corrosion_resistance of electroless nickel coatings

8/8/2019 Corrosion_Resistance of Electroless Nickel Coatings

1/13

CORROSION RESISTANCE OF

ELECTRO LESS NICKEL COATINGS

Replacement o r r e p a i r o f c o r r o s i o n damaged equipment i s t h e l a r g e s tmain tenance r e q u i r e m e n t f o r i n d u s t r y . In t h e Uni ted S t a t e s a l o n e ,t h e c o s t o f cor rOSlon i s e s t i m a t e d t o be 70 b i l l i o n d o l l a r s a n n u a l l y,o r 4 .2 p e r c e n t o f t h e g r o s s n a t i o n a l p r o d u c t . Probab ly 15 p e r c e n to f t h i s l o s s c o u l d be saved wi th b e t t e r m a t e r i a l s , p r o c e d y r e s o rd e s i g n s t h r o u g h t h e u s e o f p r es e n t l y a v a i la b le t e c h n o l o g y . Thed i r e c t c o s t o f c or r o si o n i n o t h e r i n d u s t r i a l c o u n t r i e s h as beenfound t o be s i m i l a r t o t h o s e i n t h e US A2.

On e t e c h n i q u e f o r r e d u c i n g t h e c or r o si o n o f meta l s i s t o c o a t themwi t h t h i n l a y e r s o f l e s s r e a c t i v e meta l s o r a l l o y s . Va r i o u s t e c h -n iq ue s , i n cl u d i n g e l e c t r o p l a t i n g , vapor d e p o s i t i o n , m e t a l s p r a y i n g ,and e l e c t r o l e s s d e p o s i t i o n , have been used wi th v a r y i n g s u c c e s s .U n f o r t u n a t e l y, most m e t a l l i c c o a t i n g s . a r e i n h e r e n t l y porous andh i s t o r i c a l l y have been o f l i t t l e v a lu e a s b a r r i e r s a g a i n s t c o r r o s i o n .R e c e n t l y, w i t h t h e development o f E l e c t r o l e s s N i c k e l , t h e s eprob lems have l a r g e l y b ee n o ve rc om e. Today, because o f i t s s u p e r i o rc o r r o s i o n r e s i s t a n c e and because o f t h e i n c r e as ed impor tance b e i n gg i v e n equipment c o s t and r e l i a b i l i t y , t h i s c o a t i n g i s be ingc o n s i d e r e d f o r more and more u s e s .

CORROSION RESISTANCE IS OUTSTANDING

E l e c t r o l e s s N i c k e l i s a b a r r i e r c o a t i n g . I t p r o t e c t s t h e u n d e r l y i n g

m e t a l by s e a l i n g i t o f f from t h e e n v i r o n m e n t , r a t h e r t h an by g a l v a n i co r s a c r i f i c i a l a c t i o n . Because o f i t s amorphous n a t u r e and p a s s i v i t y ,however, t h e c or r o si o n r e s i s t a n c e o f E l e c t r o l e s s N i c k e l i se x c e l l e n t and i n most env i ronments s u p e r i o r t o t h a t o f p u r e n i c k e l o rchromium a l l o y s . When p r o p e r l y a p p l i e d , t h e c o a t i n g i s a l m o s t t o t a l l yr e s i s t a n t t o a l k a l i e s , t o s a l t s o l u t i o n s and b r i n e s , t o chemica l andpe t ro leum e n v i r o n m e n t s , and t o a l l t ype s o f hydrocarbons and s o l v e n t s .

d ep os it s a ls o have good r e s i s t a n c e t o ammonia s o l u t i o n s , t oo rg a n i c a c i d s , and t o r e d u c i n g a c i d s . They a r e o n l y s i g n i f i c a n t l ya t t a c k e d by s t r o n g l y o x i d i z i n g media , such a s s t r o n g n i t r i c a c i d .

ATMOSPHERIC CORROSION

E l e c t r o l e s s N i c k e l i s a l m o s t immune t o a t t a c k i n r u r a l , i n d u s t r i a land mar ine a t m o s p h e r e s . In e xp osure s a t Kure Beach, N . C . , CorpusC h r i s t i , Texas , and Washington, D . C . , even ve ry t h i n c o a t i n g s werefound t o p r o v i d e comple te p r o ~ e c t i o nt o p r o p e r l y p r e p a r e d s u b s t r a t e sf o r p e r i o d s o f 6 t o 15 months 4 . These s t u d i e s a l s o showed 0 .5 m il(1 2 ~ m ) t h i ck E l ec tr o le s s N i c k e l c o a t i n g s t o p r o v i d e b e t t e r p r o t e c t i o nt h an 1 m il (2 5 urn) t h i c k c o a t i n g s o f e l e c t r o l y t i c n i c k e l .


2/13

An u n i q u e p r o p e r t y o f E l e c t r o l e s s N i c k e l i s i t s freedom f romu n d e r - d e p o s i t o r i n t e r t a c e a t t a c k . Unl ike e l e c t r o l y t i c c o a t i n g s ,even i f c o r r o s i o n does b e g i n th rough p in h o l e s o r damaged a r e a s ,t h e r e i s no t endency f o r i t to s p r e a d i n e i t h e r a t m o s p h e r i c o rsubmerged w a t e r env i ro nm en ts . O ften a s th e a f f e c t e d a r e a f i 1 5si th r u s t , f u r t h e r a tt a ck o f th e s u b s t r a t e s lows and may s t o p .SEAWATER

coa t ingB can p r o v i d e v e r y e f f e c t i v e p r o t e c t i o n a g a i n s tc o r r o s i o n i n seawate r sys tem s . Sh ipb oard hardware , f i t t i n g s andv a l v e s , a s w e l l a s l and b a s e d s y s t e m s , a r e commonly p r o t e c t e d wi th3 m il (75 ~ m ) t hi ck d ep o s it s. In th e Ghawar F i e l d in Saud iaA r a b i a , coa t ed b a l l v a l v e s have been u s e d i n th e s e a w a t e ri n j e c t i o n system f o r 5 y ea rs w ith ou t any c o r r o s i o n . One v a l v ewas removed and i n s p e c t e d a f t e r 4 y e a r s o f o p e r a t i o n and wasfound t o b e i n p e r f e c t c o n d i t i o n .

L a b o r a t o r y i n v e s t i g a t i o n s i n a e r a t e d , s y n t h e t i c seawate r a t temper-a t u r e s up to 20 0 0 F (95 0 C) have shown th e maximum l o s s o f . - !

E l e c t r o l e s s N i c k e l to b e only 0.04 mpy (1 ~ m / y ) .Because o f i t sh i g h h a r d n e s s , t h e d e p o s i t h as e x c e l l e n t r e s i s t a n c e to impingementand e r o s i o n / c o r r o s i o n i n s e a w a t e r and i s f r e q u e n t l y u s e d f o r pumps,v a l v e s and ~ i m i l a requipment . Vib ra to ry c a v i t a t i o n t e s t s haveshown lcoa t i n g s to have r e s i s t a n c e e q u a l to s t a i n l e s s andh i g h chromium a l l o y s in bo th sea and f r e s h w a t e r s .

NATURAL WATERS

C o r r o s i o n o f E l e c t r o l e s s N i c k e l i n f r e s h and h i g h p u r i t ywate r s i s a l s o s l i g h t . Te s t s i n ta p w a te r have shown c o r r o s i o nr a t e s o f o n l y 0 .01 to 0 .03 mpy ( 0 . 3 to 0 .8 ~ m / y )a t ambien tte mp era tu re . C orro s io n i n a i r s a t u r a t e d , de ion i zed w a t e r i si l l u s t r a t e d by F i g u r e 1 , w hic h c om pa res th e c o a t i n g to p l a i ns t e e l . Even a t t e m p e r a t u r e s a s h ig h a s 20 0 0 F (9 5 0 C) , th e maximuml o s s o f th e d e p o s i t i s o n l y 0.004 mpy (0 .1 u m/y) .

S i m i l a r l y th e r e s i s t a n c e o f c o a t i n g s to c o r r o s i o n i nc o o l i n g w a t e r , b o i l e r f e e d w a t e r , condensa te and th e l i k e i ss u p e r i o r . Even a t t e mp era tu re s o f 65 0 0 F (350 0 C ) , i t s l o s s ind e a e r a t e d , double d i s t i l l e d w a t e r i s l e s s then 0.008 mpy(0 . 2 ~ m/y) .

FIGURE 1 eo

Comparison of th e c or ro sio n of carbon s t e e l an d e l ec t ro l e s sn i c ke l in oxygen sa tura ted d e io n iz ed w at er

HNIC " . ' ' ' ' . ' ' ' ' NI,"' . " ' ' ' ' ' ' ' " " ' '0" 0


3/13

- - - - - - - - - ----------

e l e c t r o l e s s n ic ke l a l s o ha s good r e s i s t a n c e t o condens ings tea m . A utoc la ve t e s t s i n which 3600F (180C) s t eam, s a t u r a t e dw i t h 02 and C02' was condensed on h e a t t r a n s f e r p a n e l s showedt h e c o a t i n g t o be u n a t t a c k e d a f t e r 108 h o u r s . With in t h i sp e r i o d commercia l g la ss l i n in g s normal ly a r e s e v e r e l y e t c h e d .

s o l u t i o n pH ha s a pronounced i n f l u e n c e on t h e c o r r o s i o n o f mostm e t a l s , i n c l u d i n g e l e c t r o l e s s n i c k e l . The r e s u l t s o f t e s t s cond u c t e d t o e s t a b l i s h t h i s e f f e c t a r e shown i n F i g u r e 2 .

FIGURE 2Effec t of pH on thec o r r o s i o n o f

Elec t ro less Nickel "~ 0.6""~ 0.6W

~is 0.4

g~c 0.2

"\

l- ' - - . - ' - - ' - -

10 12

""

F or t h e s e t e s t s , 0 .1 p e r c e n t HCl i n d e i o n i z e d w a t e r was n e u t r a -l i z e d w i t h sodium hydrox ide t o produce s o l u t i o n s w i t h pH v a l u e sr a n g i n g from 1 .4 t o 12 . Immers ion t e s t s a t ambien t t e m p e r a t u r eshowed t h a t above 3 pH c o r r o s i o n o f t h e d e p o s i t was u n i f o r m l yl e s s than 0.08 mpy (2 ~ m / y )and averaged o n l y 0.04 mpy (1 ~ m / y ) .Atl ower pH v a l u e s a t t a c k was i n c r e a s e d , b u t d i d n o t exceed 0 .8 mpy(2 0 ~ m / y ) .

INORGANIC ACIDS

e l e c t r o l e s s n i c k e l ha s u s e f u l r e s i s t a n c e t o reduc ing a c i d s .T y p ic al l y t h e d e p o s i t ' s l o s s r a t e w i l l r ange between a b o u t 0 .3and 1 mpy (8 and 25 ~ m / y ) . A c c o r d i n g l y, t h e c o at i n g i s n o t commonlyused f o r pro longed e xp os ur es , a lt ho ug h i t can be used s a f e l y f o ri n t e r m i t t e n t o r s h o r t term s e r v i c e s such a s a c i d c l ean ing o r backwash ing . E l e c t r o l e s s n ic ke l d e p os i t s g en er a ll y have poor r e s i s t a n c et o o x i d i z i n g a c i d s . In t h e s e env i ronments , l o s s e s can be q u i t e h i g hand e l e c t r o l e s s n i c k e l i s n o t n o r m a l l y u s e d .

S u l f u r i c Acid . S u l f u r i c a c i d i s t h e most w i d e l y used m i n e r a l a c i d .I t s b e h a v i o r , however, i s u n u s u a l . At ambien t t e m p e r a t u r e , w i t hc o n c e n t r a t i o n s l e s s t h a n 85 p e r c e n t , i t i s a r e d u c i n g a c i d . A th i g h e r c o n c e n t r a t i o n s i t becomes an o x i d i z i n g a c i d . The b e h a v i o ro f e l e c t r o l e s s n i c k e l f o l l o w s t h a t o f s u l f u r i c a c i d . Belowa b o u t 85 p e rc e nt c o nc e n t ra t i on , t h es e c oa ti n g s have good r e s i s t a n c ea t ambien t te m pe ra tu re , w ith r a t e s g e n e r a l l y between 0 .3 and 1 . 1 mpy(8 and 28 ~ m / y ) .Above 85 p er c e n t, c o r r o s i o n o f t h e d e p o si t i nc re as esr a p i d l y and i n oleum may exceed 10 mpy (250 ~ m / y ) .The r e s i s t a n c eo f e l e c t r o l e s s n i c k e l t o s u l f u r i c ac id i s summarized i n F i g u r e 3


4/13

- - - - - - - - -~ ~ ~ - - - - - ~

FIGURE 3

Corrosion o f E l e c t r o l e s sNickel i n s u r f u r i c ac id I

I/

~ ' , /'.----

' ~ . J00 110 100

Hydroch lor ic Acid. At ambient t empera ture d e p o s i t s haveu s e f u l r e s i s t a n c e to h yd ro ch lo ri c a c id only up t o a b o u t 10 p e r c e n tc o n c e n t r a t i o n . In t h e s e d i l u t e s o l u t io n s l os se s a re g e n e r a l l y about1 mpy (2 5 ~ m / y ) .At h ighe r c o n c en t ra t io n s c o rr o si o n i s i nc r ea sedt o between 2 and 3 mpy (50 and 75 ~ m / y ) .Attack i s a l s o i n c r e a s e dby i n c r e a s e d tRmnerature o r by th e p re se nce o f o x i d i z i n g s a l t s . Th er e s i s t a n c e o f e l e c t r o l e s s n i c k e l to h yd ro ch lo ri c a c id i s sum-mar ized i n Fi gu re 4 .

FIGURE 4

Corrosion o f E l e c t r o l e s sNickel in hydroch lo r ic ac id

( ~. \

/ "i

r /

CONCENTRATION. wT_ PERCENT"'0'


5/13

H y d r o f l u o r i c Acid . Al though c h e m i c a l l y h y d r o f l u o r i c i s a weakera c i d t h a n h y d r o c h l o r i c o r s u l f u r i c , i t i s much more c o r r o s i v e t h a ne i t h e r and very d i f f i c u l t t o c o n t a i n . Only g o l d , p l a t i n u m and Te f l o na r e c o m p l e t e l y r e s i s t a n t t o a t t a c k . The r e s i s t a n c e o f commerc ia ln i c k e l b a s e a l l o y s , however, can be adequa te f o r most a p p l i c a t i o n s .These a l l o y s deve lop t h i n a d h e r e n t p r o t e c t i v e f l u o r i d e f i l m s on t h e i rs u r f a c e s which r e t a r d c o n t i n u e d a t t a c k .

The r e s i s t a n c e o f c o a t i n g s t o h y dr o f lu o r i c a ci d s o l u t i o n s i su s e f u l f o r d i l u t e s o l u t i o n s . Below 25 p e r c e n t c o n c e n t r a t i o n , c o r r o s i o nr a t e s a re g en er a ll y a b o u t 1 mpy (2 5 ~ m / y ) .The b e h a v i o r o fe l e c t r o l e s s n i c k e l i n h y d r o f l u o r i c a c i d s o l u t i o n s i s summarized i nTa b l e 1 .

TABLE 1

CORROSION OF ELECTROLESS NICKELIN HYDROFLUORIC ACID SOLUTIONS

AT AMBIENT TEMPERATURE

CONCENTRATION,Wt. P e r c e n t

2

10

25

40

52

CORROSION~ m / y

27

30

30

53

46

RATE,mpy

1 . 1

1 . 2

1 .2

2 .1

1 .8

P h os ph or ic A c id . The r e s i s t a n c e o f e l e c t r o l e s s n i c k e l t o

r e a g e n t p h o s p h o r i c a c i d i s good a t ambien t t e m p e r a t u r e w i t h a l lc o n c e n t r a t i o n s . At 85 p e r c e n t , i t s l o s s i s o n ly 0 .1 mpy ( 2 ~ ~ m / y ) .T h i s i s i l l u s t r a t e d by F i g u r e 5 . With commerc ia l a ci d c on ta in in gf e r r i c s a l t s , however, a c c e l e r a t e d a t t a c k ma y o c c u r and t h e c o a t i n gi s n o t commonly u s e d . C o r r o s i o n i s a l s o i n c r e a s e d i n f o r t i f i e da c i d s o r by i n c r e a s e d t e m p e r a t u r e .

FIGURE 5

Cor ros ion of E l e c t r o l e s sNicke l in phosphor ic ac id

Q ,. 20 '0 "" 50 00 , ~


6/13

..- - - - - - - -

N i t r i c Acid . N i t r i c a c i d i s a very s t r o n g o x i d i z i n g a g e n t a t a l lc o n c e n t r a t i o n s . A c c o r d i n g l y, t h e r e s i s t a n c e o f n i c k e l a l l o y s i sv e r y l i m i t e d . e l e c t r o l e s s n i c k e l can o n l y be used f o rc o n c e n t r a t i o n s up t o 10 p e r c e n t a t ambien t t e m p e r a t u r e . In t h i sr a n g e l o s s e s a r e g en er a ll y between 0 .5 and 1 mpy (1 2 and 25 vm/y) .A t h i g h e r c o nc e nt ra t i on s , c o r r os io n i nc re a s es c a t a s t r o p h i c a l l y andcan exceed 1 i n c h p e r y e a r (2 5 mm/y) i n concen t r a t ed a c i d .

CAUSTIC ALKALIS

c o a t i n g s have o u t s t a n d i n g r e s i s t a n c e t o c a u s t i c s o d a , c a u s t i cp o t a s h and most o t h e r a l k a l i s , even a t high c o n c e n t r a t i o n s ande l e v a t e d t e m p e r a t u r e s . At c o n c e n t r a t i o n s below 50 p e r c e n t , c o r r o s i o ni s n e g l i g i b l e due t o t h e fo rmat ion o f D ro te ct i v e o x i d e f i l m s . Acompar i son o f t h e per fo rmance o f c o a t i n g s w i t h p l a i n s t e e l a t2000F (95C) i n up t o 50 p e r c e n t sodium hydrox ide i s shown i n F i g u r e6 . Even a t c o n c e n t r a t i o n s as high as 72 p e r c e n t w ith te m pe ra tu re so f a b o u t 230F (110C) , e l e c t r o l e s s n i c k e l c o a t i n g s have u s e f u l r e s i s -t a n c e ; i n one f i e l d t e s t t h e l o s s r a t e o f t h e d ep os i t was o n l y 0.7mpy (18 vm/y) compared t o 0 .5 mpy (13 vm/y) f o r wrought n i c k e l and41 mpy (1000 vm/y) fo r p l a i n s t e e l .

FIGURE 6

Comparison of th e cor ros ionof carbon s t e e l an d e l ec t ro l e s sn i c k e l in caus t i c soda a t 200F

25::iw Typical Loss of carbon Steel ~, . 20a:we ,

~S 15

ui

~~ 10

oino~ 5 / ELNIC Electroless Nickel8 0.06 ropy maximum

o 10 20 30 40 50NaOH CONCENTRATION, WT. PERCENT

C h l o r a t e s and some s u l f u r compounds i n cr e as e t h e a t t a c k o f n i c k e la l l o y s i n c a us t i c soda and shou ld be removed i f p o s s i b l e . Forexample , t h e a d d i t i o n o f 0 .4 p e r c e n t sodium c h l o r a t e w i l l i n c r e a s et h e c or r o si o n o f e l e c t r o l e s s n i c k e l more than 10 t i m e s . The a d d i t i o no f s u l f u r compounds, such as sodium s u l f i t e , may a l s o i n c r e a s e a t t a c ko f t h e c o a t i n g . The p r e s e n c e o f hydrogen s u l f i d e , however, i s n o td e t r i m e n t a l and ma y improve t h e r e s i s t a n c e o f c o a t i n g s .


7/13

AMMONIA SOLUTIONS

c o a t i n g s have e x c e l l e n t r e s i s t a n c e t o ammonia and u s e f u lr e s i s t a n c e t o ammonium hydrox ide s o l u t i o n s . In pure ammonia, t h el o s s o f c o a t i n g s i s g e n e r a l l y l e s s t h a n 0 .2 mpy (5 ~ m / y ) .Whenw a t e r i s a l s o p r e s e n t , however, ammonia b e g i n s t o h y d r o l i z e , fori l l ingn i c k e l complexes and c o rr os io n i n cr e as e s as i l l u s t r a t e d by Ta b l e 2 .

TABLE 2

CORROSION OF ELECTROLESS NICKELIN AMMONIACAL SOLUTIONSAT AJ1BIENT TEMPERATURE

SOLUTION CORROSION RATE~ m / y mpy

2% NH 3 28 1 . 1

10% NH 3 23 0 .9

28% NH 3 16 0 .627% NH4Cl 8 0 .3

66% NH 4N0 3 10 0 .4

25% NH 4H 2 P 0 4 5 0 .2

43% (NH 4) 2 S0 4 3 0 .1

In ammonium s a l t s o l u t i o n s , e l e c t r o l e s s n i c k e l a l s o d i s p l a y su s e f u l r e s i s t a n c e . In ammonium c h l o r i d e and n i t r a t e i t s l o s s i sa b o u t 0 .4 mpy (1 0 ~ m / y ) ,w h i l e i n p h o s p h a t e and s u l p h a t e s o l u t i o n sl o s s e s a r e only 0 .2 mpy (5 ~ m / y ) . T h i s i s a l s o summarized i n T able 2 .

PETROLEUM ENVIRONMENTS

C o r r o s i o n i n p e t r o l e u m p roduc t i on env i ronments i s u s u a l l y complex,c o n s i s t i n g o f s e v e r a l c o r r o s i v e s , w i t h t h e a d d i t i v e e f f e c t o ft e m p e r a t u r e , p r e s s u r e , v e l o c i t y , and a b r a s i v e s . The most commonc o r r o d e n t s i n t h e o i l f i e l d a r e ' s a l t wate r, carbon d i o x i d e andhydrogen s u l f i d e , a l t h o u g oxygen, n i t r o g e n and s u l f u r compounds,and o rg a n i c and i n o rg a n i c a c i d s may a l s o be p r e s e n t .

- '"""-.

E l e c t r o l e s s n i c k e l c o a t i n g s have l ong been used i n t h e p e ~ r o l e u m

i n d u s t r y t o m i t i g a t e c o r r o s i o n and e r o s i o n . The r e s u l t s . o f ar e c e n t s t u d y t o c h a r a c t e r i z e t h e per fo rmance o f e l e c t r o l e s sn i c k e l i n o i l f i e l d env i ronments a r e summarized i n F i g u r e s 7 and 8 .In t h e s e t e s t s b r in e c o nc e nt ra t i on s from 0 t o 10 p e r c e n t TDS, andv ar i o us a c id ~ a s c o n d i t i o n s , were s t u d i e d a t t e m p e r a t u r e s up t o3500F (1800C) .


8/13


9/13

- - - - - - - - - - ~

FIGURE 8

Canparison of the corrosionof carbon s t e e l and e lec t ro l e s sn icke l in HZS conta in ing br ines

a t 95C

25

a:w>a: 20wa..

'".J

-

2

; - EN.H 2S and CO 2+H2S,I no loss _ . . . . . _ . ' I , _

o 4 6 8 10SALT CONTENT, WT. PERCENT

S i n c e mos t p r o d u c t i o n e n v i r o n m e n t s c o n t a i n a t l e a s t a t r a c e o fHzS, : c o a t i n g s p r o v i d e a l m o s t c o m p l e t e p r o t e c ti o n a g a i n s to i l f l e l d c o r r o s i o n . In one s o u r g a s sy s tem i n t h e Midd le E a s t ,c o a t e d chokes and v a l v e s hav e b e e n used f o r 8 t o 10 y e a r s a t2000F (950C) and v e l o c i t i e s up t o 50 f p s (15 mps) w i t h o u t anym e a s u r a b l e c o r r o s i o n . P r e v i o u s l y used s t e e l components e x p e r i e n c e dl o s s e s o f 80 t o 120 mpy (2 t o 3 rnm/y) 7 .


10/13

ORGANIC ACIDS

Ty p i c a l l y o rg a n i c a c i d s a r e weak and n o n - o x i d i z i n g . They normal lybecome weaker a s t h e i r m o l e c u l a r w e i g h t i n c r e a s e s and t h e ca rbonc h a i n becomes l o n g e r . Like ammonia, most o rg a n i c a c i d s have goodcomplex ing a b i l i t y w i t h n i c k e l i o n s and w a t e r ; t h u s , t h e l o s s o f

e l e c t r o l e s s n i c k e l i s a c c e l e r a t e d by i n c r e a s i n g w a t e r c o n t e n t .e l e c t r o l e s s n i c k e l c o a t i n g s have e x ce l l en t r e s is ta n c e t o a l lt y pe s o f c o nc e nt r a t e d o rg a n i c a c i d s . In d i l u te d s o l u t i o n s , however,a t t a c k i s i n c r e a s e d . C o r r o s i o n i s a l s o i n c r e a s e d by a e r a t i o n ande l e v a t e d t e m p e r a t u r e . The r e s u l t s o f t e s t s w i t h d e p o s i t s i nsome a e r a t e d o rg a n i c a c i d s a r e shown i n Table 3 .

TABLE 3

CORROSION OF ELECTROLESS NICKELIN ORGANIC ACIDS

AT AMBIENT TEMPERATURE

ACID

G l a c i a l A c e t i c

10% A c e t i c

0.25% B e n z o i c

90% C a r b o l i c

5% C a r b o l i c

5% C i t r i c

C r e s o l

88% E'brmic

85% L a c t i c

O l e i c

10% O x a l i c

HIGH TEMPERATURE ENVIRONMENTS

CORROSION RATE

)lm/y mpy

0 . 8 0 . 0 3

25 1 . 0

10 0 . 4

O. 2 0 . 0 1

5 o . 2

2 0 . 0 7

o . 2 0 . 0 113 0 . 5

1 0 . 0 5

n i l n i l

3 0 . 1

(

"

The m e l t i n g p o i n t o f e l e c t r o l e s s n i c k e l i s 1630 0F (8900C).A c c o r d i n g l y, i t s u s e f u l n e s s a t e l e v a t e d t e m p e r a t u r e s i s l i m i t e d .Like o t h e r h igh n i c k e l a l l o y s , t h e d e p o s i t i s v e r y r e s i s t a n t t on i t r i d i n g and i s somet imes used a s a maskant f o r commercia ls u r f a c e h a r n e n i n g t r e a t m e n t s . In o x i d i z i n g f l u e g a s e s , a i r o rs t eam, e l e c t r o l e s s n i c k e l c o a t i n g s e x p e r i e n c e l i t t l e a t t a c ka t ~ e m p e r a t u r e sup t o t h e i r m e l t i n g p o i n t . In s u l f u r c o n t a i n i n ge n v l r o n m e n t s , however, such a s r e d u c i n g f l u e g a s e s and somer e f i n e r y p ro ce ss s tre am s , s u l f i d a t i o n o f t h e c o a t i n g may o c c u ra t t e m p e r a t u r e s above a b o u t 5250F (2750C).


11/13

SALTS

depos i t s have exce l l en t re s i s t ance to n e u t r a l and a l k a l i n es a l t s , as wel l as most ac id s a l t s . Solu t ions of such compoundsas sodium and potassium ch lo r ides , s u l f a t e s , phosphates andcarbonates have l i t t l e e f f e c t on coa t ings . Accordingly,the depos i t has been w id ely u sed in chemica l envi ronmen ts and

fo r food , pharmaceut ica l and medical app l i ca t ions . The l o s s ofElec t ro les s Nickel in some s a l t so lu t ions i s summarizedin Table 4.

TABLE 4

CORROSION OF ELECTROLESS NICKEL

IN SALT SOLUTIONSAT AMBIENT TEMPERATURE

SALT CORROSION RATE

)lm/y ~27% A1 2(S04)3 5 0 . 2

26% BaC1 2 0 . 2 0 . 0 1

42% CaC1 2 O. 2 0 . 0 15% CUC1

225 1 . 0

5% CUS0 418 O. 7

1% FeC1 320 0 8 .0

25% KC1 n i l n i l

28 % KZC0 3

O. Z 0 .01

35% MgC1 Z Z 0 . 1

Z6% NaC1 O. Z 0 . 0 1

18% Na ZC0 31 0 . 0 5

47% NaN0 3n i l n i l

46% NaH ZP0 43 0 .1

14% NaZS n i l n i l

31% Na ZS0 40 . 8 0 .03

36% Pb (N0 3) Z O. Z 0 . 0 1

80% ZnC1 Z 70 . 3

Acid s a l t s , such as magnesium ch lo r ide .s u l f a t e , cause increased a t t ack ofdepos i t , hosever, has use fu l r e s i s t a n c eusua l ly provides s a t i s f a c t o r y se rv ice .

zinc ch lo r ide and aluminumElec t ro les s Nickel . The

to these compounds and


12/13

O x i d i z i n g h a l i d e s a l t s , such a s c u p r i c , m e r c u r i c , and p a r t i c u l a r l yf e r r i c c h l o r i d e , c a u s e s e v e r e a t t a c k o f E l e c t r o l e s s N i c k e l d e p o s i t sand sho u ld b e a v o id e d . O x i d i z i n g s o l u t i o n s c o n t a i n i n g c h l o r i n e ,such a s sodium h y p o c h l o r i t e , a l s o c a u s e r a p i d damage of th e c o a t i n g .

STRESS CORROSION CRACKING

On e i n t r i g u i n g p r o p e r t y of E l e c t r o l e s s N i c k e l c o a t i n g s i s t h e i ra b i l i t y to r e t a r d s t r e s s c o r r o s i o n c r a c k i n g o f th e u n d e r l y i n gs u b s t r a t e . They can p r e v e n t c r a c k i n g of a m e t a l i n env i ronment st h a t would n or ma lly c au se r a p i d f a i l u r e . F or i n s t a n c e , i n ones e r i e s o f t e s t s i n b o i l i n g 42 p e r c e n t magnesium c h l o r i d e , sampleso f l8C r-9N i s t a i n l e s s s t e e l c oa te d w ith 0 .4 m il (10 )lm) of h e a tt r e a t e d , E l e c t r o l e s s N i c k e l were found to be u n a f f e c t e d a f t e r 1900h o u rs o f e x p o s u r e , w h il e u nc oa te d specimens f a i l e d i n 6 to 8 h o u r s s

c o a t i n g s have a l s o been found t o be q u i t e e f f e c t i v e i np r e v e n t i n g c a u s t i c c r a c k in g o f p l a i n and low a l l o y s t e e l , p a r t i c u -

l a r l y i n stearn and c o n de n sa te e n vi ro n m en ts . Some r e c e n t t e s t simply t h a t t h e s e c o a t i n g s may a l s o be b e n e f i c i a l i n r e d u c i n gs u l f i d e c ra c k in g o f h i g h s t r e n g t h s t e e l s i n p e t r o l e u m env i ronments .

T h e . e f f e c t o f E l e c t r o l e s s N i c k e l c o a t i n g s on s t r e s s c o r r o s i o nc r a c k i n g a p p e a r s to be due to tw o s e p a r a t e f a c t o r s . F i r s t , th ec o a t i n g a c t s a s a b a r r i e r be tween th e p a r t and th e c r a c k i n genv i ronment . Second, and more im po rta nt , th e d e p o s i t a p p a r e n t l yc a u s e s a s h i f t i n th e c o r r o s i o n p o t e n t i a l o f th e s u b s t r a t e fromth e s t r e s s c o r r o s i o n r e g i o n i n t o a p o t e n t i a l r a n g e where c r a c k i n gi s n o t p o s s i b l e .

EFFECT OF HEAT TREATMENT

On e o f th e mos t i m p o r t a n t v a ri ab le s e f fe c ti n g th e c o r r o s i o n ofE l e c t r o l e s s N i c k e l i s h e a t t r e a t m e n t . As d e p o s i t s a r eh e a t e d to t e m p e r a t u r e s above a b o u t SOooF (260 0 C ) , s t r u c t u r a lchanges b e g in to occur w i t h i n th e a l l o y . F i r s t c o h e r e n t and t hend i s t i n c t p a r t i c l e s o f n i c k e l phosph ide (Ni3P) form t h r o u g h o u t th ec o a t i n g . Then a t t e m p e r a t u r e s above 650F (340 0 C ) , E l e c t r o l e s sN i c k e l b e g i n s t o c r y s t a l l i z e and to l o s e i t s amorphous c h a r a c t e r .A t h i g h e r t e mp era tu re s th e p a r t i c l e s c o n g l o m e r a t e , fo rming am a t r i x o f Ni3P, Not only do t h e s e chan ges hav e a d r a m a t i c e f f e c ton th e h ard ne s s of th e c o a t i n g , b u t t hey can a l s o c a u s e a s e v e r er e d u c t i o n i n c o r r o s i o n r e s i s t a n c e .

As th e n i c k e l phosph ide p a r t i c l e s form w i t h i n th e c o a t i n g , t heyr e d u c e th e phosphorus c o n t e n t o f th e r emain ing m a t e r i a l . Thisr e d u c e s i t s p a s s i v i t y and i n c r e a s e s i t s c o r r o s i o n . Th e p a r t i c l e sa l s o c r e a t e s m a l l a c t i v e / p a s s i v e c o r r o s i o n c e l l s , f u r t h e r c o n t r i -b u t i n g to th e d e p o s i t ' s d e s t r u c t i o n . A secondary e f f e c t of h e a tt r e a t i n g i s t h a t th e d ep o s i t s h r i n k s a s i t h a r d e n s , o f t e n r e s u l t i n gi n c r a c k s t h rough th e c o a t i n g , which can expose th e s u b s t r a t e toa t t a c k .


13/13

- - - - - - - - - - - - - - - - - - - - - - - - - - - - -

TABLE 5THE EFFECT OF HEAT TREATMENTS

ON THE CORROSION OF ELECTROLESS NICKELIN 10% HCl

HEAT TREATMENT

None

375F (190C) f o r 1 ~ h o u r s

550F (290C) f o r 6 h o u r s

550F (290C) f o r 10 h o u r s

650F (340C) f o r 4 h o u r s

750F (400C) f o r 1 h o u r

DEPOSIT CORROSION RATEHARDNESS,

VHN llm/y ~480 15 0 .6

500 20 0 . 8

90 0 1900 74

97 0 1400 56

970 900 34

1050 1200 49

The e f f e c t of these changes i s i l l u s t r a t e d by the t e s t r e s u l t s inTable 5. For these t e s t s , specimens of the depos i t werehea t t r e a t e d to r e p r e s e n t d i f f e r e n t commercial t rea tments and thenexposed to 10 percen t HCl a t ambient temperature. Baking a t 375F(190C), l i k e t h a t used fo r hydrogen embr i t t lement r e l i e f , causedno s i g n i f i c a n t i nc rease i n cor ros ion . Hardening, however, causedthe d e p o s i t ' s cor ros ion r a t e to i nc rease from 0.6 mpy (15 llm/y) tomore than 30 mpy (750 llm/y) or by 50 t imes. Tests i n o the renvironments showed a s imi l a r red uc tio n in r e s i s t a n c e a f t e rhardening. Where cor ros ion r e s i s t a n c e i s necessa ry, i t i simpor tan t t h a t hardened coa t ings no t be used 6.

CONCLUSION

E l e c t r o l e s s Nickel i s commonly used fo r a number of d i f f e r e n t ,b u t of t en complimentary reasons . In d i f f e r e n t app l i ca t ions , t h i scoa t ing can reduce cor ros ion and e ros ion , reduce f r i c t i o n and wearand avoid s t r e s s cracking . This can r ed uc e i nv es tm en t and mainten-ance cos t s and improve equipment r e l i a b i l i t y and l i f e . Because ofi t s unique combinat ion of p r o p e r t i e s , E l e c t r o l e s s Nickel hasproved to be va luab le fo r i ndus t ry, and has been f r equen t ly used tor ep lace more expensive a l l o y s .

corrosion_resistance of electroless nickel coatings

Documents