Characterization of surface layers on bronze in aqueous solutions

Surface Technology, 21 ( 1 9 8 4 ) 125 - 1 3 5

125

C H A R A C T E R I Z A T I O N OF S U R F A C E L A Y E R S ON BRONZE IN AQUEOUS SOLUTIONS G. B R U N O R O

Aldo Dacc6 Corrosion Study Centre, University of Ferrara, Ferrara 441 O0 (Italy) G. G I L L I a n d R. N A G L I A T I

Chemical Institute, University of Ferrara, Ferrara 44100 (Italy) (Received J u n e 6, 1 9 8 3 )

Summary Chemical compositions and morphological characteristics of anodic or free-corrosion products formed on bronze surfaces in HCO3--H2COa aqueous buffer solutions were studied by means of the X-ray diffraction technique and scanning electron microscopy observations. In oxygenated CO2containing solutions (pH 4), only the formation of poorly protective Cu20 layers was found. In near-neutral or slightly alkaline NaHCO3 solutions the initial formation of small crystals of cuprite was followed by the growth of adherent, continuous, compact and crystalline layers of malachite. The influence of the addition of chloride or sulphate anions on the nature of the bronze surface layers was also considered. The presence of chloride ions increased b o t h the dissolution process of the metal and the growth of the surface layers. The composition and the physical characteristics of the surface oxidation products depended on the imposed anodic potential and on the nature of the added anions (C1- or SO42-).

1. Introduction The oxidation processes of copper-base alloys in aqueous media normally cause the formation of surface layers whose chemical compositions and physical characteristics depend broadly on the nature, salinity, pH, oxygen content and temperature of the aggressive solutions. In a study of the thermodynamic stability of copper and of some of the main copper c o m p o u n d s in water containing dissolved oxygen and CO2, Ires and Rawson [1] pointed o u t the extreme vulnerability of this metal to attack and the inadequate protective properties of solid corrosion product films. It is well known, however, that some kinds of natural [2 - 4] or artificial [5, 6] surface layers show a high efficiency in reducing the corrosion rate of copper-base materials in various aggressive environments. 0376-4583/84/$3.00

© Elsevier S e q u o i a / P r i n t e d in T h e N e t h e r l a n d s

126 In the present work, the results of experimental research on the chemical and morphological characteristics of surface products formed on bronze in bicarbonate-containing aqueous solutions even in the presence of chloride or sulphate ions are reported.

2. Experimental details The cast bronze used as a test material was composed as follows: 7.4 wt.% Sn; 3.1 wt.% Pb; 1.2 wt.% Zn; 0.8 wt.% Ni; 0.03 wt.% Fe; balance, copper. Scanning electron micrographs of cross sections of the bronze rods showed a dendritic structure and the presence of roundish precipitates. Electronic microprobe maps of alloying element distributions showed an increase in the tin and a decrease in the copper and zinc content in the dendritic phase (in comparison with the average composition). In contrast, lead and zinc were present in prevailing quantities in the isolated precipitates, whereas the tin content was depleted. Disks about 20 mm thick obtained from bronze rods (diameter, 25.2 mm) were embedded in Epophix resin to obtain an exposed surface area of 5 cm 2. Aggressive media were constituted by aqueous buffer solutions of various pH values obtained by changing the HCOf-to-H2CO3 ratio according to the following scheme: pH 4.0 + 0.2, CO2 (1 atm); pH 5.0 + 0.2, CO2 (1 atm) with 0.01 N NaHCO3; pH 6.5 + 0.2, CO2 (1 atm) with 0.05 N NaHCO3; pH 6.8 + 0.2, CO2 (1 atm) with 0.1 N NaHCO3; pH 8.5 + 0.2, 0.1 N NaHCO 3. In some tests performed at pH 4 and 8.5, 0.1 N C1- and 0.1 N SO4~- ions were also added to the buffered solutions. The increases in pH values (monitored throughout the duration of the experiment) were usually found to be limited to within 0.5 pH units. The largest shifts (1 pH unit) were observed in 0.1 N NaHCO 3 solutions. Potentiodynamic anodic polarization curves (0.5 mV s-1) for various experimental conditions were recorded on bronze electrodes after immersion for 30 min in the solutions. The formation of surface layers in various aggressive solutions was obtained by anodic potentiostatic polarization (168 h ) i n the potential range between --50 and +300 mV (measured with respect to a saturated calomel electrode (SCE)}. Surface layer formation was also tested on bronze specimens in free-corrosion conditions after immersion for more than 300 h. In these test series, solutions into which oxygen was bubbled were used. At the end of both these test series, the nature of the surface products was determined by X-ray diffraction analysis. Solid phases present in the corrosion layers were identified by means of their Joint Committee on Powder Diffraction Standards (JCPDS) powder diffraction file. Formulae, c o m m o n names and conventional symbols used in the text, together with the JCPDS numbers, are listed in Table 1. The growth of anodic surface layers under the various experimental conditions was also monitored with a scanning electron microscope.

127 TABLE 1 Details for the solid phases present in the corrosion layers Formula

C o m m o n name

Symbol

JCPDS file

Cu20 Cu2(OH)2CO3 PbCO3 CuC1 Cu 2(OH ) 3Cl Cu4(OH)6SO4 Cu4(OH)6SO4* H20 PbSO4

Cuprite Malachite Cerussite Nantokite Atacamite Brochantite Posnjakite Anglesite

C M Ce N At B P An

5-667 10-399 5-417 6-344 2-146 3-282 ; 13-398 20-364 5-577

3. Results and discussion 3.1. P o l a r i z a t i o n c u r v e s

The anodic polarization curves recorded on bronze electrodes in bicarbonate solutions at pH 8.5 show a slight inflection point at 20 mV (SCE). For more noble potential values a wide interval of passivity is observed, probably due to the existence of a surface layer (Fig. 1).

0.4.

0.2

V/SCE

-0"2 f

,

~ i

c

,~)~

~,'A

,~4

Fig. 1. Anodic polarization curves on bronze in aqueous 0.1 N N a H C O 3 solution (pH 8.5): curve a, no addition; curve b, 0.1 N N a 2 S O 4 addition; curve c, 0.1 N NaCl addition.

128 0.2.

v/s c E

-0.;

-0.4 i

i

10 2

i

~A

!

10 4

Fig. 2. A n o d i c p o l a r i z a t i o n curves o n b r o n z e in CO2-saturated a q u e o u s s o l u t i o n s ( p H 4): c u r v e a, n o a d d i t i o n ; curve b, 0.1 N Na2SO4 a d d i t i o n ; curve c, 0.1 N NaCl a d d i t i o n .

The addition of chloride ions (0.1 N NaCl) to the bicarbonate solution provokes a shift in the free-corrosion potential towards more active values and an increase in the anodic current. This current increase is particularly apparent for potential values more noble than 50 mV (SCE). The acceleration of the anodic process may be attributed to the intervention of chloride ions in the alloy dissolution mechanism. The addition of sulphate ions (0.1 N Na2SO4) to the bicarbonate solution does n o t exert a notable influence on the shape of the anodic polarization curve. Only a certain reduction in the passivity range is shown. The anodic polarization curves recorded in solutions maintained at pH 4 by the bubbling of CO2 are collected in Fig. 2. In comparison with the blank test (CO2 bubbling in distilled water) the addition of sulphate ions promotes a sharp increase in the anodic current. In the presence of chloride ions the free-corrosion potential of the bronze electrode is more active and the anodic current is further increased. Since the immersion time was short, an analysis of the polarization curves only gives information about the initial anodic dissolution process of the alloy. Furthermore, it is possible to detect within what potential range anodic surface layer formation may take place.

3.2. Accelerated surface layer formation The formation of surface patinae by simple immersion of copper-base alloys in slightly aggressive solutions usually requires a very long time. There-

129 fore, accelerated tests were p e r f o r m e d either by the potentiostatic stimulation o f the anodic process or by increasing the a m o u n t of oxygen dissolved in the aggressive solutions. A first series o f potentiostatic measurements was carried o u t in bicarbonate solutions with pH 6.8 or 8.5. Table 2 shows the X-ray analysis data of the anodic products obtained at various imposed electrode potentials. These results are in reasonable agreement with the equilibrium p o t e n t i a l - p H diagrams for the ternar y system C u - C O 2 - H 2 0 at 25 °C [7] and indicate that, by increasing the anodic polarization at the examined pH values, malachite is the most stable oxidation surface product. A second series of potentiostatic measurements was p e r f o r m e d at a selected potential (150 mV (SCE)) over a wider range o f pH values. Keeping in mind the possible multilayer f o r m a t i o n on the metallic surface, the nature of the out er and inner layers was determined. The X-ray analysis data of the o u t e r and inner anodic layers f o r m e d on bronze electrodes in solutions whose pH values were determined by the HCO3--to-H2CO 3 ratio are listed in Table 3. It can be observed that only cuprite is present at lower pH values, whereas malachite is p r e d o m i n a n t at higher pH values.

TABLE 2 Nature of the surface products formed on bronze at various anodic polarization values in HCO3--H2CO3 buffer solutions a Potential

Surface products formed for the following pH values

(mV (SCE))

pH 6.8

pH 8.5

Ecorr b

--

M

--50 50 150 300

C M M M

M M M M

aTime, 168 h; temperature, 30 °C. bEcorr (30 °C) = --101 mV (SCE).

TABLE 3 N a t u r e of the anodic layers formed in solutions with various pH values determined by t h e r e s p e c t i v e HCO3--to-H2CO3buffer r a t i o a pH

4.0

5.0

6.5

6.8

8.5

Outer layer Inner layer

C C

C C

M C

M M

M M

aTime, 168 h ;controlled electrode potential, 150 m V (SCE); temperature, 30 °C.

130

.2o

60.

40 @

,c

10 u

k. Q. :3 L)

20 []

0

4•

[]

I

I

I

I

I

5

6

7

8

9

0

pH

Fig. 3. Relative amounts (arbitrary units) of cuprite (A) and malachite (D) in the outer anodic layers of the bronze electrodes estimated by X-ray analysis (see Table 3).

The relative amounts of cuprite and malachite present in the outer anodic layers as a function of pH are plotted in Fig. 3. The relative amounts are expressed as ratios of the main diffraction line heights of the products with respect to the main diffraction line height of the underlying alloy. At pH 4 the anodic oxidation produced a thick and uniform layer of cuprite. The a m o u n t of cuprite was rapidly decreased by increasing the pH of the solution; at pH 6.5 the presence of the cuprous oxide was detected by X-ray diffraction only on the inner layer of the surface products after removal of the external malachite. At pH values of 6.8 and 8.5, malachite was the only anodic product detected by X-ray diffraction analysis. The thickness of the surface layer reached its maximum value when obtained in near-neutral solutions. The growth of anodic surface layers was also monitored by scanning electron microscopy (SEM) observations. Different bronze electrodes were polarized at 150 mV (SCE) in solutions with pH values of either 4 or 8.5 for increasing immersion times {ranging from 48 to 420 h). The specimens removed at different times were dried and analysed by SEM. In agreement with X-ray data, the only observed anodic product at pH 4 was cuprite which developed in small roundish cubes that almost completely covered the metal surface in 168 h {Figs. 4 and 5). The SEM data obtained on surface layers grown potentiostatically at pH 8.5 yielded some information about the formation of multilayers. In fact, an initial corrosion attack was observed with the growth of small scattered cubes of cuprite (3 - 8 pm) on the clean surface. The first crystals of malachite appeared only after 80- 120 h as very small acicular crystals aggregated in rosettes 100 - 200 pm in size {Fig. 6). Afterwards the rosettes spread on the surface, giving a continuous layer of aggregates within 300 350 h (Fig. 7). Thus it may be concluded t h a t cuprous oxide is formed at all pH values investigated, but the quantity of this c o m p o u n d at higher pH

131

Fig. 4. An SEM view of cuprite crystals formed on a bronze surface in CO2-saturated distilled water (pH 4) (experiment time, 168 h; controlled electrode potential, 150 mV (SCE); temperature, 30 °C). (Magnification, 2680x.) Fig. 5. An SEM view of the continuous cuprite layer formed on a bronze surface: conditions as for Fig. 4. (Magnification, 134x.)

Fig. 6. An SEM view of cuprite crystals (small cubes) and aggregates of malachite crystals (rosettes) formed on a bronze surface in an HCO3--H2CO3 buffer solution at pH 8.5 (experiment time, 145 h; controlled electrode potential, 150 mV (SCE); temperature 30 °C). (Magnification, 281x.) Fig. 7. An SEM view of the continuous malachite layer formed on a bronze surface: conditions as for Fig. 6 (experiment time, 340 h). (Magnification, 281×.) v a l u e s is n o t s u f f i c i e n t f o r d e t e c t i o n b y X - r a y d i f f r a c t i o n a n a l y s i s . T h e d a t a o f T a b l e 4 r e l a t e t o t h e n a t u r e o f t h e s u r f a c e l a y e r s f o r m e d o n b r o n z e specim e n s freely c o r r o d i n g in s o l u t i o n s with b u b b l i n g o x y g e n c o n t a i n i n g bicarb o n a t e in the absence or the presence of chloride or sulphate ions.

132 TABLE 4 Nature of the corrosion layers formed in oxygenated HCO3--H:CO3 buffer solutions even in the presence of C1-or SOa2- anions a Added anion

Nature o f layers at pH 4.0

Nature of layers at pH 8.5

Outer layer

Inner layer

Outer layer

Inner layer

--

C(+)

C(+)

M(-- --)

M(-- --)

0.1 N NaC1 0.1 N Na2S04

M(++); C(-- --) C(+)

C(+); M(-- --) C(+)

M(+) M(++)

M(+) M(++)

aExperiment time, 300 h; free,corrosion condition; temperature, 30 °C. b++, +, _ and - - - indicate the amount of the compound as approximately evaluated by the height of the diffraction lines. TABLE 5 Nature of the anodic layers formed in HCO3--H2CO 3 buffer solutions even in the presence of C1- or SO42- anions a Added anion

Nature o f layers at pH 4.0 Outer layer

Nature of layers at pH 8.5 Inner layer

Outer layer

Inner layer

--

C(++)

C(++)

M(+)

M(+)

0.1 N NaC1

At(++); Ce(++)

N(++); At(++)

0.1 N Na2SO4

M(++); P(++); C(--); An(--)

C(++); P(--); An(--)

N(+); M(--); C(--); At(--) B(++) and/or P(++); M(+); An(-- _)b

N(+); M(--); C(--); At(--) C(++); An(+); M(-- --)

aExperiment time, 168 h; controlled electrode potential, 150 mV (SCE); temperature, 30 °C; symbols as in Table 4. bIncoherent layer: analysis of the powder. The d a t a o b t a i n e d for t h e same solutions b y c o n t r o l l e d p o t e n t i a l tests are listed in Table 5. The d a t a o f Tables 4 and 5 will be discussed simultan e o u s l y . It can be observed t h a t in t h e absence o f - a d d e d anions o t h e r t h a n b i c a r b o n a t e , free-corrosion e x p e r i m e n t s lead to t h e same surface p r o d u c t s as t h o s e o b t a i n e d in c o n t r o l l e d p o t e n t i a l tests. Nevertheless, higher relative a m o u n t s o f m a l a c h i t e were f o u n d in t h e p o t e n t i o s t a t i c tests as a result o f t h e s t i m u l a t i o n o f t h e a n o d i c process. In free-corrosion e x p e r i m e n t s at t h e highest p H value (pH 8.5) a t h i c k e r layer o f m a l a c h i t e was d e t e c t e d w h e n chloride ions were present t h a n w h e n t h e y were absent. This fact testifies t o t h e stimulating a c t i o n o f chloride ions in the c o p p e r alloy c o r r o s i o n . This stimulating a c t i o n increases t h e availability o f cupric ions necessary f o r m a l a c h i t e f o r m a t i o n . However, the e f f e c t o f chloride ions on t h e dissolution process is well illustrated b y t h e h e a v y

133

attack developed on the bronze surfaces. Under potentiostatic polarization at pH 8.5, chloride ions clearly influence the nature of the anodic surface layer. Relevant amounts of cuprous chloride (nantokite) were detected on the surface of the alloy, while the most heavily attacked electrodes showed significant formation of cupric oxychloride (atacamite) together with small amounts of cuprite and malachite. The composition of the surface layers formulated at the lowest pH value (pH 4) is different depending on the type of test. Under free-corrosion conditions (Table 4), the presence of chloride ions stimulates the dissolution process causing the precipitation of malachite which covers the usual cuprite layer, in spite of the low pH value. At a controlled potential (Table 5), the inner layer of copper oxidation product consists of nantokite, whereas the outer layer contains Cu2(OH)3C1 {atacamite). As a result of the heavy attack of the lead-containing alloy, PbCO3 (cerussite) is found with the atacamite. The combined influence of the chloride ions and the electrode potential on the nature and composition of the surface layers is confirmed b y a comparison of the anodic polarization curves recorded at pH 4 in the absence and in the presence of chloride ions (Fig. 2). The addition of sulphate ions to the solutions containing HCO3--H2CO 3 at both the pH values investigated does not change the nature of the surface corrosion products. In fact, malachite is found at pH 8.5, whereas only cuprite is present at pH 4.0. In the potentiostatic tests the simultaneous influence of the imposed electrode potential and the presence of sulphates p r o m o t e the formation of insoluble basic copper sulphates on the surface, mixed with malachite in the external layer. In these cases, the anodic attack on the electrode surfaces is very severe, the surface layers are loosely adherent and significant amounts of the anodic products are found on the b o t t o m of the cell.

4. Conclusions It is confirmed [8, 9] that the behaviour of copper-base alloys in aqueous solutions is strictly correlated with the nature of the surface layers. The results discussed above on the basis of corrosion measurements, electrochemical tests and SEM observations emphasize the protective properties of the surface layers obtained on bronze in HCO3--H2CO3 solutions at neutral or slightly alkaline pH values. In fact, the initial stage of formation of small crystals of cuprite is followed b y the growth of adherent, continuous, compact and crystalline layers of malachite. At the lower pH value in oxygenated CO2-containing solutions (pH 4), only the formation of Cu20 is found. Under these conditions, the protective properties of the Cu20 layer are poor. This may be explained b y the suggested mechanism involving a dissolution stage limiting the thickness of the layer according to the following reactions [ 10]:

134 Cu + ½ H 2 0 = ½Cu20 + H + + e-

(1)

½ C u 2 0 + -~ 1 02 + 2H + = Cu2+ + H 2 0

(2)

Cu2+ + 12H 2 0

(3)

+ e- = ½Cu20 + H +

i.e. the overall r e a c t i o n is

Cu + -~O21 = ½Cu20 A net increase in the corrosive a t t a c k o f the alloy is f o u n d at b o t h p H values as a c o n s e q u e n c e o f t h e a d d i t i o n o f chloride or sulphate anions. The n a t u r e o f t h e surface p r o d u c t s o n b r o n z e appears to d e p e n d o n the i m p o s e d p o t e n tial and on the t y p e o f anion. In t h e presence o f sulphate ions, c u p r i t e is still the first c o m p o u n d f o r m e d on metallic surfaces f o l l o w e d b y t h e a p p e a r a n c e o f b r o c h a n t i t e a n d / or p o s n j a k i t e ( m i x e d with malachite) on the o u t e r surface layer. This is possible o n c e the solubility p r o d u c t of the basic c o p p e r ( I I ) salts has been achieved. In c o n t r a s t , the presence o f chloride ions gives rise t o n a n t o k i t e as the first o x i d a t i o n p r o d u c t . By successive h y d r o l y s i s o f this c o m p o u n d (stable in acidic solutions up to p H 3.5 - 4 [11, 1 2 ] ) C u 2 0 or Cu2(OH)aC1 can be o b t a i n e d d e p e n d i n g on t h e pH value a n d the o x y g e n c o n t e n t o f the s o l u t i o n [ 5 ].

Acknowledgment This w o r k was s u p p o r t e d financially b y the Consiglio Nazionale delle Ricerche, R o m e .

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