MÖSBAUER INVESTIGATION OF CORROSION PROCESSES AND PRODUCTS

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MÖSBAUER INVESTIGATION OF CORROSION PROCESSES AND PRODUCTS

W. Meisel

To cite this version:

W. Meisel. MÖSBAUER INVESTIGATION OF CORROSION PROCESSES AND PRODUCTS.

Journal de Physique Colloques, 1980, 41 (C1), pp.C1-63-C1-70. �10.1051/jphyscol:1980112�. �jpa-

00219602�

JOURNAL DE PHYSIQUE

Colloque

C l

, suppl&ment au n

^O

1 , Tome

41,

janvier

1980,

page

C1-63

P~SBAUER INMSTIGATION OF CORROSION PROCESSES AND PRODUCTS

W. Meisel

Academy of Sciences of GDR, CentraZ I n s t i t u t e for PhysicaZ Chemistry, DDR-1199 BerZin-AdZershof.

Abstract

-

The qualitative and quantitative Kissbauer spectroscopic phase

analysis is a very useful tool for investigating corrosion processes and products.

Its nondestructive character as well as the fact that also amorphous products can be studied, are its most important advantages as compared with other appropriate methods, as e.g. Rijntgen diffraction. Because of the great technical importance, the corrosion of iron and steel will be considered in some detail.

Jy use of a simple model of atmospheric rusting, a survey will be given, to which of the stages of the corrosion process the different Prssbauer techniques

(transmission, gamma scattering, conversion electron detection) can be applied.

Some examples concerning problems important from a technical point of view will demonstrate successful applications of Uossbauer spectroscopy: The influence of ciimate factors on atmospheric rusting, the time dependent formation of rust beneath a protective coating, the effect of aggressive media and of corrosion inhibiting treatments, and the corrosion by lubricants under the influence of friction.

1. INTRODUCTION

In corrosion research, tfitissbauer spec- troscopy plays a more and more important role.

This is h e to the facts that the Kossbauer spectroscopy is already a routine method in qua- litative and quantitative phase anhlysis, and that its applicability is improved now by the development of scattering techniques and of the conversion electron detection. Nevertheless, considering the publications in this field, their number seems to be surprisingly small in relation to the economical importance ol mate- rial losses by corrosion. Tnis importance is illuminated by the fact that industrial nations have to spend 2...3% of their national income for the compensation of corrosion losses. In particular, in different publications was found

(losses in 10 marks per year): 9 USA 140, USSE

35,

FRG 24, GB b, GDR 2.5. A reduction of these losses as well as environmental demands show the great need of an improved protection against corrosion. lo make some progress in solving this problem, a good insight into the details of cor- rosion is necessary, bat our knowledge about corrosion processes is far from being sufficient.

In technical engineering and construction, empi- rical methods are still quite common: There are many experiences about corrosion rates (mea- sured in micrometer per year), a d , on their

basis, a surplus of the initial material inset (measured in megatons of steel per year) is usual, as e.6. the surplus thickness of hot water pipes.

In fact, corrosion processes are rather complicated, a d there are only a few aethods available for their investigation. lnese are mainly wet chemistry, electrochemistry, thermo- gravimetry, hontgen methods, ESCii, and

-

^last

cot least

-

Mossbauer spectroscopy. xll of them have specific advantages and restrictions in

their applicability in corrosion research. In the following chapters, the usefulness of Bijss- bauer spectroscopy in corrosion research is to be shown.

Jecause of their technical importance, we are interested in corrosion investigations on he-containing ~lliiterihls. There are only a iew publications concerning NiBssbauer corrosion studies by use of other elements, especially of tin [I-41.

From a practical point of vies, it is helpful to subdivide the corrosion of iron into several classes:

A. Atmospheric corrosion;

3. Atmospheric corrosion below protective coatings;

C. Corrosion in water (cold,hot,sea water,...);

D.

Corrosion in aggressive media;

g. Corrosion under the influence of pressure,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980112

Cl-64 JOURNAL DE PHYSIQUE

s t r e s s , and o t h e r mechanical t r e a t m e n t s ; F. Corrosion i n aggressive media with a d d i t i o -

n a l mechanical treatment;

G.

...

Some examples w i l l be given below.

2. k MODEL EVK ATMOSPIiEHIC CORROSION

In order t o d i s c u s s t h e a p p l i c a t i o n of Wssbauer spectroscopy i n c o r r o s i o n r e s e a r c h , t h e following simple model of atmospheric corro- s i o n is t o be considered (cf. f i g . 1 ) :

I. In a f i r s t s t e p , oqygen d i f f u s e s i n t o t h e immediate v i c i n i t y of t h e metal.

11. Oxygen i s adsorbed on t h e metal s u r f a c e . 111. Chemisorption takes place. Electrons a r e t r a n s f e r r e d by e l e c t r o s t a t i c f o r c e s . The metal i s now covered by one o r two monolayers (even- t u a l l y only i s l a n d s ) c o n s i s t i n g of pe2+ and 0 2-

.

1...2 l a y e r s

Fe3+(stable) several

Fez+, F ~ ~ + , H ~ o phases

Fe2+, ~ e ~ +

t

Fig.1. Model of atmospheric corrosion.

I V . Further o x i d a t i o n l e a d s t o t h e formation o f a l a y e r , up t o 50

2

t h i c k , containing Fe 26.

,

Fe?, and 0 2-

.

On the metal s u r f a c e , we have always t h e monolayer l i k e in 111.

V. Depending on t h e humidity, some water d i E - f u s e s i n t o t h e upper l a y e r , l e a d i n g t o a more o r l e s s porous s t r u c t u r e a f t e r phase transforma- t i o n s i n t h i s l a y e r . This porosity i s respon- s i b l e f o r t h e f u r t h e r progress of corrosion, r e s u l t i n g i n a l a y e r thickness of t h e order of magnitude of 7000

2 .

VX. A l l o x i d i c i r o n phases formed up t o s t e p V a r e not s t a b l e , although some of them can e x i s t f o r a r e l a t i v e l y long time. Nevertheless, a f t e r some time and under t h e i n f l u e n c e of atmospheric f a c t o r s , t h e upper p a r t of t h e c o r r o s i o n pro- d u c t s w i l l undergo phase transformations t o form s t a b l e phases, containing only te3+. Thus,

"old r u s t * may grow up t o a thickness of about 1 mm.

decause of t h e i r s e n s i t i v i t y l i m i t s , ltissbauer techniques a r e not a p p l i c a b l e a s a r o u t i n e method f o r s t u d i e s concerning s t e p s I, 11, and 111. Here, o t h e r methods l i k e e l e c t r o - chemistry and BSCA a r e favorable. Therefore, t h e s e s t e p s w i l l n o t be considered i n d e t a i l i n t h i s r e p o r t . For s t e p s I V and V, t h e d e t e c t i o n of Mtissbauer conversion e l e c t r o n s i s a very use- f u l t o o l . Descriptions of t h i s method a r e e.g.

C4-61. This technique i s u s e f u l i n t h e thickness range of 20.. .3000

2

( f o r i r o n oxides, f o r t i n s t u d i e s 10 times more) and allows a l s o a depth- s e l e c t i v e phase a n a l y s i s . I n t h e upper p a r t of t h i s thickness range, t h e s c a t t e r i n g of ltiss- bauer gamma r a y s becomes a p p l i c a b l e . Step V I i s b e s t s u i t e d f o r common Mtissbauer spectroscopy i n both s c a t t e r i n g and transmission geometry. Here, t h e well-known advantages of t h e iktissbauer spec-

t r o s c o p i c phase a n a l y s i s can be turned f u l l y t o account, a s f o r example i t s nondestructive c h a r a c t e r , i t s u s e f u l n e s s f o r q u a l i t a t i v e a s w e l l a s q u a n t i t a t i v e determinations, and i t s a p p l i c a b i l i t y t o highly dispersed and amorphous m a t e r i a l s (contrary t o x-ray methods). Descrip- t i o n s of such phase a n a l y s e s a r e given i n

i7l;

Although. t h e u s e f u l n e s s of B s s b a u e r measure- ments i n c o r r o s i o n r e s e a r c h i s obvious, t h e number of papers i n t h i s f i e l d i s not y e t very high (sy 100).

In t a b l e 1, t h e l a t t i c e parameters p of

m e t a l l i c i r o n and. of i t s main c o r r o s i o n products a r e shown. From a comparison of them f o l l o w s t h a t among them FeG i s most s i m i l a r i n s t r u c t u r e t o a i r o n , i f i t i s canted by an angle of 45' with r e s p e c t t o t h e cubic 6 - P e l a t t i c e , be- cause t h e plane dizgonal of *-iron i s 4.053

1.

Therefore, i t i s most prooablt t h a t t h e oxida- t i o n of peO t o 2e2+ i n i t i a l l y l e a d s t c FeO- i s l a n d s and, by s u r f a c e d i f f u s i o n , t o t h e f o r - mation of one o r a few monolayers of FeO. Up t o now, by no method t h e d s t i t e s t r u c t u r e o f t n i s f i r s t c o r r o s i o n product could be s t r i c t l y iden- t i f i e d beczuse of i t s smsli thickness and, pro- bably, i t s d e f e c t s t r u c t u r e . F u r t h e r o x i d a t i o n and growth l e a d t c t h e formation of Ee,O

.,

⁴

and/or

8

-BeZ03 up t c a t h i c k n e s s of 56.. .I00

2

( t h e t a r n i s h l a y e r )

.

Here, some words about Fe 0 and f-Fe203 3 4

a r e necessary. I n many o l d e r p u b i i c z t i o n s , only

Table 1. L a t t i c e parameters of rnetaliic i r o n and i t s main corrosion products.

-

-- -

-

^{- -. . - --}

L . ! D s t r u c -

substance 3 t u r e

c llg/cm

'

. . . . - -

Hi:b6 ^I ""

^{! ! 7.66}

I ^!

^{! bcc}

2 - F e i 3 . 6 4 ' 7 , I : f c c

I ⁱ

^{- l e}

/

^2.932

1

^bcc

p-'eOOH

^10.52

I 1

^{3.028 ' 3.55} t e t r a g . (akaga-

n e i t e ) '

j

^I

I i

FeO 4.30 !

( w i i s t i t e )

I _I

1

a-Xe00ii

!

^{4.bO 10.61}

( g o e t h i t e )

I

! : 5.67 j ^{~ C C}

j ortho- 3.04 j 4 . 2 6 i

;

^rt.omb.

:

I~e00h-&el

/

8.37

1 1

^3.8

1

^cubic

&;

;:

:: ^,

3-65 112.50

I

c r o c i t e )

j 6-F~OOH

1

^{2 * 9 4}

¹

i*-Fe203

/

^5.035

( h e m a t i t e )

I

: ortho-

3.C7 4.09 rhamb.

4.49 i

I /

hexag.

I

8

-F'e203

8.33 (maghemite)

Fe 0

3 4 8.396 (magnetite)

/

Fe(OHj2

j

3.27

t r i g . c u b i c

cubic t r i g . t r i g .

t h e appearance of Fe30d i s reported. Yut t h i s does not mean t h a t 2-Fe,O was absent. decause

2 3

of t h e i r high s t r u c t u r a l s i m i l a r i t y , magnetite and maghemite produce nearly i d e n t i c a l x-ray d i f f r a c t i o n p a t t e r n s . A s o p h i s t i c a t e d l i n e shape a n a l y s i s only y i e l d s some i n s i g h t i n t o t h e d i f f e r e n t vacancy d e n s i t i e s i n t h e s e mate- r i a l s

163.

On the o t h e r hand, due t o t h e d i f - f e r e n c e s of t h e magnetic h y p e r f i n e i n t e r a c t i o n i n t h e s e two o x i d e s , an e x c e l l e n t d i s t i n c t i o n between then! i s p o s s i b l e by iE6ssbacer spectro- scopy. I n some degree t h i s s o l d s a l s o f o r i n - t e r m e d i a t e c o ~ p o u n d s . It i s known t h a t Fe G

3 4 and x-Pe2O3 a r e only t h e f i n a l p o i n t s of a broad v a r i e t y of s p i n e l type i r o n oxides with d i f f e r e n t vacancy d e n s i t i e s . Thus, $ -Fe203 i s che s p i n e l type with a maxircum vacancy d e n s i t y , symbolized e.g. by Fe2.0700.3304 (93 a s com- pared with Pe-0 From t h i s p o i n t of view i t

2 4 .

becomes 2150 c l e a r , why t h e appearance of Ee 0 3 4 i s always an i n d i c a t i o n of an oyygen deficiency e f f e c t i v e during the c o r r o s i o n process. I n

1101

i t was found t h a t the transforniation of Fe 0 3 4 i n t o f -Fe 0 i s c o n t r o l l e d by the d i f f u s i o n of

2 3

i r c n c e t i o n s . k d e t a i l e d study of $-Fe203/

E1e30d by KossUtiuer and Rtntgen spectroscpy i s published i n i i 11.

The s t r u c t u r a l s i m i l a r i t y between d-E e and i t s i n i t i a l o x i d a t i o n products w u s t i t e and magnetite and t h e n o n s i m i l a r i t y u i t h lepidocro- c i t e a r e schernaticaliy skiown i n f i g . 2 . For PeO, Fej04, and f - F e 0 t h e d i f f e r e n c e s i n t h e

2 3'

l a t t i c e c o n s t a n t s a r e small enough t o allow an e p i t a c t i c a l growth of 10.,.?0 atomic l a y e r s without a d i s r u p t i o n due t o the r e s u l t i n g s t r e s s . U n f o r t u n a t e l y , t h i s oxide cover i s not a permanent c o r r o s i o n p r o t e c t i o n , because t h e s e oxides a r e n o t s t a b l e enough. Lnder t h e i n f l u - ence of water vapor, $-r'e2Qj i s converted i n t o f -FepOii. This phase t r a n s f o r m a t i o n i s ccnnec- t e d with a l a r g e i n c r e a s e of t h e mole volume and l e a d s t o a d i s r u p t i o n of t h e oxide l a y e r i n t o s m a l l p a r t i c l e s . decause of t h e i r h i g h d i s p e r s i t y and d e f e c t s t r u c t u r e , t h i s product sometimes behaves l i k e an amorphous o r g e l - l i k e s t a t e C121. The consequences a r e c l e a r : The d e s t r u c t i o n of t h e p r o t e c t i v e oxide l a y e r a l l o w s f u r t h e r o x i d a t i o n of t h e metal, p r o g r e s s of t h e c o r r o s i o n process and, i n p r i n c i p l e , a n i n f i n i t e growth of t h e $-FeOOIi

c1-66 JOURNAL DE PHYSIQUE

Fig.,?. Comparison of the a--Fe lattice (intersections of the straight lines) with the first lattice planes of FeO, Pe304, and y-FeOOH.

phase. The corrosion rate was found to increase with the nonstoichiometry of Fe304, reaching its maximum for nearly stoichiometric (y. -Fe2O3 C87.

Several Wossbauer investigations of corroded surfaces have demonstrated the existence of

-Fe203 as well as a relatively large quantity of $-FeOOH. This holds preferentially for cor- rosion under mild conditions 1131). Under more aggressive (corrosive) conditions, the 8-FeOOH itself will transform into the more stable pro- ducts d -PeOOH and %-Fe203. In fig.3 an energy level diagram is shown which relates the diffe- rent corrosion products to.each otner. ,The exact positions of FeO and $-Fe203 in this scheme are not yet known. Due to the local rearrangement of ions necessary for the transformation into an-

other crystalline structure, some tzansitions, in this diagram are more or less forbidden. The fact that the different oxides and oxihydroxides have a remarkably long lifetime under mild con- ditions shows that for every transition a rela- tively high activation enera is necessary. Eor example, two activation energies are given in fig.3. It should be mentioned that they have been determined from Wdssbauer measurements c 1 5 , 163. In both cases, the transition is diffusion controlled. The corrosion accelerating effect of corrosive conditions is to deliver the acti- vation energy by irradiation or heating or to reduce the activation energy by catalytic influ- ences.

Some years ago, so-called rust-transfor-

Fe(OHI2

a c t i v a t i o n energy 65 kJ/mole

r F e 2 0 3 ? Y

Fe304

0

-FeOOH ?

I

a c t i v a t i o n

g

-FeOOH 12

,

^energy

I

20 kJ/mole

s~-PeOOH 8

Fig.3. Energy l e v e l diagram Of s e v e r a l i r o n c o r r o s i o n products (cf

. ^P

^{41 )}

mers have been o f f e r e d promising a re-transfor- mation of t h e Corrosion products i n t o the pro- t e c t i v e magnetite l a y e r . It seems unreasonable t o expect t h a t the r u s t p a r t i c l e s can be t r a n s - Powed i n t o a closed e p i t a c t i c a l l a y e r of magne- t i t e by such agents. Furthermore, hlossbauer s p e c t r a showed t h a t no phase transformations a t a l l a r e induced by them c771. TEus t h e only ef- f e c t of t h e "rust transformers'' i s t h e i r pene- t r a t i o n i n t o t h e gaps between t h e r u s t p a r t i c l e s .

It i s not p o s s i b l e t o review here a l l Wossbauer s t u d i e s concerning corrosion. ,There- f o r e , a few examples a r e t o be given, which a r e of i n t e r e s t i n connection with t h e described mo- d e l of t h e i r o n c o r r o s i o n process.

F i r s t of a l l i t should be mentioned t h a t from Bnossbauer spectroscopy i t became evident t h a t t h e be-phases maghemite and l e p i d o c r o c i t e play an important r o l e i n t h e i r o n ccrroslcn process. This was' contrary t o r e s u l t s from Ridnt- gen d i f f r a c t i o n s t u d i e s showing only magnetite

i n s t e a d of t h e maghemite c o n t r i b u t i o n s . Kow, Rijntgen difl'ractometry i 8 going t o v e r i f y t h e Xdssbauer r e s u l t s .

A ⁰

-

Several i n v e s t i g a t i o n s have been per- formed by exposure of pure i r o n o r s t e e l samples t o atmospheric r u s t i n g . The Mossbauer spectro- m e t r i c phase analyses always showed t h a t t h e r u s t c o n s i s t s of more than one phase, the re- l a t i v e concentrations of which depend on t h e c o r r o s i v e c h a r a c t e r of t h e atrnos2here: The higher t h e c o r r o s i v i t y , the h i g h e r i s t h e con- c e n t r a t i o n of those phases which a r e found i n t h e lower p a r t of fig.3. Thus, f o r example, the p o r t i o n of a-PeOOEi, a s compared with 8-FeOOti, i s higher f o r exposure i n i n d u s t r i a l d i s t r i c t s than f o r exposure i n f o r e s t d i s t r i c t s C171.

Fe 0 i s t y p i c a l f o r wet conditions, and 3 4

oc-Fe203 f o r very o l d r u s t C17land f o r a higher sunshine r a t e il8J.

-

B. Studies of corrosion beneath a protec- t i v e coating a r e of g r e a t t e c h n i c a l importance.

There i s no o t h e r method but ifiossbauer s c a t - t e r i n g u s e f u l f o r a nondestructive a n a l y s i s of c o r r o s i o n products below a coating. Some exam- p l e s a r e given i n i191. I n Cl81, an i n v e s t i g a - t i o n of the forlnation of t h e d i f f e r e n t corrosion products dependent on the kind, of pigrsents i n t h e c o a t i n g i s reported. The samples were ex- posed t o an atmosphere of high moisture i n a c l o s e d v e s s e l a t room temperature. I n s c a t t e r i n g

geometry the i n t e n s i t y of t h e iliossbauer l i n e s o f t h e c o r r o s i o n products Ic was measured i n re-

l a t i o n t o t h a t of the s t e e l s u b s t r a t e I i e * I n a l l cases curves l i k e i n f i g . 4 were found: After t h e i n i t i a l formation of a doublet, a branching i n t o a doublet and a s e x t e t beginning a t expo- s u r e time to was r e g i s t e r e d , which showed t h e formation of magneticslly ordered c r y s t a l l i t e s . Of s p e c i a l i n t e r e s t i s t h e r e s u l t t h a t the mean s l o p e of t h e curves, i.e. the corrosion r a t e

( c h a r a c t e r i t e d e.g. by t h e a n g l e s ) , depends on t h e kind of pigments a s w e l l a s t h e s a t u r a t i o n value S does, but not t h e time to. to was prac- t i c a l l y the same f o r all. samples. The explana- t i o n of t h i s behaviour sup2orts t h e above men- tioned model f o r t h e corrosion process: The access of a a t e r t o the e p i t a c t i c a l l a y e r on t h e s u b s t r a t e surface depends strongly on t h e kind

(21-68 ^{JOURNAL DE PHYSIQUE}

The q u a n t i t y of loose r u s t formed a f t e r 4 . 5 y e a r s was found t o depend on t h e c o a t i n g , but the phase compositions of t h e r u s t samples d i f f e r e d only s l i g h t l y . Their Xessbauer s p e c t r a , a l l of them s i m i l a r t o those reproduced i n Pig.5, a r e t o be discussed i n more d e t a i l , because they show the problems t y p i c a l f o r many r u s t samples. I n each case a s a t i s f a c t o r y f i t was p o s s i b l e by 3 s e x t e t s ( I , I I , I I I ) and one doublet (IV). To draw c o r r e c t

f 0 f - conclusions from the s p e c t r a , t h e i r temperature

Fig.4. R e l a t i v e W6ssbauer i i n e i n t e n s i t y of cor-

rosion products below 6 coating vs. expo- dependence over a broad range was needed. Disre- s u r e time. The upper branch a f t e r t re- garding s l i g h t q u a n t i t a t i v e d i f f e r e n c e s between p r e s e n t s t h e sum of a l l corrosion p?oducts

(sextet + d o u b l e t ) , t h e l o n e r one (-.-.-) t h e samples, the measured s p e c t r a l parameters corresponds t o the doublet only. y i e l d e d the temperature dependence shown i n fig.6.

of t h e c o a t i n g , and t h e r e f o r e t h e phase t r a n s f o r - ^- The h o r i z o n t a l c h a r a c t e r of the i n t e n s i t y curves mation 8-Fe 0 -8-FeOOH r e s p o n s i b l e f o r a fur- shows t h a t superpar&mgnetic p a r t i c l e s p l w a

3 3

t h e r progress of corrosion depends on the coating. minor r o l e . E-PeOOH i s p r a c t i c a l l y absent (TN%

After t h e d i s r u p t i o n of t h i s l a y e r , a l l t h e form- 290 K ) although i t was expected t o be formed due ed r u s t p a r t i c l e s a r e p r a c t i c a l l y independent t o the i n f l u e n c e of C1-ions. Thus t h e portion from t h e s u b s t r z t e and from t h e coating. They a l l corresponding t o I V is i d e n t i f i e d a s 8-FeOOE f e e l the same conditions (e.g. temperature), which which i s a l s o v e r i f i e d by i t s t r a n s i t i o n i n t o a a r e responsible f o r f u r t h e r phase transformatiofis magnetic phase below 70

d

(TIv=73 K ) . The con- and c r y s t a l i i t e growth, and hence, t is i d e n t i - t r i b u t i o n s I1 with i t s broad and I11 with i t s c a l f o r them. very broad l i n e w i d t h s can be a t t r i b u t e d t o

m-FeOOH e x h i b i t i n g a highly d e f e c t i v e s t r u c t u r e

-

C. I n order t o study more c o r r o s i v e condi- C171. The s e x t e t I i s of some i n t e r e s t . I t s para- t i o n s , s t e e l samples with d i f f e r e n t c o a t i n g s meters d and

5

a t 80 k and 300 K could be e a s i l y were exposed t o an atmosphere kept i n equilibrium i n t e r p r e t e d i n terms of r-E1e203, but t h e para-

Fig.5 Wdssbauer s p e c t r a of r u s t from a sample exposed t o an HC1-containing atmosphere.

t h a 0.5 n HC1 s o l u t i o n a t room temperature. meter changes d r a s t i c a l l y with temperature.

P _L

- 7 - - - 7 - - -

I I 1 I I I I I 1 I I

-10 0 mm/s 10

Fig.6. Temperature dependence of the spectral parameters (cf. fig.5):

I

-

relative intensity (area), H

-

magnetic hyperfine field,

6 -

isomer shift relative to Pe metal, A - quadrupole interaction.

Y

From the latter result it is evident that the subspectrum I is due to smalls-Fe203 crystal- lites, their seizes being large enough to show no superparamagnetism, but small enough to show a reduced magnetic interaction and a reduced Morin temperature. Tinus their seizes are distri- buted mainly within 20C.. .I000

2

C201. The ap- pearance ofoc-Fe203 demonstrates the more corro- sive character of the HC1-containing atmosphere as compared with that used in series Ei described above. Obviously the HCl admixture acts as a ca- talyst in reaching the lowest level in fig.3.

-

D. Special -and very important- ~roblems of corrosion in aggressive media with mechanical load are the behaviour of materials under the in- fluence of friction in the presence of a lubri- cant, and the study of the resulting wear. Al- though WIussbauer spectroscopy is best suited for such studies, practically no application in this field is known so far. On first efforts is repor- ted in this conference (paper by B.D?ehner et al.).

Mttssbauer scattering yields information on the formation of surface compounds by reactions of the metallic substrate with the lubricant as well as on structural alterations in the upper

layers of the metal. Off-line phase analyses can be made from samples treated in standard test machines and also from real bearings. Of course, studies of wear debris are possible by simple transmission measurements. Pirst studies showed the formation of different iron suifides in the reaction layer due to additives in the lubricant 30th the surface reactions and the phase trms- formations in the surface region of the bearing metal were found to be load and temperature de- pendent.

-

E. A series of scattering spectra irom'the inner sursaces of pipes used in a hot-water heating system is reported in

C173.

Only fie 0

3 4' but no $-3e203 and $-FeCGH, was found. This is in agreement with the concept given above that Pe 0 is the corrosion product, which is typical

3 4

for wet exposure conditions leading to a high owgen deficiency. Cbviously, a phase transfor- nation Fe 0 --tFeOOH has a relatively low proba-

3 4

bility in this case. The anticorrosive effect of a phosphate admixture to the water was also in- vestigated. It is of interest that no iron phoa-, phate appeared among the corrosion products, but a large amount of iron carbonate. This behaviour is understandable by consideration of the appro- priate solubility products. They shift the ion equilibrium in the carbo~ate containing water corresponding to

3 CaC03

+

E'e3(P04)2--rc3 PeC03

+

Ca3(P0 4 2 ' ) The reducea corrosion rate due to the phosphate admixture can be drawn easily from the ldssbauer spectra by comparing the intensity ratios of the corrosion product lines and the metallic iron sextet resulting from the uncorroded pipe ma- t erial.

This heating systere mas run ~t 40...50 0 C with drinking water of 12' hardness. Another

system run at 50.. .70°c C211 yielded $-PeOOH as the &in corrosion product. This shows the in- fluence of & somewhat higher temperature (and probably of another steel composition) allowing phase transitions to a lower level in fig.3.

Another example of corrosion inkibition by phosphates is given in f221, where the sur- face layers before and after heating in air were studied by the conversion electron technique.

Here, the formation of different phosphate layers was observed. Very thin corrosion layers

c

1-70 JOURNAL DE PHYSIQUE formed on iron films in a KaN03 solution yield

conversion electron spectre interpreted by the existence o.f mainly m-FeOOIi 1233. If this inter- pretation holds, then the influence of nitrate leads to a very low level in fig.3. The effect of passivation by an alkaline bath on scale forma- tion-was studied in C241. Passivation of iron by a chromate solution results in Fe(II1) in the surface layer and in an incorporation of Cr into

the oxide lattice 1251.

These examples show that Iossbauer spectro- scopy is also a good means for investigating the effect of corrosion inhibitors and of passivation layers. Further examples are found in L261.

-

F. The corrosion of iron containing alloys is a very broad field and still far from being well understood. In C271 electrodeposited Pe-Zn alloys were found to be corrosion resistent in

[iJ M.Shibuya, ~ . ~ n h o , H.Sano, dul1.Chem.Soc.

Japan, 51(3978)1363,

[2] J .P.Schunck, J .NI.Friedt, Y .Llabador, Rev.Phys.App1. 10(:975)121,

131 Zv.Bonchev, A.Jordanov, A.Enitova, Nucl.

1nstr.leth. 70(1969)36,

141 G.P.Huffman, Nucl.Instr.Meth. 137(197b)267,

[s]

W.Jones et al., Appl.Surface Sci. 1 (1978)388, 161 G.l.duffman, H.ri.Podgurski, Oxidation of

Netals 16t1976)377,

[7] I.iLeisel, Proc.5th 1nt.Conf .ltG6ssb.Spectr.

dratislsva I(197312CC;

Proc.Int.Conf.i~cssb.Spectr.ducharest

part by the role played by the Zn-rich $-phase in sacrificial corrosion protection. Of special interest are some low alloy steels, the so-called weathering steels, which show, after initial rusting, a surprisingly high resistence against atmospheric corrosion. h few Mossbauer studies are published concerning this problem [I 2,28,29], but they differ in some results and interpreta- tions. It seems clear that the initial corrosion rate is the same like in normal carbon steels, and that a relatively large portion of a very finely dispersed oxide phase appears. The latter is interpreted as an FeOCH gel in ;I21 and in r281 as

d

-FeOOB. There is no qualitative diffe- rence to the corrosion model given above, because the sppearance of a 6-phase was not verified by further experiments.

il8] Vi.hIeise1, b.PJoln&r, to be published in Vier.kst.Korros., b( 1979):

[21] V.Hamshesh, k.Ravichandran, Radiochem. kadio- anal.Letters 29(1977)87,

[22_] F.J .derry

,

A.G.Kaddock, Workshop Chem.kpp1.

Iossb.Spectr., Seeheim 1978, Abstrscts (Y.Giitlich ed.), p. 106,

[23] ii.0nodera et al., Japan J.Appl.Phys.

;1(1972)1380,

[243 A.V&rtes et al., Proc.Int.Conf.K6ssb.Spectr.

Cracow 1(1975)317,

[8] C.-G.Kestler, dissertation TH Karl-Zarx- Stadt 1977,

[?I

G.HIgg, G.SSder*olm, Z. phys .Chem.

3 29(1935)88,

('101 K.J .Gallagher, O.Feitknecht, U.biirleiler, Nature 217(1968)1118,.

I] H.Annersten, S.S.Hafner, Z.Kristallogr.

137(1?73)321,

[72j h.Kubsch, dissertation dA Freiberg 1974;

H.Kubsch, ii.A.Schneider, Proc.1nt.Conf.lossb.

Spectr. Cracow 1(1975)3~-13, [13] iY.Xeise1, Z.Chem. 1 1 (1971 )238,

1141 H.Schwarz, Vierkst .horimos. 25( 1972)b48,

~.~.3ancroft et al., 0r.Corros.J. c(1971)119, [26] G .Yi.Simmons, ii.Leidheiser in ''Applications

of Mossb.Spectrosco?y ,,Val. I", P,.i.Cohen (edS Acad.Press 1F7b, p.85,

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i:

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,

i17j

W.Ieise1, KBm.koz1. (Budapest) 48(1977)41,