khun oxi ho sulfua hot ng tt trong mi trng axit vi pH = 0 6. C loi vi khun thc y qu trnh oxi ho ion Fe2+ thnh Fe3+, khi nng Fe2+ gim

v thc y qu trnh anot ho tan st:

Fe 2e Fe2+ Vi khun oxi ho st pht trin tt vng pH = 4 10, sn phm n mn c mu . Ti nhng ni mng sn b h hng, st b n mn v c Fe(OH)3 kt ta to ra cc g

cng l ni ch n ca cc vi khun k kh.

n mn kim loi c vi khun k kh. Vi khun k kh ch yu l vi khun kh sulfat (D.desulfuricals). Do hot ng ca chng

cc ion sulfat b kh thnh sulfua theo phn ng:

Vi khun 24SO S2 + 4O

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S c mt sn phm S2 v oxi thc y qu trnh n mn in ho, ngha l oxi sinh ra s phn ng vi nguyn t H mi sinh ca phn ng catot. Mt khc, s c mt S2 s phn ng vi Fe2+ to ra sulfua st:

Fe2+ + S2 FeS C hai yu t trn s thc y qu trnh n mn in ho st. Sn phm n mn thng

dng mu en, mu ca sulfua st, c dng xp cng gp phn lm d dng kh oxi.