1

FACTORS AFFECTING THE NANO-SCALE INVESTIGATION OF

PASSIVE LAYER FOR CORRODING STEEL BARS IN CONCRETE

UNDER SEVERE ENVIRONMENTAL CONDITIONS

Raja Rizwan HUSSAIN (1), Abdulrahman ALHOZAIMY (2), Abdulaziz Al

NEGHEIMISH (3) and Rajeh Al ZAID (4)

(1) Asst. Professor, Center of Excellence for Concrete Research and Testing, Civil

Engineering Department, King Saud University, Riyadh, Saudi Arabia.

(2) Professor, Center of Excellence for Concrete Research and Testing, Civil Engineering

Department, King Saud University, Saudi Arabia.

(3) Executive Director and Associate Professor, Center of Excellence for Concrete Research

and Testing, Civil Engineering Department, King Saud University, Riyadh, Saudi Arabia.

(4) Professor, Center of Excellence for Concrete Research and Testing, Civil Engineering

Department, King Saud University, Riyadh, Saudi Arabia.

Keywords:

Nanotechnology, Corrosion, Passive Layer, SEM, XRD, EDS, Reinforced Concrete.

Author contacts

Authors E-Mail Fax Postal address

First Author raja386@hotmail.com +966-1-4696355

PO Box: 800, CoE-CRT,

King Saud University,

Riyadh, 11421, Saudi Arabia.

Second Co-author +966-1-4696355

PO Box: 800, CoE-CRT,

King Saud University,

Riyadh, 11421, Saudi Arabia.

Third Co-author +966-1-4696355

PO Box: 800, CoE-CRT,

King Saud University,

Riyadh, 11421, Saudi Arabia.

Fourth Co-author +966-1-4696355

PO Box: 800, CoE-CRT,

King Saud University,

Riyadh, 11421, Saudi Arabia.

Contact person for the paper: Dr. Raja Rizwan Hussain, Asst. Professor, Center of Excellence

for Concrete Research and Testing, Civil Engineering Department, King Saud University.

e-mail: raja386@hotmail.com, Tel:+966-562-556969, Fax: +966-1-4696355,

Postal address: PO Box: 800, Riyadh, 11421, Saudi Arabia.

2

FACTORS AFFECTING THE NANO-SCALE INVESTIGATION OF

PASSIVE LAYER FOR CORRODING STEEL BARS IN CONCRETE

UNDER SEVERE ENVIRONMENTAL CONDITIONS

Raja Rizwan HUSSAIN (1), Abdulrahman ALHOZAIMY (2), Abdulaziz Al

NEGHEIMISH (3) and Rajeh Al ZAID (4)

(1) Asst. Professor, Center of Excellence for Concrete Research and Testing, Civil

Engineering Department, King Saud University, Riyadh, Saudi Arabia.

(2) Professor, Center of Excellence for Concrete Research and Testing, Civil Engineering

Department, King Saud University, Saudi Arabia.

(3) Executive Director and Associate Professor, Center of Excellence for Concrete Research

and Testing, Civil Engineering Department, King Saud University, Riyadh, Saudi Arabia.

(4) Professor, Center of Excellence for Concrete Research and Testing, Civil Engineering

Department, King Saud University, Riyadh, Saudi Arabia.

Abstract

The presence of ambient hot weather and chloride ions in the soil, ground water, as well as in

the concrete raw materials is a major cause of corrosion in reinforced concrete structures.

Chloride ions and high temperature break the passive film on the reinforcement steel surface

in concrete protecting the steel from corrosion which is believed to be in nanometers and

primarily composed of iron oxides. However, little is known about the chemical composition

and the structure of the passive film as well as its breaking process. This makes it difficult to

classify corrosion, proven by the fact the chloride threshold value of steel measured by

conventional electro-chemical techniques under variable temperature conditions can vary

greatly. While this technique measures corrosion in a macro scale, the growth and

deterioration of passive film actually takes place on nano-scale and is governed by the

elemental compositions and nano-microstructure of the steel as well as the chemistry of the

concrete pore solutions around the rebar. This paper focuses on factors and limitations

affecting the characterization of passive layer at the nano-scale using different techniques. To

address these key issues, factors and limitations affecting the nano-techniques (such as SEM,

XRD, EDS/EDX, XRF, Metallography) have been addressed to obtain a more precise

characterization of passive film as well as its breaking in terms of nano-micro structural

material properties. The nano-scale investigations conducted in this paper revealed that the

steel bars with passive oxide layer have an orderly structure in contrast to what was expected.

A comparison of steel bar with passive layer showed a uniform repeating pattern in contrast to

the surface without passive layer at the nano-level. This is an interesting and novel finding

which will be further explored and reported in the future. Three Iron-oxides (β-Fe2O3, Fe0.92O

3

and Fe3O4) as well as Iron hydroxide (Fe (OH)3) were identified in this passive layer.

However, the results presented in this paper are preliminary and will be elaborated in detail in

the future.

1. INTRODUCTION

Despite the substantial amount of research work reported, quantitative nano-scale

investigation of passive layer characterization and its breakdown for corroding steel in

concrete under severe coupled environmental conditions of chloride and high temperature has

yet to be fully explored. Furthermore, it was found from the previous data that there exists a

difference of opinion among various researchers. Conducted at the level of the atom and

molecule, the scale of such research is generally ten times the diameter of a water molecule,

i.e., one billionth of a meter: a nanometre. Corrosion damage investigation in RC (reinforced

concrete) structures has become important field of study in civil engineering, with the goal of

continuous and periodic assessment of the safety and integrity of the corroded civil

infrastructure. For decades, the struggle to deal with the detrimental effects of several

environmental loadings on RC structures such as hot weather in countries like Gulf region has

been a major area of concern for researchers. In the recent past, the authors of this paper have

been involved deeply in the research related to the corrosion of reinforced concrete structures

under variable environmental actions from macro to micro scale [1-11]. In this research paper,

problems regarding the study of passive layer, its break down and corrosion products at the

nano-scale level using nano technology have been analysed which has limited research data in

the past.

Several nano-techniques are available for analysing materials at the nano-scale level in

the present era. However, nano-scale investigation of passive layer complete characterization

as well as its breakdown and corrosion products formation for reinforced concrete under

several environmental actions is yet to be fully explored [12, 13]. In the previous research

[14], the behaviour and evolution of passive films generated on AISI 304L has been studied at

the nano-scale for a long immersion time in chloride containing media. A theoretical

impedance function was deduced by the use of nanotechnology [15] for a proposed

mechanism of passive film formation of steel in contact with alkaline aqueous media

involving two reaction intermediates: mixed oxide with similar stoichiometry to magnetite

and Fe(III)-oxides. In one of the previous reported researches [16], the corrosion resistance of

C+Mo dual-implanted H13 steel was studied using multi-sweep cyclic voltammetry and

nanotechnology. Evolution of the passive films formed on AISI 304L and duplex stainless

steel SAF 2205 in NaOH 0.1M was investigated by C. M Aberu et. al. at nano-scale [17]

using X-ray photoelectron spectroscopy (XPS). The effect of chloride and nitrite ions on the

passivity of steel in alkaline solutions was also investigated at the nano-scale [18]. The

influence of stress on passive behaviour of steel bars in concrete pore solution was studied

with electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy [19]. X-

ray photoelectron spectroscopy (XPS) had also been used to study the properties of passive

oxide film that form on carbon steel in saturated calcium hydroxide solution and the effect of

chloride on the film properties [20].

However, the nano-scale techniques used in all the above reported research studies have

several limitations, difference of opinion and factors affecting the quality, repeatability,

4

reproducibility and reliability of nano-scale investigation results which needs to be clearly

pointed out and remains for future studies. This leads to the objectives of this research paper.

2. EXPERIMENTATION

A general plan of experimental program is outlined including the broad design and

methodology that has been adopted in this research. Various nano-technological experimental

testing techniques were employed in this research. Scanning Electron Microscopy (SEM) -

both Tungsten and Field Emission imaging was used from micro to nano scale for steel

passive layer and corrosion products as well as the Interfacial Transition Zone (ITZ) between

the embedded steel and concrete. Energy Dispersive Spectroscopy/Energy Dispersive X-ray

(EDS/EDX) elemental analysis was carried out for passive layer and interfacial transition

zone characterization. X-ray Diffraction (XRD) peak location and compound identification of

passive layer as well as the corrosion products was performed at low angle (2θ) special

attachment for thin film analysis. FT-IR material orientation analysis was also conducted to

observe the geometry and arrangement of particles. Photo Electron Spectroscopy for material

characterization and elemental constituent analysis of metals was conducted for various types

and brands of steel available in the local market of gulf region. Along with that,

metallographic analysis for steel reinforcement x-section was also conducted to reveal TM

and PF rings in TMT bars which are very important for passive layer successive

characterization. The Nano-Experimental techniques have been used by many researchers in

the past to study the surface characteristics of many materials at the nanometer scale.

However, in this research, nano-scale studies on the characteristics of iron oxide surface

passive films formed on steel rebars in concrete pore solution have been reviewed which have

limited data in the past and there exists a difference in opinion as well between various

researchers with regards to this passive iron oxide nano-layer.

Material Properties

The materials used and their properties are as follow:

• Reinforcing Steel: Steel bars were procured from five different locally available sources

namely; 1. Sabic Hadeed Steel 2. Ittefaq Steel 3. Muhaidib Steel 4. China Steel and 5.

Korean Steel which is deliberately named as steel source A, B, C, D and E in this paper

(Details are provided in the following sections).

• 20 mm coarse aggregates, procured from Saudi Ready Mix Company, North Riyadh,

KSA, Bulk Specific Gravity: OD basis = 2.58, Absorption, % = 1.56.

• 10 mm coarse aggregates, procured from Saudi Ready Mix Company, North Riyadh,

KSA, Bulk Specific Gravity: OD basis = 2.62, Absorption, % = 1.17.

• Crushed sands, procured from Saudi Ready Mix Company, North Riyadh, KSA, Bulk

Specific Gravity: OD basis = 2.58, Absorption, % = 2.00, Fine Modulus = 4.41.

• Silica sands, procured from Saudi Ready Mix Company, North Riyadh, KSA, Bulk

Specific Gravity: OD basis = 2.58, Absorption, % = 0.376, Fine Modulus = 1.04.

• Cement, procured from Al-Yammamah Cement Company, KSA with chemical

composition satisfying ASTM C-150 for Type I cement.

5

• Sodium Chloride: 99.9% pure sodium chloride was obtained from VWR Chemicals as a

source for chloride ions in concrete and pore solution.

• Water: Tap water available at King Saud University, Civil Engineering Department,

KSA.

Specimen Preparation

Mild Steel (MS) rebar specimens were prepared from a 12 meter long as received deformed

and plain black steel rebars from five different production sources, measuring 6-14 mm in

nominal diameter. The rebars were cut into 1000mm, 10mm and 2 mm segments (Fig. 1) to

suffice the requirement of various SEM and XRD machines as well as their sample holders.

The five steel sources commercially available in the gulf region are being tested under three

types of surface conditions including as received with black oxide mill scale, polished and

brown rust condition (Fig. 2). In general, concrete ingredients were mixed by following

ASTM C-192 “Standard Method of Making and Curing Concrete Test Specimens in the

Laboratory”. Aggregates together with absorption water were added into the mixer first, and

after a few revolutions of the mixer, the cement and the remaining water were added. The

mixer was run for 3 minutes after all ingredients were added into it, then left to rest for 3

minutes, and finally was run for another 2 minutes. After preparation of concrete mixes,

standard tests were performed including: Slump test based on ASTM C-143 “Standard Test

Method for Slump of Portland Cement Concrete” and setting time tested as per ASTM C-403

“the Standard Test Method for Time of Setting of Concrete Mixtures by Penetration

Resistance”.

Preparation of Pore Solutions

Simulated Concrete Pore Solution: Synthetic pore solution was prepared consisting of 7.4 g

NaOH and 36.6 g KOH per litre of saturated calcium hydroxide solution. The solution was

saturated with Ca(OH)2 to simulate conditions in ordinary portland cement concrete. Prior to

its use, the solution was kept for 24 h under continuous magnetic stirring (Fig. 3) and then

filtered on Wattman paper of No. 15 grade. This was necessary to remove insoluble CaO from

the solution.

Cement Extracts: Ordinary Portland cement was sieved through 150µm sieve and extract was

prepared as 100 g of the cement was mixed with 100ml of distilled water and shaken

vigorously using a microid flask mechanical shaker for about 1 hour. The extracts were then

collected by filtration.

Concrete Leached Pore Solution: Another technique was considered to extract the pore

solution from the concrete itself. A concrete prism was cast with a cylindrical hole throughout

the specimen having diameter just equal to the actual steel bar to be tested. Then the steel bar

was inserted into the concrete prism hole and the hole was filled with distilled water. The idea

was to leach the actual pore solution from the concrete in the very small fitting gap between

the steel bar under investigation and the concrete prism hole inner periphery. PH meter is used

to monitor the leached pore solution.

NaCl Pore Solution: Another set of solution was prepared as above and NaCl was added to

these solutions to simulate aggressive conditions prone to corrosion. The oxygen

6

concentration of the stagnant solutions was monitored during the experiments. The

concentration of oxygen was sufficiently high in the solutions (>1 mg/L) for the oxygen

reduction reaction to take place; therefore, oxygen bubbling in the solutions was not required

in general. All reagents were of at least ACS grade and the solutions were prepared with

distilled water with conductivity of 0.8µS/cm. The pH of solutions was carefully checked by a

Methron pH meter and was found to be around 12.3.

Fig.1 1mm, 10mm and 2mm Fig. 2 As received, polished Fig. 3 Pore solution under

rebar samples and brwon rusted rebars continuous magnetic stirring

3. RESULTS AND DISCUSSIONS

The chemical composition and structure of steel rebars was determined. At first SEM-EDXA

analysis was performed for various sources of rebars to find out the elemental composition of

rebars but it was observed that the elemental composition of lighter elements such as ‘Carbon’

was difficult to determine with EDXA analysis. Therefore, Photo-Electron Spectroscopy was

conducted to find out the exact chemical composition of all the elements present in steel bars

obtained from different local production sources. Along with that, metallographic technique

was also used to further study the surface texture of steel bars. All this data is important in the

sense that the successive passive layer which will be developed on the steel after immersion in

alkaline pore solution environment will be influenced by all these properties of steel bars.

The steel samples were cut, polished and prepared in resin moulds as shown in Fig. 4.

The results of Photo-Electron Spectroscopic analysis carried out to determine the elemental

constituents of various steel sources available in the Kingdom are presented in the following

Table 1. The bar ‘A’ was found to have the lowest carbon content which makes it more

resistant to corrosion. Steel ‘C’ & ‘E’ had intermediate carbon content. While, the bar ‘B’ &

‘D’ had maximum percentage of carbon which makes them more prone to corrosion. The

sulphur content in steel source ‘D’ is 0.04 % which is relatively high and the rebar will be

even more prone to corrosion due to the presence of MnS inclusions in the steel as compared

to other steel sources. Regarding the role of steels on nature of passive film, the micro level

observations are quite evident. It can be seen from the metallographic images of etched steel

samples shown in Fig. 5 that TM (Tempered martensite) ring of rebars which plays very

important role on corrosion rate, differ to a great extent in steels produced by different

companies.

7

Table.1: Elemental Composition of Various Steel Bars

Element % A B C D E

C 0.11 0.25 0.13 0.26 0.14

Si 0.13 0.21 0.12 0.19 0.14

Mn 0.34 0.85 0.57 0.92 0.59

P 0.032 0.019 0.018 0.016 0.015

S 0.030 0.021 0.017 0.040 0.019

Cr 0.024 0.018 0.015 0.060 0.020

Ni 0.009 0.005 0.012 0.051 0.014

Mo 0.004 0.001 0.003 0.009 0.004

Al 0.001 0.001 0.002 0.001 0.002

V 0.003 0.001 0.003 0.002 0.002

Sn 0.026 0.001 0.005 0.019 0.005

Fig. 4 Various Steel Samples Prepared for Analysis

sample “A” sample “B” sample “C”

sample “D” sample “E”

Fig. 5 Metallographic Images of Various Steel Sources

Characterization of Passive Layer Developed on Steel Rebars in Pore Solutions

The characterization of passive layer developed on steel bars under pore solution environment

was carried out. The constituent compound analysis as well as growth of passive layer was

investigated under various conditions. The steel samples embedded in concrete were cut

around the steel bar in small squares to fit in the limited space of SEM (scanning electron

8

microscope) and analyzed for the passive layer development around steel bar in the natural

alkaline environment of concrete (Fig. 6).

Fig. 6 Steel embedded in Concrete Being Tested in SEM

As a first trial, SEM was employed to observe the passive layer around the periphery of steel

bar type ‘C’ and images were taken at different resolutions as shown in Fig. 7. These images

proved to be very helpful to reveal the interface of concrete and steel where passive layer is

developed. It was observed that the surface of steel bar which seemed very smooth at macro

level was still very non-uniform at micro and nano level which definitely affects the quality of

passive layer.

It was decided to carry out sensitivity analysis using EDXA technique around the passive

layer to study the surrounding area and its influence on the passive layer itself. For this

purpose, block and spot spectrums were taken at different locations around the passive layer

as shown in Fig. 8. As a first step, block spectrum was taken in the concrete area around the

passive layer as shown in Fig. 8 (a). It is to be noted that as already discussed, carbon ‘C’

being a lighter element was difficult to detect correctly by EDXA-FEM analysis and should

be ignored throughout the analysis considering the carbon content as erroneous and a

limitation of EDS analysis. Taking a spectrum on the steel surface close to the passive layer

for EDS analysis (Fig. 8(b)), it was observed that the number of elements reduced. The

amount of ‘O’ also became much less confirming the fact that the analysis was being targeted

in the steel area close but not in the passive layer. The percentage of iron also increased

substantially and that of calcium decreased as compared to the Fig. 8 (a) again confirming the

above said. This sensitivity analysis provided confidence and hands on experience on the

machine. Again, it was noticed that the amount of carbon was much higher than expected and

was therefore erroneous.

The EDS sensitivity analysis was repeated several times to obtain expertise and

confidence in the hands on experience and confidence for the exact location of analysis on the

periphery of steel concrete to obtain accurate and averaged conclusion. Another idea was to

zoom in to the nano-scale, close to 10 nm and then repeat the above steps. In the light of

observations obtained from sensitivity analysis performed in Fig. 8, a more systematic

approach was adopted to identify the passive layer. A certain point was fixed in the SEM and

then was zoomed inside the reach the passive layer without changing the coordinates (Fig. 9).

But, unfortunately it was observed that the tungsten filament SEM being used in this analysis

did not have enough resolution to show images in beyond 200 nm as shown in the Fig. 10. It

can be concluded here that tungsten filament SEM is not capable of performing the passive

9

layer analysis and need was felt to utilize FE-SEM (Field Emission scanning electron

microscope) having enough resolution to focus deep into the nano-scale. Also, XRD analysis

was tried but since it cannot be focused on a small area of periphery of steel bar having

passive layer, satisfactory results were not obtained. Raman spectra seem to be a suitable

option and should be adopted under the above circumstances.

Fig. 7 SEM Image of Steel Concrete Interface (Passive layer Orientation)

135

Element Weight% Atomic%

C K 6.49 11.32

O K 48.05 62.94

Na K 0.43 0.39

Mg K 0.91 0.79

Al K 1.68 1.30

Si K 6.26 4.67

S K 0.73 0.48

K K 0.38 0.20

Ca K 32.07 16.77

Ti K 0.14 0.06

Fe K 2.87 1.08

Totals 100.00

Fig. 8(a) EDXA Analysis of Steel Concrete Interface (Passive Layer)

136

Element Weight% Atomic%

C K 9.77 29.37

O K 6.41 14.47

Mg K 0.27 0.41

Al K 0.19 0.26

Si K 1.46 1.88

Ca K 2.57 2.31

Mn K 0.51 0.33

Fe K 78.81 50.96

Fig. 8 (b) EDXA Analysis of Steel Concrete Interface (Passive Layer)

Fig. 8 FEM-EDXA Analysis of the X-section side of passive layer

10

Fig. 9 SEM Imaging of Passive layer Fig. 10 Sensitivity Analysis of steel

existing in steel-concrete Interface concrete interface to locate the passive layer

The above discussion was for the case of steel embedded in concrete and being observed from

the X-section side. In the next step, steel bars immersed in the simulated concrete pore

solution (SPS) were analyzed under SEM from the top surface for passive layer EDXA

analysis. The corresponding systematic SEM images are as shown in Fig. 11 and 12. A

surface texture of passive layer developed on steel immersed in pore solution at high

resolution (Fig. 12) showed deposits of pore solution on the passive layer as contamination

and should be ignored during analysis. Also, it was observed that the penetration of electrons

from the SEM were deep enough to bypass the thickness of passive layer and SEM-EDS

analysis may not represent the passive layer but the layer beneath it. It should be kept in mind

that this sensitivity analysis was done merely for hands on experience and understanding of

machine operations and specimen behavior. Several EDS sensitivity analyses were conducted

as already described in the previous section and similar conclusions were drawn. To avoid

repetition, only representative results are presented in Fig. 13.

Fig. 11 SEM Imaging of Steel surface bearing passive layer in simulated pore solution

(SPS)

11

Fig. 12 SEM Imaging of Steel surface bearing passive layer in SPS

169

Element Weight%

C K 56.09

O K 15.00

Al K 2.85

Si K 0.64

Cl K 3.95

Ca K 0.66

Fe K 20.80

Totals 100.00

Fig. 13 SEM-EDXA analysis of sample with passive layer

(Washed and Desiccated)

After obtaining the above results, it was concluded that Field Emission SEM (FE-SEM) must

be fetched and involved in passive layer nano-scale analysis so that the very fine passive layer

(in nano-meters) can be completely analyzed and its structure understood properly. Fig. 14

and Fig. 15 below show the FE-SEM images of steel bar with and without passive layer at the

nano-scale. A unique and repetitive symmetrical pattern was found in the structure of passive

layer at the nano-scale developed on polished steel bars while taking images under FEG SEM.

This is a rather new and very interesting finding of this research project which has not been

reported in the past that the passive oxide layer on the steel surface actually does have a

uniform repeating pattern instead of just being a random non-uniformly structured layer.

Images (Fig. 16) taken with FE-SEM at the nano-scale revealed an interesting observation

that the steel bar even without the passive layer is not so uniformly structured over the surface

as was thought and gives a similar non-uniform surface structural look as found in case of

concrete when zoomed to the nano-scale. In fact the field emission scanning electron

12

microscope images (Fig. 15) obtained at the nano-scale revealed that the steel bars with

passive oxide layer have even more orderly structure in contrast to what was expected.

1.5kV, 60,000x, SEI,

6.3mm WD

1.5kV, 100,000x, SEI

6.3mm WD

Fig. 14 Steel Surface without Passive Layer (nano-scale) FE-SEM Images

1.5kV, 60,000x, SEI,

6.2mm WD

1.5kV, 100,000x, SEI

6.2mm WD

Fig. 15 Steel Surface with Passive Layer (nano-scale) FE-SEM Images

1.5kV, 60,000x, SEI,

6.2mm WD

(With passive layer)

1.5kV, 60,000x, SEI,

6.3mm WD

(Without passive layer)

Fig. 16 FE-EM Images of Steel Bar Surface (nano-scale)

13

After revealing the above interesting information, X-ray diffraction (XRD) analysis was

conducted on the passive layer to understand the compound analysis of this oxide layer

formed on the steel surface under alkaline conditions. The XRD pattern and possible material

identification is presented in Fig. 17. It was found that most of the materials present in the

passive layer were not easily identifiable and needed advanced and complete set of data-base

for the XRD peaks to be identified satisfactorily. This is our present limitation that the

complete database required for such sophisticated material analysis is not available. The

materials identified in the passive layer comprised of various oxides of iron in different

phases, such as Hematite, Magnetite, Wuestite, PIstite, Magnesioferrite etc. along with some

other compounds as shown in Fig. 17.

It was seen from the XRD peaks of various steel sources that the passive layers developed on

various steel sources under simulated pore solution environment are different from each other.

This opens areas of future research in this project to explore the various possibilities and

reasons for these differences in the composition of passive layers developed on different steel

sources shown in Fig. 18. The qualitative comparison of various XRD patterns show that the

peaks observed in case of steel ‘A’, ‘B’ and ‘C’ are not same but similar probably belonging

to the different phases of the same family.

Visible Ref.Code Score Compound Name Displ.[°2Th] Scale Fac. Chem. Formula

* 01-073-0603 45 Hematite, syn 0.922 0.919 Fe2 O3

01-086-2316 17 Wuestite, syn 0.720 0.053 Fe0.902 O

01-074-0953 23 Fe-Ringwoodite, syn 0.742 0.083 Fe2 ( Si O4 )

01-074-1886 32 Wuestite, syn 0.981 0.020 Fe O

00-042-1468 UM alumina 1.353 0.514 Al2 O3

01-088-0840 4 Hawaiite 0.982 0.311 ( Mg , Fe , Al , T..

01-079-1968 32 W\PIstite, syn 1.346 0.018 Fe.945 O

01-079-2177 33 W\PIstite, syn 1.232 0.019 Fe0.92 O

00-019-0605 5 Enstatite, ferroan 1.031 0.240 ( Mg , Fe ) Si O3

00-025-1376 UM Magnetite 1.119 0.034 ( Fe , Mg )

( Al ,..

00-002-1044 UM Magnesioferrite 1.049 0.480 Mg Fe2 +3 O4

Fig. 17 XRD Results for Passive Layers Developed on Steel Rebar

14

20 30 40 50 60 70 80 902Theta (°)

0

400

1600

3600

6400

10000

Inte

ns

ity

(c

ou

nts

)

Fig. 18 X-Ray Diffraction Analysis of Passive Layer (Qualitative)

Along with the qualitative analysis discussed above, quantitative XRD analysis was also

carried out to identify the exact chemical compositions and phases of peaks observed in

various spectra (Fig. 19). While going through the analysis of XRD peaks presented in Figure

19, it is found that except steel source ‘D’, where peaks of beta-Fe2O3 are observed; in no

other case any compound of iron is identified. In all the other cases, the peaks of CaO are

recorded. It is suspected that the reason for this observation lies in the presence of possible

concrete pore solution layer over the passive layer formations in which the steel samples were

immersed. In the future, samples will be washed and desiccated carefully to be analyzed under

XRD to avoid such problems. In another trial, a couple of steel samples were analyzed under

XRD for varying incidence angles of 0.5, 1.0 and 1.5 respectively as shown in Figs. 20. The

three spectra were overlapped and then compared with each other for possible variations due

to different phase angles. The identified compounds along with their chemical formulas are

shown in the figures 19 and 20. X-ray diffraction is a good technique to identify the phases in

the top layer of samples. Grazing incidence diffraction was performed on two samples with

different incidence angles. Iron (Fe), 3 Iron-oxides (Fe2O3, Fe0.92O, Fe3O4) as well as Iron

hydroxide (Fe(OH)3) were identified.

Fig. 19 XRD peak Identification for steel source (Quantitative)

15

Fig. 20(a) XRD spectra of steel ‘D’ at 0.5º Fig. 20(b) XRD spectra of steel ‘D’ at 1.0º

Fig. 20(c) XRD spectra of steel ‘D’ at 1.5º Fig. 20(d) XRD spectra of steel ‘D’ at

superimposed 0.5, 1.0 and 1.5º omega

Fig. 20(f) XRD spectra of steel ‘C’ at 0.5º Fig. 20(g) XRD spectra of steel ‘C’ at 1.0º

Fig. 20(h) XRD spectra of steel ‘C’ at 1.5º Fig. 20(i) XRD spectra of steel ‘C’ at

superimposed 0.5, 1.0 and 1.5º omega

Fig. 20 Incident angle sensitivity analysis for XRD compound and peak identification

16

4. CONCLUSIONS

Despite of the fact that different techniques have been used for nano-scale studies of

extremely thin and delicate iron oxide passive films on steel rebars in concrete, the results are

not consistent and strongly depend on the type of exposure conditions, method of sample

preparation, equipment employed, analysis scale, standard library database available, skill of

the worker, precision and accuracy of measurement, insitu testing technique etc. It should be

noted that there are a number of challenges associated with these techniques when used for

non-uniform naturally developed layers (passive layer on steel reinforcement in conrete) that

can account for the lack of consistency and accurate information. A slight negligence in any

of the above factors can lead to entirely different experiment and analysis results. Especially,

a lot of care is need for sample preparation that minimizes damage to the specimen,

conducting in-situ test, proper interpretation of results and inherited limitations of the devices

used in these nano-techniques. Even though the minimization of the errors associated with

these challenges may provide an insight into the properties of passive oxide film developed on

steel reabrs in concrete and its behavior under circumstances comparable to the actual

exposure conditions, it is not practically possible to eliminate all these experimental

limitations. Therefore, multiple techniques that complement each other must be used to study

these phenomena for future concrete.

5. ACKNOWLEDGEMENTS

This research project has been supported by King Abdulaziz City for Science and Technology

(KACST), Long Term Comprehensive National Plan for Science, Technology and Innovation

(NPST), Project No. 09-NAN674-02, Riyadh, Saudi Arabia, 2009-2011.

6. REFERENCES

[1] Hussain Raja Rizwan, Tetsuya Ishida (2010) Influence of Connectivity of Concrete Pores

and Associated Diffusion of Oxygen on Corrosion of Steel under High Humidity,

Construction and Building Materials Journal, Vol. 24, Issue 6, pp.1014–1019.

[2] Hussain Raja Rizwan and Tetsuya Ishida (2010) Development of Numerical Model for

FEM Computation of Oxygen Transport through Porous Media Coupled with Micro-Cell

Corrosion Model of Steel in Concrete Structures, Computers and Structures Journal, Vol. 88,

Issues 9-10, pp.639–647.

[3] Hussain Raja Rizwan and Tetsuya Ishida, Experimental Investigation of Time Dependent

Non Linear 3D Relationship Between Critical Carbonation Depth and Corrosion of Steel in

Carbonated Concrete, Journal of Corrosion Engineering, Science and Technology (2010),

doi: 10.1179/147842210X12659647007086.

[4] Hussain Raja Rizwan and Tetsuya Ishida (2010) Investigation of Volumetric Effect of

Coarse Aggregate on Corroding Steel Reinforcement at the Interfacial Transition Zone of

Concrete, submitted to KSCE Journal of Civil Engineering, Vol. 15, Issue 1, pp. 153-160.

[5] Hussain Raja Rizwan and Tetsuya Ishida, Enhanced electro-chemical corrosion model for

reinforced concrete under severe coupled environmental action of chloride and temperature,

Construction and Building Materials Journal (2010), Vol. 25, Issue 3, pp. 1305-1315.

[6] Hussain Raja Rizwan and Tetsuya Ishida (2009) Critical Carbonation Depth for Initiation

of Steel Corrosion in Fully Carbonated Concrete and the Development of Electrochemical

17

Carbonation Induced Corrosion Model, International Journal of Electrochemical Science

IJES, Vol. 4, Issue 8, pp. 1178-1195.

[7] Hussain Raja Rizwan, Tetsuya Ishida (2010) Novel Approach Towards Calculation of

Averaged Activation Energy Based on Arrhenius Plot for Modeling of the Effect of

Temperature on Chloride Induced Corrosion of Steel in Concrete, Journal of ASTM

International, Vol. 7, Issue 5, pp. 1-8, doi: 10.1520/JAI102667.

[8] Hussain, R.R., Effect of moisture variation on oxygen consumption rate of corroding steel

in chloride contaminated concrete, Cement & Concrete Composites (2010), Vol. 33, Issue 1

(January 2011) pp. 154–16.

[9] Hussain Raja Rizwan and Tetsuya Ishida (2010) Induced Macro-Cell Corrosion

Phenomenon in the Simulated Repaired Reinforced Concrete Patch, Australian Journal of

Civil Engineering, Vol. 8, No. 1, pp. 53-60, Australia.

[10] Raja Rizwan Hussain, Underwater Half-Cell Corrosion Potential Bench Mark

Measurements of Corroding Steel in Concrete influenced by a Variety of Material Science and

Environmental Engineering Variables, Measurement Journal (2010), Vol. 44, pp. 274-280.

[11] Hussain Raja Rizwan (2010) Enhanced Mass Balance Electrochemical Computational

Model for Corrosion Rate of Steel coupled with CO2 Transport Model in Extremely

Carbonated Concrete, International Journal of Computers and Concrete, Vol. 8, No.2, April

2011, pp. 177-192.

[12] Rita Ghosh and D.D.N. Singh, Kinetics, mechanism and characterization of passive film

formed on hot dip galvanized coating exposed in simulated concrete pore solution, Surface &

Coatings Technology 201 (2007) 7346–7359.

[13] DDN Singh and Rita Ghosh, Corrosion Performance of Steel Rebars Embedded in

Chloride Contaminated Concrete Mortars-Role of Surfance Pre-Treatments, Metals A4aterials

And Processes, 2003, Va/. 16, No. 2-3, pp. 313-326.

[14] C.M. Abreu, M.J. Cristobal, R. Losada, X.R. Novoa, G. Pena, M.C. Perez, Long-term

behaviour of AISI 304L passive layer in chloride containing medium, Electrochimica Acta 51

(2006) 1881–1890

[15] M. Sanchez-Moreno, H. Takenouti, J.J. García-Jareno, F. Vicente, C. Alonso, A

theoretical approach of impedance spectroscopy during the passivation of steel in alkaline

media, Electrochimica Acta 54 (2009) 7222–7226.

[16] Zhang Tonghe, WU Yuguang, YI Zhongzhen, Zhang Xu & Wang Xiaoyan, Nano-

phases and corrosion resistance of C +Mo dual implanted steel, Vol. 44 No. 4 Science in

China (Series E) August 2001.

[17] C.M. Abreu, M.J. Cristóbal, R. Losada, X.R. Nóvoa, G. Pena and M.C. Perez,

Comparative study of passive films of different stainless steels developed on alkaline medium,

Electrochimica Acta 49 (2004) 3049–3056.

[18] M.B. Valcarce, M. Vazquez, Carbon steel passivity examined in alkaline solutions: The

effect of chloride and nitrite ions, Electrochimica Acta 53 (2008) 5007–5015.

[19] Xingguo Feng, Yuming Tang, Yu Zuo School of Materials Science and Engineering,

Beijing University of Chemical Technology, Beijing 100029, China Corrosion Science 53

(2011) 1304–1311.

[20] P. Ghodsa, O.B. Isgora,∗, J.R. Brownb, F. Bensebaac, D. Kingstonc a Carleton

University, Ottawa, Canada b CANMET Materials Technology Laboratory, Ottawa, Canada c

National Research Council, Ottawa, Canada Applied Surface Science 257 (2011) 4669–4677.