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The Relationship Between Surface Roughness and Corrosion

Conference Paper · November 2013

DOI: 10.1115/IMECE2013-65498

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1 Copyright © 2013 by ASME

Proceedings of the ASME 2013 International Mechanical Engineering Congress & Exposition IMECE2013

November 13-21, 2013, San Diego, California, USA

IMECE2013-65498

THE RELATIONSHIP BETWEEN SURFACE ROUGHNESS AND CORROSION

Alisina Toloei University of Windsor, Dept. of Mechanical,

Automotive and Materials Engineering Windsor, Ontario, N9B 3P4, Canada Tel: +1 (519) 253 -3000 Ext.(5980)

Email: [email protected]

Vesselin Stoilov University of Windsor, Dept. of Mechanical,

Automotive and Materials Engineering Windsor, Ontario, N9B 3P4, Canada

Tel: +1 (519) 253-3000 Ext.(4149) Email: [email protected]

Derek Northwood University of Windsor, Dept. of Mechanical,

Automotive and Materials Engineering Windsor, Ontario, N9B 3P4, Canada Tel: +1 (519) 253-3000 Ext.(4785)

Email: [email protected]

ABSTRACT

There are different parameters which can

affect electrochemical reactions such as type of

electrolyte, velocity, temperature, oxidizing

agents, impurities, anode material type and

surface treatment. It has been shown that pre-

treatment of working electrode (anode) through

abrasion techniques is one of the most important

parameters affecting on Tafel slopes and

consequently corrosion rate. Surface roughness of

the metal surface is a major influence on general

corrosion, nucleation of metastable pitting and

pitting potential as well.

In this study different surface roughnesses

were created on nickel surface by SiC papers and

corrosion properties were compared.

Electrochemical impedance spectroscopy (EIS)

and profilometry tests were carried out on all the

samples and the results were compared with

another sample prepared through laser ablation

method. Corrosion rate values were calculated and

were compared with EIS results for all the

samples and a trend in the effect of roughness on

corrosion protection of nickel was introduced. SEM

and 3D roughness images were taken and

compared for all of the samples before and after

corrosion tests. Different mechanisms were

distinguished for samples created through

different methods. The lower the roughness

values, the more the corrosion resistance. Sample

with patterns created through laser ablation

method showed the best protection properties

compared to other samples.

INTRODUCTION

Nickel is an important metal and it is used in a

large number of industrial applications such as

rechargeable batteries, coinage, filters and is

widely used as an alloying element in ferrous and

nonferrous alloys due to its strength, toughness

and corrosion resistance [1-3]. The corrosion

resistance of nickel is due to the formation of a

passive film on its surface upon exposure to the

corrosive media. Nevertheless, nickel could be

attacked by acidic media at a considerable rate.

2 Copyright © 2013 by ASME

This is why nickel has been the subject of

significant number of studies related to dissolution

and passivation mechanisms in acid medium [4-

6].

Material composition, manufacturing geometry

and roughness have been considered as important

parameters in determining corrosion properties of

the materials [7, 8]. Surface roughness plays a

significant role on the corrosion behaviour of

metals. It has been reported that an increase in

the surface roughness of magnesium alloy AZ91

[9], stainless steels [10, 11], copper [12],

aluminum and titanium-based alloys [13]

increases the pitting susceptibility and general

corrosion rate. Typically, the general and localized

corrosion behaviour of alloys would depend on

their passivation properties. Hence, it is important

to know the passivation behavior of alloys with

different surface finish to correlate the surface

roughness to their general corrosion and pitting

tendency [7, 9]. For metals with the ability to

form a passive layer, a decrease in surface

roughness increases the corrosion resistance but

for the ones with no passive film a reverse trend

has been observed e.g. mild steel [14] and AE44

magnesium alloy [7].

Surfaces with different roughness finishes showed

that increase in the roughness lowers the pitting

potential in 304L stainless steel in chloride

solution [15]. Burstein et.al [16] showed that the

smoother surface in stainless steel is less capable

of propagating metastable pits than the rougher

one, mainly because the sites of pitting on the

smoother surface are on average more open.

Sharland [17] suggested that the local

concentration of a solution was influenced by the

geometry of surface’s peaks and valleys. This in

turn, affected the diffusion of active ions during

corrosion process He also suggests that the

corrosion resistance is closely related to the

distribution of the valleys on the surface. The

significant influence of the valleys on corrosion

resistance is related to the depth of the valleys

which affects the diffusion of active ions during

corrosion [17-20].

There are different methods to evaluate the effect

of surface roughness on corrosion behavior of

metals such as potantiodynamic polarization

method [21] and electrochemical impedance

spectroscopy (EIS). EIS is the most suitable

method for a detailed analysis of electrochemical

reactions mechanisms and kinetics. Impedance

diagrams provide data on the elementary steps

occurring in an electrochemical reaction and on

their kinetics. They also allow a thorough study of

the role of intermediate species adsorbed on the

surface and of reaction mechanisms, as well as a

study of the properties of passive films [22]. The

impedance characteristics of an electrode in acid

solutions depend largely on the type of the surface

pretreatment and surface roughness of the

electrode [10, 23]. In this study the corrosion

behavior of aligned roughness and patterned

surfaces of nickel created through laser ablation

were investigated in dilute sulphuric acid and EIS

was used to study the effect of the formation of

passive layer in both cases.

MATERIALS AND METHODS

Figure 1: 3D topography image for samples G180 (a) before and (b)

after corrosion tests

a

b

3 Copyright © 2013 by ASME

Sample Preparation

15 × 15 × 1 mm high purity (99.7%) nickel

sheets were used as the test material. For

obtaining different surface roughness, the samples

were unidirectionally grinded with different grits of

silicon carbide (SiC) (i.e., 60, 180, 320 and 800)

and the samples were named with letter “G”.

Before use, they were embedded in mounting

materials and mechanically polished. Their active

test area is 1 cm 2 . An additional sample with

predetermined surface pattern was created by

laser ablation method. Patterns were labeled using

DxLy format where D is the hole diameter and L is

the inter-hole spacing (D10L20). To create the

holes, a copper bromide laser was used and single

pulse was applied. Nitrogen gas was blown in

order to protect the surface from oxidation.

All electrochemical experiments were conducted at

room temperature. A classical three-electrode cell

was used with a saturated calomel electrode as

reference and platinum wire as counter electrode.

Prior to testing, the samples were allowed for 30

minutes to reach a stable open circuit potential.

Electrochemical measurements (open circuit

potential, potentiodynamic polarization, and

electrochemical impedance spectroscopy) were

carried out in a 0.5 M H2SO4 solution using a

CHI660D, Electrochemical Workstation Beta,

(version 11.17). A potentiodynamic polarization

technique was used to evaluate the parameters

related to corrosion and EIS measurements were

made in order to investigate the effect of surface

modification on the general corrosion resistance of

the surface. EIS diagrams were then recorded at

the open circuit potential for nickel samples. To

ensure a complete characterization of the

electrode/electrolyte interface and corresponding

processes, EIS measurements were made over six

Figure 2: SEM of the sample G320 (a) before and (b) after corrosion and patterned sample (c) before and (d) after corrosion.

4 Copyright © 2013 by ASME

frequency decades from 10 mHz to 10 kHz at

open circuit (i.e., corrosion) potential and the best

equivalent circuit model was selected. The main

focus of this study is to compare samples with

roughnesses prepared by using SiC papers and

the patterned sample from corrosion point of view,

formation of passive layer and evaluate corrosion

properties of different samples using EIS method.

Equivalent circuit is determined, EIS curves are

fitted and corresponding values of the equivalent

elements are calculated.

In order to measure surface roughness

parameters and obtain 3-dimensional images of

the samples a Wyko Surface Profiling System NT-

1100 was used. Average surface roughness (Ra)

has been used as a roughness parameter in order

to describe the unique features of the surfaces.

Finally to compare the surface structure and

composition before and after potentiodynamic

testing, scanning electron microscopy (SEM) was

performed.

All testing procedures were validated by using a

reference system of pure Ni(99.7%) with surface

roughness <50nm. A detailed description of the

validation procedures can be found in [21]. RESULTS AND DISCUSSION

Roughness measurements:

It is seen in Table 1 that by increasing the grit number, the average roughness value decreased

both before and after corrosion testing but higher

roughness values have seen for all samples after

corrosion testing. The patterned sample however

had higher roughness values and significantly

lower roughness change after corrosion testing

compared to the samples with the unidirectional

grinding roughness.

Table 1: Roughness values for samples before and after corrosion.

Sample

Roughness values

Ra (Before Corrosion

Testing)

(nm)

Ra (After

Corrosion Testing)

(nm)

G60 704.00 1680.00

G180 276.00 802.28

G320 193.80 552.10

G800 41.10 289.58

D10L20 526.10 778.23

For sake of brevity a sample 3D topography image

for samples G180 has been shown as an example

before and after corrosion tests in Fig.1(a) and

(b). As a result of corrosion the uniform area has

changed to a surface with deeper grooves with

higher roughness value. Rougher surfaces with

deeper groves have lower openness (ratio of width

to depth at opening of the grooves) which limit

the diffusion of the corrosive ions out of the

formed grooves, hence have a higher possibility to

grow larger [11]. On smooth surfaces however the

formation of stable passive film is more possible

to occur which will result in less corrosion [20]. In

the patterned sample also there was an increase

in surface roughness after corrosion testing and in

the 3D roughness image there was no significant

change in the surface appearance which will be in

agreement with the results of the corrosion rate,

EIS and SEM analysis. Scanning electron microscopy

Figures 2(a, b). illustrate SEM images of

sample G320 before and after corrosion as one of

the examples of samples with unidirectional

roughness. It is obvious that the corrosion is more

significant along the grooves. Lee, et.al, also

reported that the deep valleys on the ground

surface are favorable sites for pit nucleation [18].

For rougher surfaces more surface degradation

was observed after corrosion testing. The results

are in agreement with the corrosion rate, EIS and

roughness observations. Figures 2(c) and 2(d)

show the microstructure of patterned sample

before and after the corrosion tests. No severe

corrosion on the surface is observed in the SEM

images. This is also in a good agreement with the

measured electrochemical values. The reason for

this phenomenon is due to the formation of

passive oxide layer and existence of air pockets

inside the holes [24] Electrochemical measurements:

Corrosion rate (CR) values were calculated

using the Tafel extrapolation method and are

summarized in Table 2. As it is seen, by

decreasing the roughness value from sample G60

to G800 the CR values decreased. It has been

suggested that this variation in CR were primarily

due to the anodic behavior of the alloy [9, 21].

5 Copyright © 2013 by ASME

Table 2: Corrosion parameters for samples with different

roughness

Sample Ecorr

(mV)

βa

(mV)

βc

(mV)

Corrosion

Rate

(mil/year)

G60 -319 104.7 114.6 9.53

G180 -325.5 111.2 113.8 8.35

G320 -300.8 100.0 112.1 7.96

G800 -272.9 82.3 107.9 5.48

D10L20 -327.4 153.4 128.5 2.61

The patterned sample created by laser ablation

method had the lowest value of the corrosion rate

compared to the samples with unidirectional

grinding roughness (Blue point in Fig.3). The

authors have shown that in patterned sample

alternating solid/liquid zones, stable air/vapour

pockets and a passive oxide layer has been

formed and the existence of air/vapor pockets

prevented the dissolution of this layer. Therefore,

the contact area between the corrosive solution

and the substrate has decreased which

consequently results in decrease in the corrosion

rate [24, 25].

Nyquist and Bode plots are the two common

methods for displaying EIS data. The Nyquist

representation has the real part of the complex

impedance plotted on the X-axis and the

imaginary part on the Y-axis. Figure 4 shows the

Nyquist impedance plots of nickel with different

surface roughness in 0.5M sulphuric acid. The

plots of nickel shown in this figure consist of

distorted semicircles. A similar behavior is

observed for all samples. As seen, the size of the

semicircle, increases with decreasing roughness.

The Nyquist plots were analyzed by fitting the

experimental data to the equivalent circuit model

shown in Fig.5. In this circuit Rs represents the

solution resistance; Rct is the charge transfer

resistance and CPE is constant phase element

related to the double-layer capacitance (Cdl).

Figure 5: Schematic for the equivalent circuit of nickel [26-28].

Rs is in a series with a parallel combination of a

constant phase element CPE (Q) and Rct. Q is used

instead of Cdl to account for the depression of the

capacitive loop [29]. The CPE is a generalized

frequency dependent element which impedance is

given by:

Figure 4: Nyquist plots of nickel for different surface roughness

Figure 3: Change of corrosion rate for samples with different

roughness values

6 Copyright © 2013 by ASME

ZCPE=1/(Q(iω) n

)

(1)

Where, i = (-1) 1/2

, ω is the angular frequency, ω =

2πf and f is the frequency. n=0 corresponds to a

pure resistor, n=1 to a pure capacitor and n=0.5

to a Warburg type impedance [30]. Changes in n

values have been related to diffusion process,

porosity and roughness [31].

By plotting the Nyquist diagrams, it is seen that all

diagrams are characterized by depressed

capacitive loop with the theoretical center located

below the real axis. This feature reflects surface

inhomogeneity which results from surface

roughness of structural or interfacial region. These

results are consistent with literature [23, 27]. This

shape of the Nyquist plots suggests that charge

transfer controls the corrosion of Ni in acid

solutions [28]. The capacitance loops in Figure 4

enlarge by the decrease of roughness which

indicates that the corrosion is mainly a charge

transfer process [32]. It means that the resistance

is proportional to the decrease of roughness.

Namely, the lower the roughness, the higher the

resistance. It is worth noting that the decrease of

roughness in H2SO4 solution does not change

substantially the shape of the plots but increases

the impedance. This observation confirms the

suggestion that roughness does not alter the

mechanism of corrosion reaction but decrease the

corrosion by retarding the charge transfer. [28]

The same can be seen in the Bode plots which

represents a log/log plot of the magnitude of

impedance plotted on the Y-axis and the angular

frequency plotted on the horizontal axis. As it is

seen in Fig.6, the total impedance of smooth

nickel samples in solutions is relatively high

compared with rough samples. The patterned

sample again shows highest impedance value.

Bode plots show only one phase maximum at

intermediate frequencies. This result indicates that

the corrosion process occurs via one step

corresponds to one time constant [28]. The Bode

plot show that by decreasing the roughness value

maximum phase angle shifts to lower frequencies

and the polarization resistance increases resulting

in less corrosion [27]. According to Fig.7, for

lower roughness values the phase angle is around

80° suggesting that the electrochemical process

occurring at high frequency decreases the

corrosion rate [27]. It is also said that the

maximum phase angle θmax is less than 80° as a

result of the roughness of the electrode surface

[28]. Corrosion of Ni in H2SO4 solution also

enhances the roughness of the electrode surface

and therefore reduces the value of θmax.

Therefore, higher corrosion rate is related to

Figure 6: Bode plots for nickel for different surface roughness

7 Copyright © 2013 by ASME

higher roughness value. Thus, the lowest value of

θmax is related to more corroded sample which is

the one with the highest roughness [28].

The EIS parameters of Ni in 0.5 M H2SO4 with

different roughnesses are given in Table 3.

Increasing Rct values with decrease of the

roughness, for nickel, suggests a decrease of the

corrosion rate since the Rct value, is a measure of

electron transfer across the surface, and it is

inversely proportional to the corrosion rate [26].

Similar trend has been reported by Hong et.al. in

their potantiodynamic tests and EIS analysis that

says the total number of the surface sites

available for metastable pits on the electrode at a

given potential decreases with increasing the grit

number of the silicon carbide paper for final

surface grinding and it implies that metastable pit

or pits starting to grow on the smoother surfaces

is more difficult than that on the rougher surfaces

[10]. The smooth surfaces also have fewer places

for pit nucleation and can quickly form a passive

film [7]. Rough surfaces also limiting diffusion out

of the grooves or forming pits and also are able to

trap the corrosive ions and the corrosion products,

allowing for more pitting to occur on the semi-

polished samples [10, 11, 20]. In the fitting

method, a combination of randomize followed by

Levenberg-Marquardt fitting, was used [33].

Based on equivalent circuit model in Figure 5, the

plots are best fitted and the fitting results are

shown in Fig.8. as solid lines passing through the

testing results.

There is a good agreement between experimental

and fitted impedance spectra as well as a good

correlation between the corrosion rate and 𝑅ct values.

In the case of the patterned sample, according to

the author’s work, the retention of the passive

oxide layer inside the holes doesn’t let the fluid

reach the bottom of the patterned hole. It has

been resulted in less contact area between

solution and the substrate and consequently will

result in smaller corrosion rate of the patterned

sample [24].

Table 3: values of the equivalent elements

Sample Rs

(Ω)

Rct

(Ω)

CPE

(Ω -1

cm -2

S n )

n

G60 7.99 1664.2 5.58E-05 0.898

G180 9.21 3514.7 3.76E-05 0.922

G320 8.56 3764 4.51E-05 0.920

G800 8.68 4463.1 4.57E-05 0.920

D10L20 8.98 4733.5 4.52E-05 0.922

Figure 7: Bode phase plots for nickel for different surface roughness

8 Copyright © 2013 by ASME

Figure 8: Fitted curve for the selected equivalent circuit

CONCLUSIONS:

The effect of different roughness created on

nickel surface was investigated through EIS, Tafel

extrapolation method, roughness measurement

and SEM. A smoother surface in the case of the

unidirectional rough surfaces will result in lower

corrosion rate of nickel samples. So, samples with

lower roughness act as better barrier to

penetration of the aggressive electrolyte to the

metal substrate. This is mainly due to the fact that

nickel forms a stable passive layer/film which

significantly reduces further mass loss(corrosion).

The introduction of the unidirectional roughness

effectively increased the area of the contact

surface between the electrolyte and the metal

surface, which in turn led to increase of the

corrosion rate. In contrast, in the patterned

surface has the highest roughness but showed the

smallest corrosion rate compared to the

unidirectional roughness samples. This result was

consistently repeated in the roughness

measurements, EIS analysis, potentiodynamic

polarization tests and SEM images. The clear

conclusion based on these results is that the

corrosion mechanism governing the

electrochemical process in the patterned surface

and the unidirectional surface roughness are

different. The most likely explanation of the

superior performance of the patterned surface is

the formation of heterogeneous wetting interface

(trapped air pockets within the surface holes),

which significantly reduced the electrolyte/metal

surface area and the corresponding corrosion rate

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