Polymeric Materials for Corrosion Control - American Chemical Society

4 Application of Electrochemical Noise Measurements to Coated Systems Downloaded by UNIV OF NEW SOUTH WALES on September 5, 2016 | http://pubs.acs.org Publication Date: October 14, 1986 | doi: 10.1021/bk-1986-0322.ch004

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D. A. Eden , M . Hoffman , and B. S. Skerry 1

Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, Sackville Street, Manchester, M60 1QD, England Sherwin Williams Company Research Center, 10909 South Cottage Grove Avenue, Chicago, IL 60628

2

This paper describes the application of novel electrochemical techniques to studies of paint films on steel substrates exposed to aqueous environments. Simultaneous monitoring of the self-generated electrochemical potential and current noise using analogue and digital techniques has been evaluated as a tool for monitoring coating performance. These data obtained have been compared with those from a.c. impedance techniques. Laboratory measurement procedures used for electrochemical data acquisition and analysis during the monitoring exercise are outlined, and particular emphasis is placed on the electrochemical noise techniques. Electrochemical current noise has been monitored between two identical electrodes and the potential noise between the 'working' electrodes and a reference electrode. Digital noise measurements have been obtained by use of a microcomputer controlling the sampling rate of a sensitive digital voltmeter employed to measure the potential or current fluctuations. The subsequent analysis of the derived time records is described. Analogue noise measurements have been made using high gain amplifier/ filter circuits which permit examination of low frequency fluctuations on a 'real-time' basis. Electrochemical noise monitoring techniques have been used p r e v i o u s l y i n studies of c o r r o s i o n processes o c c u r r i n g on metals i n a v a r i e t y o f environments. I n i t i a l l y , work was d i r e c t e d towards t h e m o n i t o r i n g of p o t e n t i a l noise f l u c t u a -

0097-6156/ 86/ 0322-0036S06.00/ 0 © 1986 American Chemical Society

Dickie and Floyd; Polymeric Materials for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Downloaded by UNIV OF NEW SOUTH WALES on September 5, 2016 | http://pubs.acs.org Publication Date: October 14, 1986 | doi: 10.1021/bk-1986-0322.ch004

EDEN ET AL.

Application of Electrochemical Noise Measurements

t i o n s , and was u s e d p a r t i c u l a r l y i n t h e i d e n t i f i c a t i o n o f t h e o n s e t o f l o c a l i s e d a t t a c k ( i . e . p i t t i n g o r c r e v i c e type a t t a c k ) Q-.3] . C u r r e n t n o i s e measurements have been u s e d i n t h e s t u d i e s o f e l e c t r o c r y s t a l l i s a t i o n [4] and p i t t i n g [5] w i t h t h e specimens b e i n g h e l d under p o t e n t i o s t a t i c c o n t r o l . Recent work [ 6 ,_7 ] has been d i r e c t e d towards t h e s i m u l t a neous m o n i t o r i n g o f p o t e n t i a l and c u r r e n t n o i s e , where t h e c u r r e n t n o i s e s i g n a l i s g e n e r a t e d by c o u p l i n g two n o m i n a l l y i d e n t i c a l e l e c t r o d e s w i t h a z e r o r e s i s t a n c e ammeter (ZRA), and the p o t e n t i a l n o i s e of the couple i s monitored with r e s p e c t t o a reference electrode. In t h i s manner no e x t e r n a l l y a p p l i e d signal i s required. The potential n o i s e s i g n a l provides i n f o r m a t i o n pert a i n i n g t o t h e t y p e o f a t t a c k , whereas t h e c u r r e n t n o i s e p r o v i d e s d a t a which i n d i c a t e t h e r a t e o f c o r r o s i o n and t h e t y p e of attack. When used i n p a r a l l e l , t h e two n o i s e measurements may be used t o e s t i m a t e t h e p o l a r i s a t i o n r e s i s t a n c e o f t h e i n t e r f a c e b e i n g examined. When a p p l i e d t o c o a t e d m e t a l s , t h e f l u c t u a t i o n s o b s e r v e d i n the c u r r e n t n o i s e s i g n a l are g e n e r a l l y low i n magnitude w i t h t h e b a s e l i n e o f d e t e c t i o n e s s e n t i a l l y b e i n g l i m i t e d by the s e n s i t i v i t y o f t h e e l e c t r o n i c i n t e r f a c e . For t h e s t u d i e s c i t e d , t h e lower l i m i t o f t h e c u r r e n t n o i s e s i g n a l i s some 10 pico-amps. F o r t h e p u r p o s e s o f t h i s study t h e r e s p o n s e s o f a v a r i e t y o f i n t a c t and d e f e c t i v e c o a t i n g s were m o n i t o r e d and the r e s u l t s a r e compared w i t h a.c. impedance d a t a . The a.c. impedance t e c h n i q u e i s u s e f u l f o r m o n i t o r i n g changes o c c u r r i n g i n c o a t e d systems, and t h e v a r i o u s t y p e s o f response may be summarised b r i e f l y as f o l l o w s : a)

b) c) d)

I n t a c t c o a t i n g s (no pores) v e r y h i g h impedance p r o d u c e s almost p u r e l y c a p a c i t i v e r e s p o n s e , d i f f i c u l t y i n e s t i m a t i n g d.c. component o f r e s i s t a n c e . I n t a c t c o a t i n g s (as ( a ) ) , w i t h water uptake c a p a c i t a n c e i n c r e a s e s due t o d i e l e c t r i c c o n s t a n t changes. C o a t i n g s w i t h minor d e f e c t s u s u a l l y produce w e l l d e f i n e d r e s p o n s e w i t h r e s i s t i v e as w e l l as c a p a c i t i v e components. C o a t i n g s w i t h major d e f e c t s show response i n which comp l e x b e h a v i o u r i s observed, t h e c o a t i n g r e s p o n s e moving t o h i g h e r f r e q u e n c i e s due t o s m a l l e r v a l u e s o f r e s i s t a n c e , and i n a d d i t i o n , charge t r a n s f e r and diffusion e f f e c t s may become e v i d e n t .

Instrumentation D i g i t a l electrochemical noise. The d i g i t a l i n s t r u m e n t a t i o n used f o r t h e n o i s e s t u d i e s comprised t h e f o l l o w i n g : A H e w l e t t P a c k a r d HP85 Microcomputer A Hewlett Packard 3478A D i g i t a l V o l t m e t e r A "custom b u i l t " m u l t i p l e x e r A schematic diagram f o r t h e e x p e r i m e n t a l i n F i g u r e 1.

s e t up

is illustrated


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POLYMERIC M A T E R I A L S FOR CORROSION

Multiplexer and Zero Resistance Ammeter

CONTROL

Microprocessor


8 sets of inputs

To working electrodes

*To reference

electrode

F i g u r e 1. S c h e m a t i c diagram using multiplexed electrodes.

for digital

noise

measurements

The i n p u t m u l t i p l e x e r was d e s i g n e d t o a l l o w m u l t i - c h a n n e l c a p a b i l i t y and was c o n f i g u r e d t o m o n i t o r b o t h p o t e n t i a l and c u r r e n t n o i s e f l u c t u a t i o n s s e q u e n t i a l l y on a maximum o f e i g h t p a i r s o f samples. The s a m p l i n g r a t e o f t h e d i g i t a l v o l t m e t e r (DVM) was cont r o l l e d by t h e m i c r o p r o c e s s o r and c h a n n e l s e l e c t i o n f o r monit o r i n g was o b t a i n e d by u t i l i s i n g a p u l s e o u t p u t from t h e DVM. Time r e c o r d s o f t h e c o u p l i n g c u r r e n t and p o t e n t i a l f o r t h e r e s p e c t i v e samples were o b t a i n e d and s t o r e d f o r f u r t h e r analysis. Analogue electrochemical noise. Analogue instrumentation m o n i t o r i n g low f r e q u e n c y n o i s e i n a s p e c i f i e d bandwidth was u s e d f o r t h e a n a l o g u e measurements. The s c h e m a t i c diagram ( F i g u r e 2) i l l u s t r a t e s t h e b a s i c c o n f i g u r a t i o n o f t h e i n s t r u mentation. The rms v a l u e s o f t h e n o i s e s i g n a l s were l o g g e d and s e n t as a 0 - 10V s i g n a l t o a c o n v e n t i o n a l c h a r t r e c o r der. The s i g n a l s e n s i t i v i t y c o r r e s p o n d i n g t o t h e f u l l s c a l e

To workina electrodes

Zero Resistance Ammeter

High Impedance Buffer

To reference

F i g u r e 2. monitoring.

Recorder

Filter

electrode

System

f o r analogue

current

and

potential


noise

EDEN ET AL.



response o f the c h a r t r e c o r d e r was 1uV minimum (OV o u t p u t ) t o 10mV (10V output) c o v e r i n g f o u r decades a t 2.5V p e r decade. The c u r r e n t n o i s e s i g n a l was m o n i t o r e d by u s i n g a s e n s i t i v e , low n o i s e z e r o r e s i s t a n c e ammeter (ZRA) t o c o u p l e p a i r s o f i d e n t i c a l e l e c t r o d e s ; the ZRA a c t i n g as a c u r r e n t t o v o l tage converter. T h i s d e r i v e d p o t e n t i a l s i g n a l was t h e n f e d i n t o a p o t e n t i a l noise monitor. A.c. impedance. Impedance measurements were made u s i n g a S o l a r t r o n 1250 f r e q u e n c y r e s p o n s e a n a l y s e r under computer con t r o l u s i n g a Hewlett Packard HP85 microcomputer and commer c i a l l y a v a i l a b l e software. The c o a t i n g s were s t u d i e d i n t h e t h r e e e l e c t r o d e mode u s i n g a Thompson M i n i s t a t . Figure 3 i l l u s t r a t e s s c h e m a t i c a l l y t h e e x p e r i m e n t a l arrangement.

F i g u r e 3. Working arrangement f o r 3 e l e c t r o d e e l e c t r o c h e m i c a l impedance s t u d i e s .

Ssmple p r e p a r a t i o n F o r t h e p u r p o s e s o f t h i s study a v a r i e t y o f c o a t i n g s a p p l i e d t o m i l d s t e e l s u b s t r a t e s were u s e d . The c o a t i n g s were chosen t o p r o v i d e a range o f p r o t e c t i o n from p o o r t o e x c e l l e n t . The c o a t i n g s s t u d i e d were: 1.

Polyurethane

(unpigmented)

2.

Polyurethane

(pigmented)

3. 3.

Bitumen Bitumen o v e r z i n c r i c h

paint

1 2 1 2 1 1

c o a t ~ 40μπι c o a t s ~ 80urn c o a t ~ 45um c o a t s ~ 90μπι c o a t ~ 20um c o a t ~ 20μπι

Expérimental P l a s t i c c e l l s o f d i m e n s i o n s 5 χ 5 χ 7.5cms were f i x e d t o t h e c o a t e d specimens u s i n g s i l i c o n e r u b b e r s e a l a n t . The s i l i c o n e


P O L Y M E R I C M A T E R I A L S FOR CORROSION C O N T R O L

40


r u b b e r was a l l o w e d t o c u r e f o r a t l e a s t two days p r i o r t o f i l l i n g with e l e c t r o l y t e . Three p e r c e n t sodium c h l o r i d e s o l u t i o n i n d e m i n e r a l i s e d water was added t o t h e c e l l s which were p r e p a r e d as i d e n t i c a l p a i r s . C o u p l i n g between t h e p a i r s o f e l e c t r o d e s was a c h i e v e d u s i n g a sodium c h l o r i d e / a g a r salt bridge. P o t e n t i a l s o f t h e specimens were m o n i t o r e d u s i n g silver/silver chloride reference electrodes. A platinum c o u n t e r e l e c t r o d e was i n t r o d u c e d i n t o i n d i v i d u a l c e l l s when m o n i t o r i n g t h e a.c. impedance r e s p o n s e . A typical c e l l ar rangement i s shown i n F i g u r e 4. Platinum Reference

electrode

electrode NaCl/Agar s a l t

bridge

Cell

— Coated specimen

F i g u r e 4.

C e l l arrangement f o r e l e c t r o c h e m i c a l s t u d i e s .

D u r i n g t h e p e r i o d o f immersion o f t h e samples i n sodium c h l o r i d e e l e c t r o l y t e , e l e c t r o c h e m i c a l n o i s e measurements were made u s i n g t h e e l e c t r o n i c a p p a r a t u s p r e v i o u s l y d e s c r i b e d . The time r e c o r d s o b t a i n e d were a n a l y s e d u s i n g s t a t i s t i c a l t e c h n i q u e s t o d e r i v e mean, s t a n d a r d d e v i a t i o n and c o e f f i c i e n t o f variance. The d e r i v e d v a l u e o f p o l a r i s a t i o n r e s i s t a n c e was e v a l u a t e d from t h e r a t i o o f t h e s t a n d a r d d e v i a t i o n o f t h e p o t e n t i a l noise s i g n a l t o the standard d e v i a t i o n of the current noise signal, i.e. :

OV oi Data i s presented g r a p h i c a l l y t o i l l u s t r a t e the v a r i a t i o n i n d.c. potential ^Figure 5) mean d.c. c o u p l i n g c u r r e n t , i ( F i g u r e 6) and — τ « Rp ( F i g u r e 7) w i t h t i m e . T y p i c a l anaîogue n o i s e t r a c e s a r e i l l u s t r a t e d i n F i g u r e s




EDEN ET AL.

-0.5

15 Time (days) Figure 5. Potential vs time for coated specimens in 37 NaCl. Key: · , bitumen;A> Zn r i c h + bitumen;Q , polyurethane, one coat (unpigmented); ψ , polyurethane, two coats (unpigmented); 0, polyurethane, one coat (pigmented); and X, polyurethane, two coats (pigmented). c

-5

Time (days) Figure 6. Log i vs time for coated specimens in 3% Key: same as for Figure 5.



42

POLYMERIC M A T E R I A L S FOR CORROSION C O N T R O L

8 and 9. Impedance d a t a F i g u r e s 10, 11 and 12.

for typical

cells

are

presented

in

Discussion From t h e d a t a o b t a i n e d f o r t h e d i f f e r e n t specimens i t can be seen t h a t t h e r e i s s i g n i f i c a n t l y d i f f e r e n t b e h a v i o u r between t h e p o o r , p o r o u s c o a t i n g s (bitumen) and t h e p o l y u r e t h a n e p a i n t samples. Of t h e p o l y u r e t h a n e samples o n l y one showed any e v i dence o f c o r r o s i o n b e n e a t h t h e c o a t i n g d u r i n g t h e d u r a t i o n o f the t e s t and t h i s was an unpigmented s i n g l e c o a t specimen.

Potential (rms)

L

1mV

Current (rms)

1uA

.

L 100nA

I IOOuV J"

10nA

10pV

ο Time

F i g u r e 8. Analogue p o t e n t i a l bitumen on m i l d s t e e l . Day 1.

and

current

(hours)

noise

traces


for

.

4.

EDEN ET AL.



potential (rms)

Current (rms)

-

1mV

-

-

1nA

-

-

100μν

-

- 100pA

-

.

10μν

.

.

3 Time

(hours)

Figure 9. Analogue p o t e n t i a l and c u r r e n t p o l y u r e t h a n e 2 c o a t s unpigmented. Day 2.

noise

2

10pA .

traces f o r

Z Imaginary (ohms)

15000 Ζ Real (ohms)

Ζ Imagi nary (ohms) 500 r -

1000 Ζ Real (ohms)

Figure 10. Nyquist Day 0 and Day 50.

plots

f o r bitumen

coated

mild

steel.


44

P O L Y M E R I C M A T E R I A L S FOR CORROSION C O N T R O L


Z Imoglnory (ohms)

150000 Ζ Real (ohme) Ζ Imaginary (ohms)

300

Figure steel.

11. Day

Nyquist p l o t s 0 and Day 50.

for zinc

rich

400 Ζ Real (ohme)

and

bitumen

on

mild

Even so, t h e low f r e q u e n c y impedance as d e r i v e d from t h e n o i s e measurements was s t i l l two t o t h r e e o r d e r s o f magnitude h i g h e r t h a n t h e bitumen c o a t e d system. The b e t t e r c o a t i n g s e x h i b i t e d low f r e q u e n c y impedances some f o u r t o f i v e o r d e r s o f magnitude h i g h e r t h a n t h e bitumen. The d.c. p o t e n t i a l s , however, o n l y i n d i c a t e d whether t h e m a t e r i a l b e i n g s t u d i e d was i n a c o r r o s i o n regime, both t h e b i t u m e n and unpigmented s i n g l e c o a t p o l y u r e t h a n e assumed v e r y s i m i l a r p o t e n t i a l s o v e r the p e r i o d o f the t e s t , even though t h e c o r r o s i o n r a t e s were g r o s s l y d i f f e r e n t . The impedance d a t a i l l u s t r a t e d i n F i g u r e s 10, 11 and 12 have been chosen t o i l l u s t r a t e t h e w i d e l y d i f f e r i n g b e h a v i o u r o f the d i f f e r e n t c o a t i n g systems. In F i g u r e 10, which shows t h e impedance b e h a v i o u r o f t h e bitumen c o a t i n g on m i l d s t e e l , i t i s a p p a r e n t t h a t a t day 0, t h e c o a t i n g i s immediately showing s i g n s o f major d e f e c t i v e a r e a s , w i t h the impedance response b e i n g governed by what ap p e a r t o be d i f f u s i v e e f f e c t s . The response a t high fre q u e n c i e s p r o b a b l y b e i n g due t o t h e c o a t i n g i t s e l f . The r e s i s tance o f t h e system a t t h i s s t a g e i s g r e a t e r t h a n 15000 ohms. A f t e r 50 days' exposure, the impedance response has changed t o one i n d i c a t i n g charge t r a n s f e r and d i f f u s i o n e f f e c t s w i t h a r e s i s t a n c e g r e a t e r than 1000 ohms. In c o m p a r i son, t h e b e h a v i o u r o f t h e bitumen c o a t i n g on z i n c r i c h p a i n t ( F i g u r e 11) i n d i c a t e s t h a t a t day 0, the c o a t i n g i s showing


EDEN ET AL.


2 Imaginary (ohms)


100000 r

50000

50000

200000 Ζ Real (ohme)

2 Imaginary (ohme) 50000

50000

100000 Ζ Real (ohme)

F i g u r e 12. N y q u i s t p l o t s f o r p o l y u r e t h a n e (1 coat) u n p i g mented system a f t e r 50 d a y s , i l l u s t r a t i n g c o a t i n g breakdown on P a n e l B. m i n o r d e f e c t s w i t h an impedance o f some 150,000 ohms t y p i c a l of a c o a t i n g response, although e x h i b i t i n g a f r e e c o r r o s i o n p o t e n t i a l (-806mV), which i s i n d i c a t i v e o f a porous c o a t i n g . A f t e r exposure f o r 50 days t h i s system i s showing t o t a l l y d i f f e r e n t b e h a v i o u r w i t h charge t r a n s f e r and d i f f u s i o n e f f e c t s becoming much more e v i d e n t . Figure 12 i l l u s t r a t e s t h e d i f f e r e n c e i n impedance be h a v i o u r between t h e two samples o f unpigmented p o l y u r e t h a n e ( a p p l i e d a t ~ 40um) a f t e r 50 days exposure. P a n e l A gave a r e s p o n s e i n d i c a t i v e o f a good, i n t a c t c o a t i n g (almost p u r e l y



46

P O L Y M E R I C M A T E R I A L S FOR CORROSION

CONTROL

c a p a c i t i v e ) , whereas p a n e l Β was showing s i g n s o f breakdown, w i t h an e s t i m a t e d r e s i s t a n c e o f some 75,000 ohms. W i t h t h e n o i s e t e c h n i q u e s , b o t h a n a l o g u e and d i g i t a l , no e x t e r n a l l y a p p l i e d s i g n a l i s r e q u i r e d , and measurement o f t h e f l u c t u a t i o n s around t h e f r e e c o r r o s i o n p o t e n t i a l p r o v i d e s a l l the i n f o r m a t i o n . Hie n o i s e t e c h n i q u e i s u s e f u l i n t h a t i t a l l o w s a f a i r l y r a p i d e s t i m a t i o n o f t h e e l e c t r o c h e m i c a l im pedance o f t h e system b e i n g s t u d i e d , whereas, w i t h f o r i n s t a n c e , a.c. impedance t e c h n i q u e s , v e r y o f t e n t h e minimum f r e quency s t u d i e d i s s t i l l n o t low enough t o p r o v i d e s u f f i c i e n t i n f o r m a t i o n t o a l l o w an a c c u r a t e e s t i m a t i o n o f t h e impedance. With e l e c t r o c h e m i c a l n o i s e measurements t h e d.c. p o t e n t i a l o f two c o u p l e d i d e n t i c a l e l e c t r o d e s i s governed by t h e sample w i t h t h e l o w e s t impedance. I t i s t h i s lower v a l u e o f impedance w h i c h i s m o n i t o r e d by t h e n o i s e t e c h n i q u e , i . e . t h a t o f t h e worst c o a t i n g o f t h e p a i r . F o r t h e systems s t u d i e d , i t i s i n t e r e s t i n g t o note t h a t the mean l e v e l o f c o u p l i n g c u r r e n t a l s o appears t o be v e r y u s e f u l a s a means o f s t u d y i n g h i g h impedance systems, b u t t h i s can cause problems i f t h e c u r r e n t f l u c t u a t e s around z e r o and changes p o l a r i t y . G e n e r a l l y , i t would appear t o be a b e t t e r approach t o u t i l i s e t h e v a l u e o f s t a n d a r d d e v i a t i o n o f t h e c u r r e n t s i g n a l as a measure o f c o r r o s i o n r a t e . The c o e f f i c i e n t o f v a r i a n c e f o r t h e c u r r e n t s i g n a l g i v e s some i n d i c a t i o n o f t h e s t a b i l i t y o f t h e d.c. c o u p l i n g c u r r e n t . I f we c o n s i d e r t h e a n a l a g o u s n o i s e e q u a t i o n s d e r i v e d f o r e l e c t r o n i c components a t t h e low f r e q u e n c y end o f t h e spec trum, one o f t h e e q u a t i o n s u s e d t o d e s c r i b e t h e n o i s e i s :

1 V where : l"d.c. Rs Kl f

η

= Κχ/— . I . Rs r d.c.

= d.c. c u r r e n t f l o w i n g t h r o u g h = source r e s i s t a n c e = constant = frequency

Correspondingly I

n

(1)

device

the equation f o r the current noise i s : = Ki/J . I. „ f d.c. A

(2)

I f we u t i l i s e t h e above e q u a t i o n s t o d e s c r i b e t h e low f r e quency n o i s e s i g n a l s o b s e r v e d w i t h e l e c t r o c h e m i c a l systems, i t i s apparent t h a t t h e p o t e n t i a l n o i s e s i g n a l w i l l p r o v i d e i n f o r m a t i o n p e r t a i n i n g t o t h e v a l u e o f t h e S t e r n Geary c o n s t a n t since: =r

corr where: i corr Β

—

R Ρ

corrosion current S t e r n Geary c o n s t a n t polarisation resistance


4.

EDEN ET AL.

and hence:


Vn = K^/*

. Β

whereas t h e c u r r e n t n o i s e s i g n a l w i l l p r o v i d e i n f o r m a t i o n r e l a t i n g to the c o r r o s i o n r a t e . I t i s t h e r e f o r e , not s u r p r i s i n g t h a t t h e low f r e q u e n c y p o t e n t i a l n o i s e s i g n a l s o n l y t e n d t o v a r y o v e r a few d e c a d e s , whereas t h e c u r r e n t n o i s e s i g n a l s may v a r y o v e r many o r d e r s o f magnitude.


Conclusions 1.

2.

3.

E l e c t r o c h e m i c a l n o i s e measurements have shown g r e a t p r o mise as a m o n i t o r i n g t o o l i n s t u d i e s o f c o r r o d i n g m e t a l s i n a v a r i e t y of environments. The a p p l i c a t i o n o f t h e s e s e n s i t i v e t e c h n i q u e s t o e v a l u a t e t h e p e r f o r m a n c e o f c o a t e d specimens would appear t o be a p p r o p r i a t e f o r t h e s t u d y o f slow c o r r o s i o n p r o c e s s e s and also f o r t h e m o n i t o r i n g o f c o a t i n g breakdown/degrada tion. S i n c e the n o i s e s i g n a l s a r e g e n e r a t e d by the specimens t h e m s e l v e s c o a t i n g f a i l u r e i s accompanied by a change i n t h e e l e c t r o c h e m i c a l n o i s e s i g n a l which g i v e s a r a p i d i n d i c a t i o n o f the s t a t e of the c o a t i n g . Statis t i c a l a n a l y s i s o f t h e d a t a p r o v i d e s a r a p i d method o f a s s e s s i n g the noise levels without the n e c e s s i t y f o r t r a n s p o s i t i o n o f t h e d a t a i n t o t h e f r e q u e n c y domain by, f o r i n s t a n c e , FFT t e c h n i q u e s . Simultaneous m o n i t o r i n g o f c u r r e n t and p o t e n t i a l n o i s e and d e r i v a t i o n o f low f r e q u e n c y v a l u e s o f impedance a l lows, i n some i n s t a n c e s , d i r e c t comparison w i t h p o l a r i s a t i o n r e s i s t a n c e v a l u e s d e r i v e d from, f o r example, a.c. impedance t e c h n i q u e s .

Literature Cited 1. 2. 3. 4. 5. 6. 7.

Hladky, Κ., and Dawson, J.L., Corr. Sci 22, p317 (1981). Hladky, Κ., and Dawson, J.L., ibid, 23, p231 (1982). Dawson, J.L, Hladky, Κ., and Eden, D.A., Paper presented at "On line Monitoring of Continuous Process Plant", London, June 1983. Bindra, P., Fleischmann, Μ., Oldfield, J.W. and Singleton, D., Discussion of Faraday Soc. 56 (1974). Williams, D.E., Westcott, C., Fleischmann, Μ., Passivity of Metals and Semi Conductors, p217-228, Elsevier Science publishers, Ed. M. Froment. Farrell, D.M., Cox, W.M., Stott, F.H., Eden, D.A., Dawson, J.L., and Wood, G.C., High Temperature Technology Vol. 3, No. 1, February 1985. John, D.G., Hladky, Κ., Eden, D.A., and Dawson, J.L., Paper presented at Research Sciences Symposium NACE/Corrosion 84, New Orleans, April 1984.

RECEIVED March 6,

1986


Polymeric Materials for Corrosion Control - American Chemical Society

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