the effect of corrosion on the wear rate of steel pipelines conveying backfill slurry

8/15/2019 The effect of corrosion on the wear rate of steel pipelines conveying backfill slurry

1/9

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~

Synopsis

The w ear qf steel

p ip elin es u sed fo r the

tra nsp orta tio n q f

b ac /Y ill s lu rr ie s i s

freq ue ntlY reg arde d a s

an ero sio n p rob le m o nlY .

A n i nves ti gat ion

w as undertaken qf the

lik elY c on trib utio n to th e

to ta l w ea r ra te m ad e

by co rro sio n. U se w as

made

qf

a pipejl ow loop

w i th cathodical{ y

p rote cted sp ecim en s a nd

corr osi on i nhi bit ors .

L a bo ra to ry e le ct ro -

c he mic al te sts u til iz in g

a rotat ing-cylinder

e le ctro de w ere a lso

c on du cte d. T he s lu rr ie s

u se d w er e c la ss jfte d

ta iling sfro m tw o m in es

at a relative density qf

1,1. T he v elo citie s w ere

between 2 an d 4,9 m/s.

su bstan tia l de-

creases in the w ear rate

qfup to 80 per cent for

one slurry and up to 67

per centfor the other

w ere obtained w ith the

u se q f c orro sio n c on tro l

sh ow in g th at c orro sio n

and m ore i m por tantl Y

i nt er ac ti on b e tw e en

e ro si on a nd c or ro sio n

c o nt ri bu te a s ig n jf tc a nt

p ortio n qfthe o vera ll

w ea r in s te el p ip elin es .

*

A nd rew B uch an

Assoaates Roodepoort

Transvaal.

t

A ng lo A m er ic an

C o rp o ra ti on q fS o ut h

A fiic a L td P . O. Bo x

61587, Marshalltown

2 10 7 T ra ns va al.

@

T he S ou th A fr ica n

Institute qf M ining and

Metallurgy 1994. SA

I SS N 0 0 38 - 22 3 X /3 .0 0

+ 0 .0 0. P ap er s ub mitte d

Sep. 1993; revised

paper received D ec. 1994.

he effect o f c o r r o s io n on the

we r r te o f stee l p ipelines

c o n v e y in g c k f i l l s lu r r y

by A .lJ. B uchan* and A .lLS. S pearingt

Introduction

T h e w e a r

of

p ip e l i n e s t r a n s p o rt i n g b a c k f i l l

s l u r r i e s i s o f t e n s e e n t o b e o n l y a n e r o s i o n

p r o b l e m . T h e r o l e p l a y e d b y c o r r o s i o n i s

f r e q u e n t l y n o t u n d e r s t o o d a n d o ft e n i g n o r e d .

S e v e r a l a u t h o r s h a v e e x a m i n e d t h e c o r r o s i o n

c o n t r i b u t i o n t o w e a r r a te s i n p ip e s t r a n s p o r t i n g

s l u rr i e s , f i n d i n g t h a t c o r r o s i o n c o n s t i t u t e s a

s i g n i f i c a n t p a r t o f t h e t o t a l w e a r r a t e .

E c o n o m i c m e t h o d s f o r c o n t r o ll i n g t h e m a s s

l o s s f r o m p i p i n g i n b a c k f i l l s y s t e m s d e p e n d o n a

d e t e rm i n a t i o n o f t h e r e la ti v e e ff e c t s o f t h e

d if f e r e n t c o m p o n e n ts of t o ta l w e a r . I n s lu rr y

p ip e l i n e s , t h e r e a re t h re e p o s s i b le m o d e s of

m a te ri a l l o s s f r o m t h e p ip e w a l l : e ro s i o n ,

c o r r o s i o n , a n d a s y n e r g y b e t w e e n w e a r a n d

c o r r o s i o n e r o s io n - c o r r o s io n ) .

C o r r o s i o n a l o n e c a n b e m e a s u r e d f a i r l y e a s i l y

b y t h e u s e o f e l e c t r o c h e m ic a l m e t h o d s , o r o f

m a s s - l o s s m e t h o d s u s i n g c o u p o n s o r e v e n t e s t

p ip e s i n s e rt e d i n w a t e r p ip e l i n e s . T h e q u e s t i o n

a r i s e s a s t o w h a t p o r t i o n o f t h e t o t a l w e a r r a t e

i n a s l u r r y p i p e i s d u e t o c o r r o s i o n , e i t h e r a s

c o r r o s i o n a l o n e o r a s s o m e s y n e r g y b e t w e e n

w e a r a n d c o r r o s i o n . M a d s e n

1

d e f i n e s t h is

s y n e r g y a s e x i s t i n g w h e n t h e w e a r r a t e i s g r e a t e r

t h a n t h e s u m o f t h e e r o s i o n a n d t h e c o r r o s i o n

c o m p o n e n t s a c t i n g i n d i v i d u a l l y . T h i s m a y b e d u e

t o t h e r e m o v a l

of

c o r r o s i o n p r o d u c t s f r o m t h e

s u r f a c e o f t h e c o r r o d i n g m e t a l , t h u s p r o v i d i n g a

c l e a n , u n p r o t e c t e d s u r f a c e t h a t a l l o w s t h e

m a x i m u m r a t e of c o r r o s i o n , o r t o t u r b u l e n c e i n a

s l u rr y , i n c r e a s i n g t h e m a s s t r a n s p o r t of o x y g e n

t o t h e c o r r o d i n g s u r f a c e . C o r r o s i o n p ro d u c t s o n

s t e e l p i p i n g r u s t ) m a y b e e a s i e r t o r e m o v e

t h r o u g h a b r a s i v e a c t i o n t h a n t h e s t e e l i t s e lf .

L u i a n d H o e y 2 s t a t e t h a t , w h e n t h e w e a r r a t e

d e p e n d s o n l y o n e r o s i o n , t h e r a t e of s u r f a c e

r e m o v a l i s g o v e r n e d b y t h e c o n d i t i o n s of f l u i d

f l o w , t h e p r o p e r t i e s o f t h e s o l i d p a r t i c l e s a n d t h e

m a te ri a l s u rf a c e , a n d t h e m e c h a n is m

of

m a t e r i a l

r e m o v a l . T h e a d d it i o n

of

c o r r o s i o n , h o w e v e r ,

a d d s f u r t h e r v a r i a b le s t h a t a f f e c t t h e r a t e

of

m a t e r i a l l o s s , t h e s e b e i n g t h e n a t u r e a n d

p r o p e r t y o f t h e c o r r o s i o n p r o d u c t s f o r m e d a n d

t h e r a t e a t w h i c h t h e y a r e f o r m e d . T h i s r a t e

d e p e n d s v e r y m u c h o n t h e c o r r o s i v i t y o f t h e

l i q u i d m e d i u m o f t h e s l u r r y .

T he J ou rn al o f T he S ou th fric an n stitu te o f M in in g a nd M eta llu rg y

I

P o s t l e t h w a i t e

e t a l

f o u n d t h a t , i n s l u rr i e s

m a d e u p o f s i l i c a s a n d 2 0 p e r c e n t b y v o l u m e ) ,

I

c o r r o s i o n i s t h e d o m in a n t m o d e of m e t a l l o ss .

T h e y c o n c l u d e d t h a t e ro s i o n p r e v e n t s t h e

f o r m a t i o n o f a c o m p l e t e f i l m

of

r u s t t h a t s ti f l e s

o r, a t l e a s t , s l o w s d o w n t h e c o r r o s i o n r e a c t i o n .

P o s t l e t h w a i t e a n d H a w ry l a k 4 f o u n d t h a t , e v e n

w h e n t h e s o l i d p a r t i c l e s i n a s l u r r y w e r e s o f t e r

t h a n s t e e l, t h e y w e r e a b l e t o a c c e l e r a t e t h e

c o r r o s i o n p r o c e s s .

E x p e r i m e n t a l M e t h o d

I n a n e x a m i n a t i o n o f t h e c o n t r i b u t i o n t o t h e t o t a l

w e a r r a t e m a d e b y c o r r o s i o n , u s e w a s m a d e o f a

p ip e f l o w lo o p , c a t h o d ic p ro te c t i o n , c o r r o s i o n

i n h i b it o rs , a n d e le c t r o c h e m i c a l c o r r o s i o n t e s t i n g .

T w o s l u r r i e s w e r e u s e d i n t h e t e s t s , b o t h a t a

s o li d s c o n c e n t r a t i o n o f 4 2 p e r c e n t

by

v o l u m e ,

d e t a i l s of w h i c h a r e g i v e n i n T a b l e I a n d

F i g u r e s 1 a nd 2.

T a bl e I

Param eters of the test slurries

Vaal

R e e f s V R )

s l u r r y

9 ,4

4 0 7

8 5

1 2 0

1 5 4 5

7 0

1 6 1 5

3 6 1 5

< 1 0

1 9 5

2200

3 6 5

F E B R U A R Y 99 4

37


2/9

ell

C

80..

..

..

Co

..

60

el l

..

C

..

~4 0

..

IL

20

0

1

10 0

el l

C

..

80

..

..

Co

..

60

ll

..

C

..

~4 0

..

IL

20

0

1

E ffect of corrosion on w ear rate

12 0

10 0

10

Particle size ,.vm

10 0

1000

12 0

F ig ure 1 -P artic le -siz e d istrib utio n o f th e slurry fro m W este rn D ee p L ev els

10

Particle size ,.vm

10 0

1000

F ig ure 2 -P artic le -siz e d istrib utio n o f th e slurry fro m V iiI R ee fs S ou th

Acknowledgements

The authors thank M r NHs

Steward for his advice and

assistance in the perform ance

of the pipeloop tests;

Dr P.V . Scheers of Mintek for

the electrochem ical testing;

and The Gold and Uranium

Division of Anglo American

Corporation for permission to

publish this paper.

References

1 . M AD SE N,B .W .

M easurem ent of

erosion-corrosion

synergism w ith a slurry

w ea r t es t a pp ar atu s.

Wear

if

materials

Ludem a, R .C . ed. . N ew

Y ork, A SM E, 1987.

~

ow oop Tests

A p ip e l o o p c o m p le te w i t h

a

v a r i a b l e - s p e e d

p u m p , m a g n e t i c f l o w m e t e r, an d m e c h a n i c a l l y

a g i t a t e d s t o r a g e t a n k w a s u s e d i n t h e t e s t s

F i g u r e 3 ) , a s d e s c r i b e d p r e v i o u s l y b y S te w a r d

a n d S p e a ri n g S ,

T h e w e a r s p e c i m e n s w e r e m a d e f r o m

S c h e d u l e 4 0 I D 5 2 , 5 m m ) A S T M A I 0 6 G r a d e B

s e a m l e s s t u b e o f 5 0 m m n o m in a l b o r e , w i t h a

n o m in a l c o m p o s i t i o n o f 0 , 3 0 p e r c e n t C , 0 ,2 9 t o

1 , 0 6 p e r c e n t M n , 0 , 1 0 p e r c e n t m in . ) S i, 0 , 0 4 8

p e r c e n t m a x . ) P , a n d 0 , 0 5 8 p e r c e n t m a x , ) S .

T h e h a rd n e s s

as

m e a s u r e d w a s 1 5 2 V ic k e r s

h a r d n e s s v a l u e H v ) . L e a d - i n a n d l e a d - o u t

l e n g t h s o f 5 0 0 m m w e r e p r o v i d e d t o s t r a i g h t e n

t h e f l o w . T h e a r r a n g e m e n t o f t h e t e s t s e c t i o n s i s

s h o w n i n F ig u r e 4.

F E B R U A R Y 1 9 9 4

T h e p l a i n - w e a r s p e c i m e n s w e r e 2 0 0 m m

l e n g th s o f t u b e , w h i l e t h e c a t h o d ic a ll y p ro te c t e d

s p e c i m e n s c o n s i s t e d o f a 2 0 0 m m l o n g t u b e

s e c t i o n e d l o n g it u d i n a l l y i n h a l f . T h e t w o h a l v e s

w e r e i n s u l a t e d f r o m e a c h o t h e r , w it h o n e h a l f

f o r m in g a c a t h o d e a n d t h e o t h e r

an

a n o d e . T h e

c a t h o d e w a s d r i l l e d t o r e c e i v e a 3 m m p o r o u s

t a p p in g f o r a c o n n e c t i o n t o a r e fe re n c e e le c t r o d e ,

T h e a n o d e -- c a th o d e a s s e m b l y i s i l l u s t r a te d i n

F ig u re 5 . I n p r e p a ra ti o n f o r t h e t e s t , t h e

s p e c i m e n s w e r e a b r a d e d w it h a 2 0 O - g r i t f l e x i -

h o n e a n d t h e n c l e a n e d i n i n h i b it e d 1 O p e r c e n t

s u l p h u r i c a c i d a s s p e c i f ie d 6 i n A S T M

G1 - 1 9 8 1 .

T h i s e n s u r e d t h a t a l l t h e s p e c i m e n s h a d t h e s a m e

s u rf a c e f i n i s h ,

as

w e l l

as

r e m o v i n g an y s c a le o r

c o r r o s i o n p ro d u c t s . T h e s p e c im e n s w e r e s t o re d i n

a n a ir t i g h t c o n t a in e r w it h d e s i c c a n t b e f o re b e i n g

w e i g h e d a n d i n s t a ll e d i n t h e f l o w lo o p . A ft e r b e i n g

e x p o s e d , t h e s p e c i m e n s w e r e a g a in c l e a n e d w it h

i n h i b it e d a c i d i f a n y c o rr o s i o n p ro d u c t w a s v i s i b le

o n t h e i r s u rf a c e s ; i f n o t , t h e y w e r e r i n s e d , w ip e d

c l e a n , a n d d r i e d w it h w a r m a i r .

A v a r ia b l e -v o l t a g e d.c. p o w e r s u p p l y w a s

a p p l i e d a c r o s s t h e t w o a n o d e -c a th o d e p a ir s . T h e

p o t e n t i a l o n t h e c a t h o d e s u r f a c e a t t h e p o i n t

m o s t d i s t a n t f r o m t h e a n o d e w a s m e a s u r e d w it h

r e f e r e n c e t o a s a t u r a t e d c a l o m e l e le c t r o d e S C E ).

T r i a l s w e r e c o n d u c t e d w it h a r a n g e o f

d if f e re n t p o t e n t i a l s a t t h e c a t h o d e . A t a p o t e n t i a l

o f - 1 , 2 V S C E ) , n o m a s s l o s s a t a l l o c c u r r e d a t

t h e c a t h o d e . T h i s w a s d u e t o a b u i l d - u p o f s c a l e ,

w h i c h a p p e a r e d t o c o n s i s t m a i n l y o f c a r b o n a t e s

w it h s l u r r y p a r t i c l e s e m b e d d e d i n t h e s c a l e .

A ft e r f u rt h e r t r i a ls ,

a

p o t e n t i a l o f - 0 , 7 0 t o

0 7 1

V S C E ) w a s u s e d i n t h e t e s t s . T h i s

p o t e n t i a l , a l t h o u g h b e i n g s l i g h tl y a b o v e t h a t

r e c o m m e n d e d f o r u s e i n c a th o d ic p ro te c t i o n 7 .s

m in i m u m o f - 0 , 7 7 V S C E ) b y N A C E ) , w a s u s e d

f o r t h e f o l l o w in g r e a s o n s . F ir s t l y , m o r e n e g a t i v e

p o t e n t i a l s r e s u lt e d i n t h e d e p o s i t i o n o f c o p p e r

a n d s c a le o n t h e s u rf a c e s o f t h e c a th o d ic a ll y

p ro te c t e d s p e c i m e n s . S e c o n d ly , t h is v o l t a g e

w o u l d h a v e b e e n t h e l e a s t n e g a t i v e p o t e n t i a l o n

t h e s p e c i m e n s u r f a c e o w in g t o i t s c u r v a t u r e ; t h e

v o l t a g e o n o t h e r p a r t s o f t h e s p e c i m e n n e a r e r t h e

a n o d e w o u l d b e m o r e n e g a t i v e , T h e c a l c u l a te d

c u r r e n t d e n s i t i e s o n t h e c a t h o d e s u r f a c e s w e r e i n

t h e r a n g e 0 , 7 5 t o 3 m A / c m 2 .

T h e t e s t c o n d i t i o n s a r e s u m m a r i z e d i n

T a b l e H.

Th e Jo urnal of The South frican Institute of M ining and M etallurgy


3/9

E f f e c t

of corrosion on wear ra te

1 S lu rry re .e rv olr

2 Centrlfugll pump

3

Mlgne tl c f lowme te r

4 Test pipeline

5 He lt -e xchl nger .

61 BllI-vlln Iw ltch-onr

6b Byp... pipeline

6c W eigh tlnk

6d Jet Im plct nozzle

T ab le

Flowloop tes t conditions

@

F ig ure 3 -L ay ou t o f th e s lu rry flo wlo op a fte r S tewa rd a nd Spe arin g~

50 0

20 0

200

.. .. ... . J

500

Tie bar

pvc colla r

BS10 table D

5 0 N B flang e

Pipe: ASTM A108 Or. B 50NB Sch.40

F ig ure 4 -S ch ematic d ia gram o f th e p ip e te st s ec tio ns d im en sio ns in m illim etre s)

2. LUI. AW .. and HoEY. G .R .

Co rro sive a nd erosive we ar

of m eta ls in m in eral

slurr ies.

C a n. M e ta ll

vol.

1 2. n o. 2 .1 97 3.

3 . P OS TL EW A IT E. J ..

et al.

Th e

ro le of oxygen mass

transfer in the

e ro sio n-< :o rro sio n o f

s lu rr y p ip e li ne s.

Corrosion.

vo l. 42. no. 9 .

Sep.1986.

4. POSTLEWAITE, J.. and

HAWRYLARK, M.W. Effect of

slurry abras ion on the

anodic disso lu tion of iron

i n w a te r.

Corrosion

vol.

31. no. 7. Ju l. 1975.

5 . ST EW AR D. N .R .. an d

SPEAR ING.A . J .s . Combat ing

p ip el in e w ea r-a n

a dv an cin g te ch no lo gy .

j.S.

4/r. Inst. M in. M etall.

.

vol. 92. no. 6. Jun. 1992.

6. AMERICAN SooErv FOR

T Es TI NG A ND M AT ER IA L5 .

A STM S tandord G I 1981.

Stan da rd p ractic e for

prep arin g. cle an in g an d

eva lu atin g c orrosion test

spe cim ens. A STM , 1 981 .

~

Reference

electrode

tipping

\

In.ulllion

F ig ure -S ch em atic d ia gra m o f th e a no de -ca th od e

assembly

More lim ite d te stin g w as a lso p erfo rm ed in

w hich the sam e tw o slu rrie s w ere u se d w ith th e

a dd itio n o f a c orro sio n in hib ito r. T he in hib ito r

used w as N aN O2 w ith an in itial do sa ge of 2 g/l

of water, followed by 0,17 g/l of water after 1, 2,

and 3 hours of pum ping the slurry. W ork

c on du cte d b y W hite 9 fo rm ed th e b as is fo r th e

s ele ctio n a nd u se o f th is in hib ito r.

T he J ou ma l o f T he S ou th A fric an In stitu te o f M in in g a nd M eta llu rg y

Value

Description

2.75 3,77 4,88

Vaal R eefs South Plant VR)

W estern D eep Levels W est

P la nt W DL )

1.7

4

22 to 27

Table 11/

C ond ition s fo r the electro ch em ical te sts

~I

Value

Description

1433 2168 2913

eq uivalen t to 2 . 3. a nd 4 rn /s

with sluny in a pipe of ID 52 m m

an d 22 79 e qu iva le nt to 3 rn /s

w ith w ate r in th e sa me

pipe)

VR and W DL

1,7

22

Unear po la r iza ti on

r es is ta nc e LPR ) a nd Taf el

e xt ra po la tio n o ve r t he r an ge

E~ :t

200 m V

Nitrogen

2 g II N aN O,

le c t r o c h e m ic a l e s t s

E le c t r o c h e m ic a l t e s t s w e re c o nd u c t e d w it h a

r o ta tin g - c y l in d e r e le c t r o d e R C E m a ch in e d f r o m

ASTMA106 Gr.B pipe.The cylinderw a s 2 0 m m

in d ia m e te r a n d 1 0 m m in h e ig h t .

T h e c o m p o s it io n o f th e pipe w as 0 ,1 3 p e r

c e n t C , 0 , 2 3 p e r c e n t S i , 0 ,9 6 p e r c e n t Mn 0 , Q 1 7

per

c e n t S , a n d 0 , 0 1 2 p e r c e n t

P.

T e s ts w e re

c o n d u c t e d b o t h in t h e s lu r r ie s a n d in th e w a te r

u s e d fo r th e m a k e -u p o f th e s lu r r ie s . T h e s p e e d

o f t h e e le c t r o d e w as c a lc u la t e d b y u s e o f a

c o r r e la tio n d e te rm i n ed a t M in te k , which is b a s ed

o n t h e a c h ie v e m e n t o f e q u a l s h e a r s t r e s s e s a t t h e

s u r f a c e o f a p ip e a n d th e R C E . T h is in c lu d e d

s u c h fa c t o r s a s v is c o s i t y a n d p ip e d ia m e te r to

p r o v id e a n e q u iv a le n t s e t o f s p e e d s u s e d in t h e

e le c t r o c h em ic a l te s t s t o th re e p ip e f lo w v e lo c i t ie s .

F E B R U A R Y 1994

39

....


4/9

Effect of corrosion on wear rate

Figure 7-C orrosion pits in a plain specim en V R slurry 2,75 m /sI

7.

NACEStflndordRP OI 69.

e o m m e n d e d practice,

co ntro l o f ex tern al

co rro sio n o n u nd erg ro un d

or sub merg ed m etallic

piping systems. 1983

R evision , N AC E, H ouston,

1983.

8. GU MM OW ,R.A. Ca thod ic

p rot e ct ion c r it er ia - a

c ri ti ca l r ev ie w o f NACE

S ta nd ar d RP -D I -- 69 .

Ma ter ia l s Pe rfo nn a nc e

S ep . 1 98 6.

9. W HIT E,R .T . A n

a ss es sm en t o f a va il ab le

c or ro si on i nh ib it or s f or

th e r eticu la tio n s ys te ms o f

S ou th A fr ica n g old m in es.

Mintek Report n o. M 1 80 .

R an db ur g S ou th A fric a

1985.

The te st c ond ition s are giv en in T ab le Ill. T he

te sts w ere c on du cte d w ith a n e le ctro ch em ic al c ell

con sis ti ng o f t he RCE, th ree car bon counte r-

e le ctro de s, a nd an SCEco nn ec te d v ia a Lug gin

p ro be . Mech an ic al a gita tio n was u se d to

ma in ta in th e s lu rr y in suspens ion.

T ests of m ass loss w ere co ndu cte d o ver 14 0

h ou rs o n th e same R CE i n a gita te d co ntaine rs o f

slurry.

s u l t s

ip Tests

A fte r th e spe cim ens h ad b ee n e xp ose d to th e

s lu rr y, a v is ib le d if fe rence was appar en t b etween

th e ca th ode s, th e an ode s, a nd th e p la in

specimens.

~ 40

F EB RU A RY 1 99 4

The c ath od es were c le ar o f c orro sio n

pro du cts an d, a t th e high er ve lo citie s, h ad a

polis hed appea rance. The sur face s ti ll s howed

man y o f th e o rig in al ma rk in gs . The ir s urfa ce s

F ig ure 6 we re e ss en tia lly th e s ame in

appea rance a s thos e obser ved by Po stle thwai te

e t a l. 3 in th eir w ork, viz a smoo th rip pled

appearance.

T he p la in sp ec im en s te nde d to h ave a

ro ug he r s urfa ce w ith n ume ro us p its . T he se p its

con ta in ed cor ro sion p roducts . F igur e 7 shows

corro sio n p its o n th e s urfa ce o f a n u np ro te cte d

sp ec imen VR s lu rry 2 ,8 m /s . S ome p olis hin g

mark s were s till v is ib le in th e d ire ctio n p erp en -

d ic ula r to th e d ire ctio n o f flow. A t th e lowe r

sp ee ds a nd o n sp ec im en s u sed o nly o nc e, th e

p olis hin g ma rk s were s till v isib le while , a t th e

h ighe r f low vel oc it ie s, th e polis hing mark s h ad

a ll b ee n remov ed , e ve n o n th e c ath od ic ally

p ro te cte d s pe cimens . F ig ure 8 s hows th e in te rio r

o f a ty pic al p it o n th e p la in s pe cimens . F urth er

e xp os ure o f th e p la in s pe cimens a t h ig he r

ve lo citie s re sulte d in a surfa ce a s sh ow n in

Figure 9, again w ith large num bers of pits and a

p olis he d s urfa ce e xp os ed th re e time s, th e la st

tim e being W DL s lurry at 4,88 m /s . N o

d iffe re nc e in ma ss lo ss was fo un d b etw een

n ew ly pre pa red spe cim en s a nd tho se th at h ad

be en re -use d, L e. n o e ffe ct o f surfa ce fin ish w as

o bs erv ed o ve r th e p erio d o f th e te sts .

T he a nod es w ere c ov ered w ith o ra ng e to

b rown corro sio n p ro du cts , a nd th ere was

ev idence o f p it ti ng , a lt hough genera l d is so lu ti on

o f the surfac e w as p erha ps more usu al. a lo ng

w ith th e fo rmatio n o f lo ng itu din al g ro ov es in th e

d ire ctio n o f flow. The g ro ov es a pp ea re d to b e

m ain ly d ue to th e lin king -u p o f pits th at h ad

exte nd ed in th e d ire ctio n o f flow. Wh at is p artic -

u la rly in te re stin g is th at th es e g ro ov es h ad th e

same g ene ra l ap pe ara nc e a s th at see n in v ertic al

b ac kfill sh aft p ip in g, a lb eit o n a sma lle r s ca le .

T he im pressed current at the anode has the

eff ec t o f acce lera ti ng co rros ion . The observa ti ons

o f th e a no de s w ere on ly o f a q ualita tiv e na tu re

b ec au se o f th e u nre alis tic s itu atio n a ris in g from

th e a pp lie d c urre nt. Howev er, th e fa ct th at

a ccel er ated cor ro si on p roduces a s im ila r sur face

to th at o bse rv ed in ve rtica l pipin g th at ha s b een

exp os ed to b ac kfill s lu rr ie s o ve r a n e xte nd ed

p erio d m ay b e an in dica tion of the e ffe ct th at

c orro sio n p la ys in th e ty pic al g ro ov in g s ee n in

shaf t p ip ing .

T he J ou rn al o f T he S ou th fric an n stitu te o f M rlln g a nd M eta llu rg y


5/9

ffe t of corrosion on wear rate

Figure 9-Pits and worn surface in a plain specim en (VR s lurry 3,77 m /s and

W DL slurry 2,77 m /s and 4,88 m /sI

14

12

. VRtotal wear

*

VRcath . prot .

10

.. VR E. chem

~

..

CD

>-

E

E

-

8

6

10

110

0

...I

,

*

---

..

4

2

0

0

0,5 1,0 1,5 2,0 2,5 3,0

Velocity [m /s]

3,5 4,0 4,5

F ig ure 1 O-W ear rates in V aal R eefs slu rry

T he J ou rn al o f T he S ou th fric an n stitu te o f M in in g a nd M eta llu rg y

Figure 10 shows the loss in thickness for the

plain and the cathodically protected specim ens in

the V R slurry extrapolated back to no flow .

Thickness loss due to corrosion w as obtained by

Tafel extrapolation using an R CE (denoted V R. E.

chem on the figure) for a speed of 3,0 m/s in clear

slurry make-up water. As can be seen, the

ca th od ic ally pro tecte d sp ec im en s (VR . cath. p ro t.

in the figure), have a substantially low er m ass

loss. The value obtained for 3,77 m /s cathodically

protected specim ens appears to be low since it

does not follow the pattern of the other VR or

WDL results. Excluding this result, the effect of

cathodic protection w as to low er the w ear rate to

between 40 and 45 per cent of the wear rate

w ithout protection. This im plies that corrosion or

corrosion-erosion w as responsible for at least

55 to 60 per cent of the total mass loss in this

situation. W hen the data w ere converted to a w ear

rate per kilotonne of dry solids transported, a

linear relationship em erged (Figure 11). The

dotted lines indicate the effect of ignoring the

questionable result obtained at 3,77 m /s (denoted

V R CP m od. and VR total m od. in the figure).

The results obtained w ith WDL slurry are

shown in Figure 12. In this case, the use of

cathodic protection caused the total w ear rate to

decrease to a value of between 67 and 86 per

cent of the total loss in mass. This effect is not as

m arked as w ith the V R results. There is concern,

how ever, that the cathodic protection w as not as

effective at the potential used as it w as in the VR

tests, since the W DL water was found to be

approxim ately 40 per cent m ore corrosive in the

electrochem ical tests using an RCE in clear w ater

fi lte re d f rom th e s lu rry . ( Th e e le ctr oc hem ic al

corrosion tests in slurry are indicated as W DL E.

chem . in slurry and are discussed later.) A reas of

corrosion pitting w ere also found on the

specim ens when under exam ination with a

scanning electron m icroscope (SEM). W hen the

data w ere converted into m illim etres of w ear per

kilotonne of dry solids, a straight line w as

produced (Figure 13). C orrosion data obtained

from the electrochem ical tests in slurry w ere also

converted and w ere then superim posed on these

results (denoted E. chem corr.).

The lead-in and lead-out pieces w ere

weighed before the beginning of the tests, and

were exposed in all the tests. In betw een tests,

they w ere left partially w et, as w ould happen

w ith backfill piping betw een cycles of slurry

pum ping. A s expected, a loosely adherent layer

of orange-brow n corrosion product form ed

during this period. This layer w as rem oved

during each period of slurry pum ping. A

com parison of the total m ass losses from the

individual tests and the m ass loss from the lead-

in and lead-out lengths is given in Table IV. This

shows that there was an apparent increase of 45

per cent in the total mass loss. This was

probably due to the corrosion occurring during

the 20 hours betw een cycles of slurry pum ping.

F E R U R Y

1994 41

~


6/9

0,03

0

0

0

.. .

E

0,02

E

-

.

..

0

.J

0,01

3

~. .

..

>-

E

2

E

-

.

..

0

.J

0,010

0,009

0,008

0,007

0

0

0,008

...

E

0,005

E

..

0,004

..

0

.J

0,003

0,001

0

0

E ffect of corrosion on wear rate

0,04

V R 101.1 1 088

--

VR CP mod.

*

V R C P 1088

'

.. E.ch.m corr.

. OR ...

m .~

.

---

----

----

---

--

---

'It---

*

0

0

3,00

Velocity [m /sI

3,77,75

Figure 11-W ear rates in V aal R eefs slurry expressed in term s of tonnes of dry solids

4

0

0

I

,

,

,

,

,

,

,

,

,,

,'

'

,'

W DL E.

'

ch em In slurry

,'

./

...~

'

*...........

.~: ..~......................

; ~ '-

0,5

1,5 2,0 2,5 3,0

V elocity [m /s]

3,5 4,0,0

4, 5

F ig ure 1 2-D ata for WDL slu rry

, '

,'

,'

,'

,'

.,..'

,' ~

,,' ,

,'

WDL CP re sults

, '

,'

'~

, '

...,.. , E. chem c orr.

,'

,..........

,'

,'

2,75 3,77

V elocity [m /sI

~

42

Figure 13-W DL data expressed as a function of the m ass of solids pum ped

F E R U R Y

1994

4,88

R E s t s

In the se tests, th e e ffec t on th e co rro sio n ra te o f

solids a dd ed to th e w ate r w as d ete rm in ed . T he

exp ec te d re su lt w as a n in cre as e in c orro sio n ra te

a s measur ed e lectr ochemical ly when soli d

p artic le s w ere a dd ed . Howev er, a s T ab le V

illu str ate s, th er e was, in f ac t, a sub stanti al

d ec reas e in e lectr ochemical cor ro sion r ate s a fte r

solid p artic le s h ad b ee n a dd ed to th e w ate r. T his

r esult was comp le te ly unexpec ted. The RCEte st

h ad b ee n fo un d b y M in te klO t o c orre la te w ell

w ith c orro sio n ra te s in a ctu al p ip es . Howev er,

th is c orre la tio n h ad b een d ev elo ped fo r c le ar

w ater o nly. A s the u ne xpe cte d re sult h ere m ay

b e th e s ub je ct o f fu rth er in ve stig atio n, n o

attemp t is m ad e to e xp la in it in th is p ape r.

T he effe ct o f d e-a era tio n an d th e a dditio n of

a cor ro sion i nh ib ito r (NaNOz)on th e cor ro si on

rate w ith b oth c le ar w ate r a nd slu rry w as

examin ed . Ve ry sub stanti al d ec reas es in

cor ro sion r ate we re obta in ed . The inhi bi ted

c orro sio n ra te was a pp ro ximate ly th e s ame fo r

slurry and clear w ater in both the VR a nd the

WDL slurries.

T he RCEw as the n u sed fo r the m easu remen t

of m ass losse s in slu rry , b oth w ith a nd w ith ou t

th e add itio n o f i nh ib ito r. Sub st an ti al d ec reas es

(a ppro xim ate ly 9 6 p er c en t) in th e m ass loss

w ere a ch ie ved th rou gh the u se o f in hibito r, as

shown in T ab le V I. T h is im plies th at th e m ass

lo ss is du e a lm ost so le ly to a c orrosio n

mechanism, a nd th at e ro sio n p la ys little p art in

th e p ro ce ss. T he c on clu sion d rawn from this is

th at, w hile th e RCEmay b e su itab le fo r ra pid

c orro sio n te stin g in c le ar w ate r, th e wea r

mechanisms a ss oc ia te d w ith it a re n ot e qu iv ale nt

to tho se in a p ip e tra nspo rting slu rrie s. It w as

also found that the m ass loss w as linear w ith

time , e ve n th ou gh th e s lu rry was n ot re ple nis he d

ov er a pe rio d of 1 40 ho urs. A p ure c orro sio n

situ ation is m ore lik ely to h av e g iv en a

de crea sin g ra te o ve r tim e, as fo und by

Higginson

11

in h is wo rk on minewa te rs .

Deg ra da tio n o f th e s lu rry o ve r time wou ld a ls o

be e xp ected to pro vide a d ec re ase in w ear ra te

w ith tim e. T hu s, it a pp ea rs th at th e le ve l o f

mechanic al w ea r was s uffic ie nt o nly to ma in ta in

a 'c le an' surfa ce so tha t the c orrosion rate

rema in ed con sta nt ra th er th an d ec re as in g w ith

time d ue to th e fo rmatio n o f c orro sio n p ro du cts

o n th e su rfa ce .

I n an examin atio n o f t he v ar ia tio n o f cor ro sion

r at e w ith v elo city in WDLs lu rr y, a l in ea r

relation sh ip w as fou nd b etw ee n 2 an d 4 m /s,

whic h a ga in d id n ot fo llow th e re la tio ns hip s ee n

w ith p ip e wea r.

T hat there w as som e com ponent of w ear w as

ap pare nt in th at th e spe cim ens in th e te sts

c on du cte d in s lu rry were b rig ht a nd 'p olis he d' a t

the end of the tests, w hereas in clear w ater the

s p e c i m e n s

h ad a v is ib le

c o r r o s i o n l aye ron the

surface.

88

T he J ouma l o f T he S ou th fric an n stitu te o f M in in g a nd M eta llu rg y


7/9

E ffect of corrosion on w ear rate

Table V

The m ass loss in lead-in and lead-out

pieces, and the total m ass loss in plain

spe cim ens ov er all the tests

M ass loss, g/m

Mean

=

27 1

cr

=

1,16

18,65

Table V

R es ults of the R CE e lec troch em ical tes ts

Test

3,52

0,06

0,29

0,35

0,47

0,08

Corrosion

rate

mm/year

W ater, 3 m /s

W ate r 3 m /s d e -a era te d + in hib ito r

S lu rry 2 m /s

S lu rry 3 m /s

S lu rry 4 m /s

S lu rry 3 m /s d e-a era te d + in hib ito r

Table V

Results of mass-loss tests w ith the RCE

T hic kn es s lo ss , m m/y ea r

VR WDL

5,11

0,21

5,34

0,18

Table V

In hib ito r re su lts in th e flo wlo op

Velocity

m/s

3,77

3,77

T he J ouma l o f T he S ou th fric an n stitu te o f M in in g a nd M eta llu rg y

Substantial d if f e r e n c e s w e r e s e e n o n t h e

s u rf a c e s o f t h e s p e c i m e n s e x p o s e d t o t h e d if f e re n t

c o n d i t i o n s . T h o s e e x p o s e d t o W D L s l u r r y i n t h e

m a s s - l o s s t e s t s h a d n u m e r o u s e l o n g a t e d p i t s o n

t h e s u r f a c e t h a t t e n d e d t o l i n k u p t o f o r m s e m i-

c o n t i n u o u s g r o o v e s r u n n i n g a r o u n d t h e c i r c u m -

f e r e n c e o f t h e c y l i n d e r F i g u r e 1 4 ) . T h e V R

s p e c i m e n s h a d d is c r e te p it s w it h l i m i t e d

e l o n g a t i o n i n t h e f l o w d i r e c t i o n F ig u r e 1 5 ) . T h e

s p e c i m e n s e x p o s e d t o t h e i n h i b it e d s l u rr i e s w e r e

v e r y s h i n y w it h n o m a c r o s c o p i c e v i d e n c e o f

p i t t i n g . U n d e r t h e S E M , s o m e s m a l l a r e a s w h e r e

b r e a k d o w n o f t h e s u r f a c e h a d o c c u r r e d w e r e

v i s i b l e F i g u r e 1 6 ) , a l t h o u g h t h i s m a y h a v e b e e n

d u e m o r e t o t u r b u l e n c e a t r o u g h p o i n t s o n t h e

s u rf a c e t h a n t o c o rr o s i o n a lo n e .

lo wlo op T ests in n h ib ite d S lu rry

T h e p r o c e s s e d r e s u l t s f r o m t h i s w o r k a r e g i v e n i n

T a b l e V II . T h e g r e a t e r d e c r e a s e i n w e a r r a t e w h e n

a c o r r o s i o n i n h i b i t o r w a s u s e d t h a n w a s o b t a i n e d

w it h c a th o d ic p ro te c t i o n w a s e x p e c t e d s i n c e , a s

p re v i o u s l y d is c u ss e d , i t i s b e l i e v e d t h a t f u ll

c a th o d ic p ro te c t i o n w a s n o t a c h ie v e d . T h e

c a th o d ic p ro te c t i o n t h u s a p p e a rs t o h a v e b e e n

o n l y 7 0 t o 7 5 p e r c e n t e f f e c t i v e a t t h e p o t e n t i a l s

u s e d .

T h e d e c r e a s e i n m a s s l o s S T e s u lt i n g f r o m

p r o t e c t i n g t h e p i p e s a g a i n s t c o r r o s i o n g i v e s a n

i n d i c a t i o n o f t h e e f f e c t o f c o r r o s i o n o n t h e t o t a l

w e a r . P re v i o u s w o r k d o n e f o r A n g l o A m e r i c a n

R e s e a r c h L a b o r a to ri e s 1 2 i n d i c a te d t h a t , i n V a a l

R e e f s s l u rr y , t h e c o r r o s i o n r a t e i n c l e a r w a t e r w a s

a p p r o x i m a t e l y 3 3 p e r c e n t o f t h e t o t a l w e a r r a t e

i n s l u r r y t e s t s d o n e i n a n A S T M A 1 0 6 t u b e o f I D

2 2 , S m m ) . T h o s e r e s u l t s t o o k i n t o a c c o u n t o n l y

p u r e c o r r o s i o n a n d d i d n o t a l l o w f o r t h e s y n e r g y

b e t w e e n e r o s i o n a n d c o r r o s i o n . A ls o , t h e

c o rr o si o n r a te g iv e n w a s t h e a v e ra g e c o rr o s i o n

r a t e , w h i c h w o u l d h a v e d e c r e a s e d o v e r t i m e s i n c e

t h e b u i l d - u p o f c o r r o s i o n p r o d u c t s o n t h e s u r f a c e

w o u l d h a v e p r o b a b l y s l o w e d d o w n t h e c o r r o s i o n

r e a c t i o n o v e r t i m e . T h u s , i t w o u l d b e e x p e c t e d

t h a t t h e t o t a l c o r r o s i o n - r e l a t e d c o m p o n e n t i n t h e

w e a r r a t e w o u l d b e s u b s t a n t i a l l y g r e a t e r t h a n

3 3 p e r c e n t . T h i s w a s b o r n e o u t i n t h e p r e s e n t

t e s t s s i n c e , i n t h e V R s l u r r y , c o r r o s i o n a n d i t s

r e l a t e d e f f e c t s w e r e s e e n t o a c c o u n t f o r a t o t a l o f

5 5 t o 6 0 p e r c e n t o f t h e w e a r r a t e w h e n c a t h o d i c

p r o t e c t i o n w a s u s e d , a n d 8 0 p e r c e n t w h e n a

c o r r o s i o n i n h i b it o r w a s u s e d . P o st l e th w a i t e 3

f o u n d t h a t , i n a s l u r r y c o n s i s t i n g o f 2 0 p e r c e n t

b y v o l u m e ) s i l i c a s a n d 3 0 t o 5 0 m e s h ) a n d

w a t e r c o n t a i n i n g 3 1 3 0 p p m o f c h l o r i d e s ,

c o r r o s i o n w a s t h e d o m in a n t m o d e o f m e t a l l o s s .

H is f i n a l c o n c l u s i o n w a s t h a t m e a s u r e s t o c o n t r o l

w e a r i n s t r a ig h t r u n s o f s l u rr y p ip e l i n e s h o u ld

f o c u s o n c o r r o s i o n c o n t r o l . r a t h e r t h a n o n t h e u s e

o f e ro s i o n - r e s i s ta n t p ip in g .

F EB RU AR Y 1 99 4

43

....


8/9

ffect of corrosion on w e r r te

F igu re 1 6 -S urface of a n R C E sp ecim e n a fter e xp os ure fo r 14 0 ho u rs in in h ib ited slurry,

sh o w in g ve ry sm all pits

~

44

F EB R U AR Y 1 99 4

In a slurry

consisting

of 31 per cent by

m ass) alum inium oxide 200 to 250 m esh) and

w ater containing 1000 ppm of N a2S 04 at a pH of

9, inhibition and cathodic protection reduced the

m ass loss by 43 per cent and 48 per cent respec-

tivel/. In that w ork, the authors used these

corrosion-control techniques so that a direct

com parison could be m ade of the erosion

resistance of various m aterials. T his is, of

course, relevant to the backfill situation, w here

m aterials are frequently com pared on the basis

of their erosion resistance, w hereas they are

really being com pared in term s of their

resistance to the com bined effects of erosion and

corrosion. Thus, in a backfill slurry, any

polym eric m aterial w ould gain an advantage

over m ild steel, even if its erosion resistance w as

only the sam e as that of m ild steel. A survey on

w ear and corrosion in the m ining industry13

indicated that, because of the difficulty in

recognizing corrosion as a separate entity in

slurry-transportation system s, m ost personnel

identified failures as being due to erosion. De

Jongh and M orris14, in reporting B rabyn s

results, point out that corrosion had been found

to account for 70 per cent of the w ear caused by

rod-m illed w aste, and 95 per cent of the wear

c aus ed b y c la ss ifie d ta ilin gs.

os t Imp li ca ti on s

If corrosion accounts for up to 80 per cent of the

m ass lost by steel piping for the slurries

considered in the velocity range up to 4,9 m /s,

there is a strong indication that corrosion control

m ust be investigated w hen carbon-steel piping is

to be used. If corrosion accounts for approxi-

m ately 60 to 80 per cent of the m ass loss in the

transportation of VR slurry and 33 to 47 per cent

in the transportation of W D L slurry, the costs

associated w ith pipewall w ear due to corrosion

c an b e c alc ula te d.

T he beneficial effects of backfilling in deep-

level gold m ines have been established, and

depend on three m ain operational param eters:

. the fill-to-face distances 6 m or less)

. good fill contact w ith the hangingw all

. a high percentage of filled area 60 per

cent of the m ined area as a m inim um ).

In addition, the distribution system m ust be

designed so that slurry velocities are m inim ized

and therefore so that full-flow conditions exist).

T his w ill reduce the effect of wear to a m inim um

although it could still contribute significantly to

the to ta l c os t o f b ack fillin g).

T he actual recorded cost per tonne of backfill

associated w ith w ear is difficult to estim ate

reliably in the short term . T his is because the

w ear-associated cost is not very significant until

the pipe ranges actually start failing. T he costs

are also strongly and adversely influenced if

there are free-fall conditions in the pipe range.

T he J ou rn al o f T he S ou th fr ic an In stitu te o f M in in g a nd M eta llu rg y


9/9

E f f e c t

of corrosion on wear ra te

10. U npublished w ork,

M in te k, R an db urg S ou th

Africa).

11. H IG GIN SO N, . T he effect of

p hy si ca l a nd c he mi ca l

f ac to rs o n t he c or ro siv it y

o f a s yn th etic m in e w ate r.

MintekRepoTtno M140.

R an db ur g S ou th A frica ),

1984.

12. U npublished w ork. A nglo

Amer ic an Re se ar ch

Laboratories,

J oh an ne sb urg S ou th

Africa),1990.

13. C AR TE R,A .M .F . E r os io n,

co rro sio n a nd th e

a br as io n o f m at er ia l

handling system s in the

m in in g i nd u st ry . IS Aft

Inst Mill Metall

vo\. 86, no. 7. Ju\. 1 986.

14. D E JO NG H,CL, and

MORRIS,A.N. Guide lines

fo r c on sid er atio n in th e

design and operation of

b ac kfill s ys te ms fo r

n arr ow ta bu lar o re b od ie s,

par t 1 ). B ac k F il li ng

~mposium Johannes-

b urg S ou th A fric a) ,

A ss ocia tio n o f M in e

M an ag er s o f S ou th A fr ica ,

1986.

AtVaa l

R e e f s

where free-fallconditions

e xisted in sch edu le 80 p ipe s w ith a n om in al

bore of 50 mm), the replacem ent cost due to pipe

w ear e xc ee de d R 3,00 pe r ton ne o f fill p la ce d. A t

othe r V aal R ee fs site s, th e co sts w ere le ss tha n

RO ,10 p er to nn e p la ce d, b ut th e o ve ra ll to nn ag e

p er r an ge was s till re la tiv ely low.

A t We ste rn Deep Lev els , whe re s ch ed ule

X XS pipes w ith a nom inal bore of 80 mm) are

u sed, th e cur rent r ep lacemen t cos t a ssoc ia ted

w ith the w ea r o f b ac kfill p ip es is a rou nd RO,OS

pe r ton ne, b ut th is fig ure w ill rise a s th e ton nag e

in cre as es a nd mo re fa ilu re s o cc ur.

It sho uld b e no te d tha t a ll th ese co sts are

a ss oc ia te d w ith m ild -s te el g ra de A I0 6) p ip es ,

a nd th at fig ures a re n ot ye t av aila ble fo r the

othe r pipe s use d owing to th e low to nn age

plac ed via ea ch ra ng e to d ate .

on lusions

The re sea rc h h as sh ow n th at c orrosio n an d,

more import an tl y, t he co rros ion -erosion

in te racti on con tr ib ute a s ig ni fic an t por tio n o f t he

ov erall w ea r loss in pip e m ass) in b ack fill

pipe lin es u p to 80 pe r ce nt). T his c on clusion is

d rawn for two typi ca l b ackf ill ma te ri al s under

various f low cond it ions.

Wh ere s te el p ip in g is u se d, a ttemp ts to re du ce

th e o ve ra ll w ea r ra te s ho uld in clu de meas ure s to

re du ce th e c orro sio n e ffe ct. A v ia ble a nd cost-

e ff ec tiv e a lt er na tiv e is th e u se o f a cor ro si on -

r es is tant l in in g such as polyur et hane ).

C ath od ic p ro te ctio n o f th e te st s amp le s o f

ste el pip e w as fo und to be p rob lematic in th e

tr an spor ta tio n o f b ackf ill s lu rr ie s ow ing t o th e

p la tin g-o ut o f meta llic io ns a nd th e fo rmatio n o f

scale.

W ith a ny b ackfill p ip elin e, it is e ss en tia l th at

th e flow v elo citie s a re re du ce d to a m in imum

Le . f ull- fl ow conditi on s must p reva il) . Th is w ill

a ss is t in r educ ing t he wear r ate s ig nif ic an tly

since

Wear a Velocity

c c

the effect of corrosion on the wear rate of steel pipelines conveying backfill slurry

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