Abstract of thesis: Study on the formation and corrosion protectiveness of rust layer on weathering steel in tropical atmosphere of Vietnam

MINISTRY OF EDUCATION
AND TRAINING

VIETNAM ACADEMY OF
SCIENCE AND TECHNOLOGY

GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY

……..….***…………

HOANG LAM HONG

STUDY ON THE FORMATION AND CORROSION
PROTECTIVENESS OF RUST LAYER ON WEATHERING STEEL
IN TROPICAL ATMOSPHERE OF VIETNAM

Major: Metallography
Code number: 9.44.01.29

ABSTRACT OF THESIS

Hanoi – 2019


The Thesis finished at Graduate University of Science and
Technology - Vietnam Academy of Science and Technology

Supervisors:

Assoc. Prof. Dr. Le Thi Hong Lien
Dr. Pham Thy San

Opponent 1: Assoc. Prof. Dr. Nguyen Xuan Hoan
Opponent 2: Assoc. Prof. Dr. Nguyen Van Tu
Opponent 3: Prof. Dr. Mai Thanh Tung

The Thesis will be defended at the doctoral council which organizes
by Graduate University of Science and Technology - Vietnam
Academy of Science and Technology at ….. h…’ date ………..2019

The Thesis can find out at:
- The Library of Graduate University of Science and Technology
- National Library of Vietnam


THE LIST OF PUBLISHED PAPERS
1. Le Thi Hong Lien and Hoang Lam Hong, “Characteristics of corrosion product
layer formed on weathering steel exposed to the tropical climate of Vietnam”,
Material Science and Application, Vol. 4, 7A, 2013, pp. 6-16, USA.
2. Le Thi Hong Lien and Hoang Lam Hong, “Study on atmospheric corrosion of
weathering steel in Vietnam”, Proceeding of JSCE Material and environments
2014.
3. Hoang Lam Hong, Le Thi Hong Lien and Pham Thi San, “Atmospheric
corrosion of weathering steel in marine environment of Viet Nam”, Tạp chí
Khoa học và công nghệ, tập 53-1B (2015).
4. Thy San P., Hong Lien L.T., Lam Hong H., Trung Hieu N., Thanh Nga N.T.,
“Establish mathematical models to predict corrosion of carbon steel and
weathering steel in atmosphere of Viet Nam”, Tạp chí Khoa học và công nghệ,
tập 53-1B (2015).
5. Le Thi Hong Lien and Hoang Lam Hong, “Corrosion behavior of weathering
steel in atmosphere of Vietnam”, Proceeding of JSCE Material and

environments 2015.
6. Le Thi Hong Lien, Hoang Lam Hong, Pham Thi San, “Corrosion behavior of
weathering steel in tropical atmosphere of Vietnam”, International Journal of
Engineering Research and Science (IJOER), Vol.2, Issue 11 (2016)
7. Le Thi Hong Lien, Hoang Lam Hong, Pham Thi San, Nguyen Trung Hieu and
Nguyen Thi Thanh Nga, “Atmospheric corrosion of Carbon steel and
Weathering steel – Relation of corrosion and environmental factors”,
Proceeding of JSCE Material and environments 2016.



PREFACE
Weathering Steel (WS) is low alloy steel that has been using as un-painted
steel in mild atmosphere due to forming protective layer of corrosion product
(rust) on steel substrate, which can act as a protective barrier against the inward
transportation of corrosive species. Using the WS on steel structures reduces the
material cost, especially reduces maintenance cost of anti-corrosion whichis
effectively applied WS to works of transportation, bridges, works of the
century.
WS has been used popular in temperate zones due to its corrosion resistance.
However, the WS types have just stated using in Vietnam. The possibility to
apply of the weathering steel in the humid tropical climate with heavy rain of
our country has not been fully and systematically researched. So, in order to
build the scientific bases of applying WS to construction works, highway
bridges, works of the century, ... in Vietnam, and provide the necessary
information for selecting and using effectively WS (Corten B) in each climatic
region, the author selected the thesis titled: “Study on the formation and
corrosion protectiveness of rust layer on weathering steel in tropical
atmosphere of Vietnam”
 The purpose of thesis:

 Research on the influence of climatic and environmental parameters to the
formation, structure and properties of corrosion products formed on the
WS surface in different climatic regions of Vietnam.
 Research on the mechanism of forming protective rust layer on WS and
corrosion resistance of WS in tropical climate of Vietnam.
 The research object: Corten B steel.
 Methods for research:
 In order to research the ability of forming protective rust layer on WS in
the Vietnam climate, WS was exposed to three different climate zones: (1)
Northern (Hanoi) with four seasons, the temperature in the year fluctuates
sharply, high humidity, long time of wetness and cold winter; (2) The
humid marine atmosphere at Dong Hoi with seasonal temperature changes
and high content of airborne sea salt; (3) Dry marine atmosphere at Phan
Rang with low rainfall and high temperature in the year. The impacts of
climatic and environmental parameters on atmospheric corrosion of WS
were studied using the samples exposed at 15 difference climatic zones in
Vietnam: Son La, Yen Bai, Tam Dao, Cua Ong, Con Vanh, Hanoi, Dong
Hoi, Quang Ngai, Pleiku, Phan Rang, Bien Hoa, Ho Chi Minh City, Can
Tho, Rach Gia and Ca Mau.

1


 The corrosion rate is determined by mass lost method. The parameters of
the environment are collected and determined simultaneously in the same
test period.
 The physical methods (SEM-EDX, X-ray diffraction, Raman scattering,
optical microscopy) are used to study the morphology, structure, chemical
composition and phase composition of the rust formed on WS.
 The electrochemical methods (polarization curves, electrochemical

impedance spectroscopy EIS) were applied to study the corrosion
protectiveness able of rust that formed on WS.


The scientific and practical values of the thesis:
 This is the first time the study of atmospheric corrosion of WS in
Vietnamese tropical climate was implemented systematically. The effect of
atmospheric conditions on corrosion dynamics and the formation of
protective rust layer on WS is discussed. Some critical values of using
unpainted WS in Vietnamese climate are initially mentioned in the thesis.
 The research results can be referred as the scientific bases for the
application of WS in Vietnam, and provide the necessary information for
effective selection and us of WS types in each climatic zone.
 The research results will contribute scientific valuable data to the world’s
database of WS in humid tropics, especially in Asia.

 The composition of thesis: the thesis including 113 pages, 17 tables and 94
figures was separated to 4 chapter:
 Chapter 1: The overview of atmospheric corrosion of WS.
 Chapter 2: Experience and methods of research
 Chapter 3: The characteristic of WS corrosion in the Vietnamese climate.
 Chapter 4: The formation and protectiveness of corrosion product formed
on WS in climate of Vietnam
 Conclusion
 List of published papers and references.
Chapter 1. THE OVERVIEW OF ATMOSPHERIC CORROSION OF WS
 Introduce the history of WS development.
 The overview of effect of atmospheric environmental factors (temperature,
humidity, rainfall and atmospheric pollutions) and alloying elements to WS
corrosion resistance.

 Summarize the characteristics of corrosion product (CP) formed on WS
(phase composition and structure of rust)
2


 The mechanisms of forming and growing corrosion protectiveness rust layer
on WS were discussed.
 Some critical values of atmospheric environment factors for using uncoated WS in some atmosphere in the world were shown.
 Introduce the characteristics of climate of Vietnam.
 Summary the study’s results of WS atmospheric corrosion in Vietnam
Chapter 2. EXPERIMENTAL AND METHODS OF RESEARCH
2.1. Material
Tab. 2.1. The chemical composition of materials, mass %
WS
CS

C
0.111
0.135
V

Mn
1.06
1.35

Si
Ni
0.236 0.1860
0.341 0.0597

Cr
0.528
0.024

Cu
0.3200
0.0616

Mo
0.048
0.048

Ti

Al
W
Co
Fe
<
<
WS
0.0158 < 0.05 < 0.005
97.4
< 0.005
<
CS 0.005
0.0243 < 0.05 < 0.005
97.9
0.005B type
0.005

WS of Corten
and CS with chemical compositions shown in tab.2.1
were used for study. The samples with size 100x75x2mm were exposed at 3 test
site: Hanoi (HN), Dong Hoi (DH) and Phan Rang (PR).
2.2. Methods of research
2.2.1. Exposed test
The samples were exposed to atmosphere following ISO 8565 standard
[118]. The corrosion product (rust) was removed according to the standard ISO
8407 [119]. The corrosion rate (CR) were determined by mass loss of tested
samples (ISO 9226 [120]). The exposure period at HN, DH and PR was divided
into two kinds: long term period (1, 3, 6, 12, 24 and 36 months) and short term
period (1, 3, 5, 7 and 14 days). The 12-month exposure were repeated three
times at 15 different climatic zones in Vietnam.
The environmental factors are collected at the same time of exposure:
temperature, humidity, rainfall and atmospheric pollutions.
2.2.2. Study on structure and characteristic of the corrosion product by
physical methods
 The morphology and chemical conposition of corrosion product (CP) was
investigated by Scaning Electron Microscope - SEM.
 The chemical compound of CP was detected by X-ray diffraction and
Raman spectroscopy.
3


 The cross-section structure of CP was investigated using Optical
Microscope.
2.2.3. Study on corrosion protective property by electrochemical methods
 EIS method
 Polarization curves method
2.2.4. Chemical methods for atmospheric impurities analysis

Chapter 3. THE CHARACTERISTIC OF WS CORROSION IN THE
VIETNAMESE CLIMATE
3.1. The corrosion mass lost of WS
During the periods less than 6-12 months, the corrosion mass losses (ML) of
WS and CS are similar because the rust is porous and uncover entire of surface.
Tab. 3.1: The environmental parameter of the exposed test (annual value, 20102013)
Time of wetness TOW
Total
Cl-,
SO2,
Time
Kind of o
RH,
2
Site
T, C rainfall
TOW,
of
w , % mg/(m mg/(m2
atm.
%
 (*)
mm
h/y dryness d/w
(**) .day) .day)
h/y
HN
urban 24,2 1606 78,6 4615 4145 0,898 52,7 3,16 1,835
DH
marine 24,9 2445 83,1 5705 3055 0,535 65,1 17,55 0,575

PR
marine 26,7 1130 76,8 3719 5041 1,35 42,4 8,77
0,87
*d/w: ratio of drying time /TOW
**w: ratio of TOW/total exposure

Fig. 3.1. ML of WS and CS at
the exposed test sites

4


After 6 months (in marine sites) or 12 months (in urban sites), ML of WS
becomes lower than that of CS. The difference in ML between the WS and CS
at the test sites in Hanoi and Dong Hoi where the climate is humid, - is greater
than that in the dry climate of Phan Rang.

Fig.3.2. ML and CR of WS and CS at the exposed test sites
The ML and CR of WS at the test sites show that the corrosion is most severe at
Dong Hoi site, at Hanoi is lower and it is lowest at Phan Rang site.
3.2. Kinetics of WS atmospheric corrosion

300
250

200

200
150

100

100

0

50

0 3 6 9 12
Exposed time, month

0
0
3
6
9 12
Exposed time, month

Fig.3.3. The variable of ML of WS by
exposure time in urban of HN

Fig.3.4. The variable of ML of WS by
exposure time in marine of DH

160

120
80
40

0
0 3 6 9 12
Exposed time, month

Fig.3.5. The variable of ML of WS by
exposure time in marine of PR.

Fig.3.6. The ML of CS by exposure
time at test sites
5


The variation of ML depending exposure time was built from the experimental
and fitted using Excel software, the results show that:
 The variation of ML with the exposure time conforms to the power law with
coefficient of n < 1, it shows the evidence of the existence and gradual
improvement of protective layer of CP on the WS. The n coefficient of
equation at Dong Hoi > at Phan Rang > at Ha Noi which shows that the time
to form protective CP at Dong Hoi is longer than that of two other sites.
 During exposure time < 6-12 months, because either the CP have not
covered fully WS substrate, or the structure of CP was porous, t CP doesn’t
express protectiveness and the variation of ML with exposure time conform
to linear law. When the exposure time >12 months the CP fully covered
WS surface and the dense rust layer was formed at very close WS substrate,
this restrained corrosion and the ML variation with exposure timeconforms
to power law.
 A and n coefficients in kinetic equations of WS are lower than that in
kinetic equations of CS. It shows that the ML of WS and CS are more and
more diffirent over exposure time andthe corrosion protectiveness of WS in
atmosphere of Vietnam is better than CS clearly.

Tab.3.2. The kinetic equations of WS at test sites
Test site

Equation

A

n

R2

Hanoi

M = 182,31*t0,5363

182,31

0,5363

0,981

Dong Hoi

M = 236,67*t0,6172

236,67

0,6172

0,974

Phan Rang

M = 142,46*t0,5653

142,46

0,5653

0,982

Tab.3.3. The kinetic equations of CS at test sites (calculated from fig.3.6)
Test site

Equation

A

n

R2

Hanoi

M = 210,02*t0,660

210,02

0,660

0,993

Dong Hoi

M = 340,67*t0,711

340,67

0,711

0,960

Phan Rang

M = 156,66*t0,693

156,66

0,693

0,978

The ML of WS for 20 year- exposure in atmosphere of Vietnam is calculated
from the kinetic equations of test site. According to [31,110], the corrosion
process reaches steady state when the first one year CR increases <10%.
Accordingly, the process of atmospheric corrosion of WS in Vietnam will reach
steady state after 7 years. Following the guidelines of US Steel Association for
6

using un-coated WS (the ML after 20 years is 120 μm [45]), it is possible to use
bare WS in Hanoi and Phan Rang atmosphere.
Tab.3.4. The ML of WS that is calculated from the kinetic equation for 20 years
Hanoi
Time
,
ML,
year g/m2

ML,
m

Dong Hoi

Increment
of ML, %
(*)

ML,
g/m2

ML,
m

Phan Rang

Increment
of ML, %
(*)

ML,
g/m2

ML,
m

Increment
of ML, %
(*)

5

432,2 54,99

12,71

639,1 81,31

14,77

353,8 45,02

13,44

6

476,6 60,63

10,27

715,2 90,99

11,91

392,3 49,91

10,86

7

517,6 65,86

8,62

786,6 100,1

9,98

428,0 54,45

9,11

8

556,1 70,75

7,42

854,1 108,7

8,59

461,5 58,72

7,84

9

592,3 75,36

6,52

918,5 116,9

7,54

493,3 62,76

6,88

12

691,1 87,93

4,78

1097 139,6

5,52

580,4 73,85

5,04

13

721,5 91,79

4,39

1152 146,6

5,06

607,3 77,26

4,63

14

750,7 95,51

4,05

1206 153,5

4,68

633,3 80,57

4,28

18

859,0 109,3

3,11

1409 179,3

3,59

729,9 92,87

3,28

19

884,1 112,5

2,94

1457 185,3

3,39

752,6 95,75

3,10

20

909,0 115,6

2,79

1504 191,3

3,22

774,7 98,57

2,94

…

…

3.3. The role and influence of environmental parameters on atmospheric
corrosion of WS
For studying the influence of environmental parameters on atmospheric
corrosion of WS, the test samples were exposed for one year which were
repeated three times at different time in the year (February, May and
November) at 15 test sites (Son La, Yen Bai, Tam Dao, Cua Ong, Con Vanh,
Hanoi, Dong Hoi, Quang Ngai, Pleiku, Phan Rang, Bien Hoa, Ho Chi Minh
City, Can Tho, Rach Gia and Ca Mau). The seasonal impacts on atmospheric
corrosion of WS were studied on samples exposed every month in year at the
test sites of Hanoi, Dong Hoi and Phan Rang.
3.3.1. The effect of temperature
 T < 200C: The ML of WS increases with the increasing of air temperature

because air temperature accelerates corrosion reactions.
7


 T > 200C: The ML of WS decreases with the increasing of air temperature
because TOW and solubility of oxygen as well as other gases into water
thin film on WS surface are decreased when air temperature increases.

Fig.3.7. The effect of temperature to
the ML of WS.

3.3.2. The effect of wet-dry cycle
The ratio of drying time and wetting ness time: d/w = (t-TOW)/TOW, here
t is exposure time.
-

d/w < 0,5 means TOW is too long (w > 67%), the surface of WS is too
wet.; When the drying time is longer, the evaporation of water only makes
oxygen dissolved easily into water film and makes the concentration of
electrolyte layer increases, accordingly, the ML increases with d/w.

-

0.5 < d/w < 1: the ML decreases when d/w increase at non-marine test sites
because the drying time is long enough in order to dry the WS surface.

-

d/w > 1: the drying time is too long which makes the ML decreased when
d/w increases .

d/w < 0,5
0,5 < d/w < 1
d/w > 1

Fig.3.8. The relationship between ML and d/w
3.3.3. The effect of rainfall total

8


Fig.3.9. The relationship between ML and rainfall total
 At the inland test sites: the ML increases with the increasing of rainfall
total either because the thickness of water film increases, or because rain
water is kept in the porous rust layer which prolongs the time of wetness
on WS surface.
 At the coastal test sites: the ML increases with the increasing of rainfall
total because the air pollutions (including ion Cl-) are washed away by
rain and it makes corrosion reduced.
3.3.4. The effect of airborne salinity (Cl-)
The airborne salinity accelerates the ML of WS in the atmospheric regions
where the Cl- concentration is higher than 5 mg/m2.day. When Cl-< 5
mg/m2.day it doesn’t expresses the corrosion accelerated role.

Fig.3.10. The relationship between
ML and airborne salinity

3.3.5. The effect of SO2 deposition rate
The ML of WS shows the trend of increasing when SO2 concentration in
atmospheric increases, however this relationship is not apparent.

3.3.6. The effect of season
3.3.6.1. Hanoi test site:
Hanoi has four seasons: high temperature and rainfall in the summer, low
temperature and little sunshine in the winter, therefore, the environment shows
the different effects to corrosion of WS depending the exposure duration in the
9


year. The monthly of ML shows that the TOW dominates atmospheric
corrosion of WS in Hanoi: the ML increases in Spring when w is highest and
reduces in Summer when w is lowest.

c)

d)

Fig.3.12. The relationship between ML and: (c) – ratio ư and (d) – ratio d/w in
Hanoi
3.3.6.2. Dong Hoi test site:

c)

d)

Fig.3.14. The relationship between ML and: c) – ratio ư and d) – ratio d/w in
Dong Hoi

Fig.3.15. The relationship
between ML and airborne
salinity in Dong Hoi

Dong Hoi is effected by two main directions of monsoons: the south-west
monsoon blows from inland to the sea (in the end of April to middle of
September) and the north-east monsoon blows from sea to inland, thus the
climate in Dong Hoi depends on monsoons. The monthly ML shows that the
TOW (w and d/w) and airborne salinity dominates atmospheric corrosion of
WS in Dong Hoi.
10


Chapter 4. THE FORMATION AND PROTECTIVENESS OF
CORROSION PRODUCT FORMED ON WS IN CLIMATE OF
VIETNAM
The results presented in the chapter 3 showed that after 36 months of testing,
the ML of CS and WS are in accordance with the power law with n exponents
<1, the CR decreases time by time. This proves that the rust on the steel surface
has the effect of protecting the steel substrate, thus it inhibits the corrosion
process. Therefore, WS has shown the corrosion resistance is better than that of
CS. The durability of rust depends on the structure, properties as well as the
process of forming the protective layer on the steel surface. Thus, in Chapter 4,
the mechanism of formation and development of corrosion products on WS was
discussed in order to elucidate the protective possibility of rust formed on WS
in the atmosphere of Vietnam.
4.1. The formation of corrosion product on WS substrate in early stage of
exposure
In order to study the formation of CP on WS substrate, the samples were
exposed at Hanoi, Dong Hoi and Phan Rang for the short periods: 1, 3, 7, and
14 days. The variation of environmental parameters over time are shown on the
fig.4.1 and 4.2.

Fig.4.1. The variation of
temperature and humidity at
the test sites during short
periods of exposure

Fig.4.2. The variation of w and d/w during short periods of exposure
11


In atmospheric condition, the moisture and impurities from atmosphere
adsorb to the steel substrate and the electrolytic solution film was formed which
corrodes steel substrate as follows:
Fe + ½ O2 + H2O  Fe(OH)2

(4.1)

4Fe(OH)2 + O2  4 -FeOOH + 2H2O

(4.2)

4Fe(OH)2 + O2  2Fe2O3 + 4H2O

(4.3)

4.1.1. The morphology of corrosion product
Period

Hanoi

Dong Hoi

Phan Rang

1 day

3 days

7 days

14
days

Fig.4.3. The surface of samples after short exposed time
 The spots of rust formed and developed on WS substrate, the CP covered
almost surface after 14 exposed day (fig.4.3).


In every exposed period, the size of rust spots reduces in the range Hanoi –
Dong Hoi - Phan Rang. In Phan Rang, the high temperature and long drying
12


time delayed the corrosion process occurred, which formed the rust with small
size and dense CP (fig.4.4).
 The relative humidity in Dong Hoi is similar to Hanoi, however, the
temperature is higher (highest difference is 100C), which promoted the
water evaporation and formed the rust with smaller size in comparison with
Hanoi.
4.1.2. The formation the chemical compound of CP in the early exposed
stage

The anlysis results of micro Raman and X-ray were detected Goethite,
Lepidocrocite, Maghemite in bulk of CP after short time exposure in Hanoi and
Dong Hoi; Especially, the goethite α-FeOOH – most stable phase appeared
after only one day- exposure at Dong Hoi and after three day-exposure at Hanoi
test site.

Fig.4.5. The Raman spectrum of WS samples after 1 and 3 day exposed in
Hanoi and Dong Hoi.
G – Goethite (-FeOOH); L – Lepidocrocite (-FeOOH);
A – Akaganeite (-FeOOH); M – Maghemite.
The variations of w and d/w (fig.4.2) in Hanoi and Dong Hoi show that in the
first day -exposure in Dong Hoi, humidity is high but w reduced and d/w
increased, as a results, the drying time promoted the transformation of phases
13


Lin (Cps

800

700

400

d=1.7313

d=2.2458

d=1.5329

d=2.6599

d=2.7578

d=3.629

d=1.9294

d=2.4682
d=2.7578

d=2.6599

d=4.188

d=3.629

100

d=6.373

200

d=3.303

300

700

d=2.4682

d=3.303

VNU-HN-SIEMENS D5005 - Mau QB1d

d=2.2458

200

d=4.188

400

d=2.3533

800

d=6.373

Lin (Cps)

500

d=2.3533 d=1.4361

during the surface300changes from wetting to drying stage. In Hanoi, this variation of
w and d/w occured after two days exposure.

d=1.9294

600

100

0
11

0
11

20

30

VNU-HN-SIEMENS
D5005
- Mau QB1d
30
40

20

40

50

60

800

2-T heta - Scale
KHVL-HN3d.raw
Type:
2Th/Th
- Start:
10.000
° Cu
- End:
70.000
- Step:
File: Hong-Vien KHVL-HN3d.raw - Type:File:
2Th/ThHong-Vien
locked - Start: 10.000
° - End: 70.000 ° - Step: -0.030
° - Step
time: 1.0locked
s - Temp.: 25.0
°C (Room)
- Anode:
- Creation:
05/10/10°11:09:48
06-0696 (*) - Iron, syn - Fe - d x by: 1.000 - WL: 1.54056
06-0696 (*) - Iron, syn - Fe - d x by: 1.000 - WL: 1.54056
29-0713 (I) - Goethite - FeO(OH) - d x by: 1.000 - WL: 1.54056
08-0098 (N) - Lepidocrocite - FeO(OH) - d 29-0713
x by: 1.000 - WL:
(I) -1.54056
Goethite - FeO(OH) - d x by: 1.000 - WL: 1.54056
38-1479 (*) - Eskolaite, syn - Cr2O3 - d x by: 1.000 - WL: 1.54056
08-0098

- Lepidocrocite
- FeO(OH) - d x by: 1.000 - WL: 1.54056
11-0196 (D) - Copper Oxide Sulfate - CuSO4·CuO
- d x(N)
by: 1.000
- WL: 1.54056

700

400

0.030 ° - Step time: 1.0 s - Temp.: 25.0 °C (Room) - An

d=1.4361

d=5.764

d=6.263

500

2-T heta - Scale

38-1479 (*) - Eskolaite, syn - Cr2O3 - d x by: 1.000 - WL: 1.54056
11-0196 (D) - Copper Oxide Sulfate - CuSO4·CuO - d x by: 1.000 - WL: 1.54056

600

Fig.4.6. The X-ray spectrum of WS sample after 3 days exposed in Hanoi.
d=1.7258

d=1.8414

d=1.9342
d=1.8414

d=1.9342

d=1.7258

d=2.5050

d=3.281

200

d=2.5050

100

d=3.281

300

d=5.764

200

400

d=6.263

300

Lin (Cps)

Lin (Cps)

500

5
70

d=1.4361

600

100

0
11

20

0
11

30
20

40

30

40

50
50

2-T heta - Scale

60

60
70

2-T heta - Scale

File: Hong-Vien KHVL-QB1d.raw
- Type: -2Th/Th
locked
10.000
° -70.000
End:° -70.000
° °- -Step:
0.030
- Step
1.0 -sAnode:
- Temp.:
25.0 °C

(Room)
- Anode: Cu - Creation: 05/10/10 11:13:46
File: Hong-Vien KHVL-QB1d.raw
Type: 2Th/Th
locked-- Start:
Start: 10.000
° - End:
Step: 0.030
Step time:
1.0 s °
- Temp.:
25.0time:
°C (Room)
Cu - Creation:
05/10/10
11:13:46
06-0696
(*) --Iron,
- d x by:- 1.000
WL: 1.54056
06-0696 (*) - Iron, syn
- Fe
d xsyn
by:- Fe1.000
WL:- 1.54056
39-1346 (*) - Maghemite-C, syn - Fe2O3 - d x by: 1.000 - WL: 1.54056
39-1346 (*) - Maghemite-C,
syn - -Fe2O3
1.000
- WL: 1.54056

29-0713 (I) - Goethite
FeO(OH) -- ddx x
by:by:
1.000
- WL: 1.54056
08-0098
(N) - Lepidocrocite
FeO(OH)
- d x- by:
1.000
- WL: 1.54056
29-0713 (I) - Goethite
- FeO(OH)
- d x -by:
1.000
WL:
1.54056
25-1437 (I) - Guyanaite - Cr2O3·1.5H2O - d x by: 1.000 - WL: 1.54056
08-0098 (N) - Lepidocrocite
- FeO(OH) - d x by: 1.000 - WL: 1.54056
36-1330 (N) - Chromium Oxide - Cr3O8 - d x by: 1.000 - WL: 1.54056
25-0269-(I)Cr2O3·1.5H2O
- Atacamite - Cu2Cl(OH)3
x by:1.000
1.000 - -WL:
1.54056
25-1437 (I) - Guyanaite
- d x- dby:
WL:
1.54056

33-0645 (*) - Hydromolysite, syn [NR] - FeCl3·6H2O - d x by: 1.000 - WL: 1.54056
36-1330 (N) - Chromium Oxide - Cr3O8 - d x by: 1.000 - WL: 1.54056
01-0132 (N) - Iron Chloride Hydrate - 2FeCl3·7H2O - d x by: 1.000 - WL: 1.54056
25-0269 (I) - Atacamite - Cu2Cl(OH)3 - d x by: 1.000 - WL: 1.54056
33-0645 (*) - Hydromolysite, syn [NR] - FeCl3·6H2O - d x by: 1.000 - WL: 1.54056
01-0132 (N) - Iron Chloride Hydrate - 2FeCl3·7H2O - d x by: 1.000 - WL: 1.54056

Fig.4.7. The X-ray spectrum of WS
sample after 1 day exposed in Dong Hoi

Lepidocrocite -FeOOH
Wet stage

Amorphous compounds FeOx(OH)3-2x
Stage of change from
wet to dry in the surface

Fig.4.8. The transformation
in CP of WS
[57,63,64,67,68,99-107]

Goethite α-FeOOH

According to [57, 63, 64, 67, 68, 99-107], the transformation of -FeOOH to
-FeOOH phase is described in fig.4.8. Therefore, drying stage promoted
formation of -FeOOH and earlier apperance of this phase in Dong Hoi in
comparison with Hanoi.
14

300

300

d=2.5359

d=2.0281

d=2

200

Li

300

100
200

0
5

200

d=1.4333

d=2.5359

200

Lin (Cps)

10

20

30

40

0

5

10

20

30

40

50

60

2-T heta - Scale

70

2-T heta - Scale

File: Hong-Vien KHVL-WS-PR-3d.raw - Type: 2Th alone - Start: 5.000 ° - End: 70.010 ° - Step: 0.030 ° - Step time: 1.0 s - Temp.: 25.0 ° C

File: Hong-Vien KHVL-WS-PR-3d.raw - Type: 2Th alone - Start: 5.000 ° - End: 70.010 ° - Step: 0.030 ° - Step time: 1.0 s - Temp.: 25.0 °C (Room) - Anode: Cu - Creation: 07/31/12 10:42:55
06-0696
06-0696 (*) - Iron, syn - Fe - d x by: 1.000 - WL:
1.54056 (*) - Iron, syn - Fe - d x by: 1.000 - WL: 1.54056
26-0508 (D) - Copper Chromium Oxide - CuCr2O4 - d x by: 1.000 - WL: 1.54056
26-0508 (D) - Copper Chromium Oxide - CuCr2O4 - d x by: 1.000 - WL: 1.54056
04-0755 (D) - Maghemite, syn - Fe2O3 - d x by: 1.000 - WL: 1.54056

100

04-0755 (D) - Maghemite, syn - Fe2O3 - d x by: 1.000 - WL: 1.54056

0

d=1.4320

d=1.9343

PR 7 days

d=1.9343

d=2.5498

d=2.4643

d=2.5498

d=2.4643

d=3.317

100

d=3.317

Lin (Cps)

PR 3 days
100

0

5

5

10

10

20

20

30

30

40

40

50

60

50

70

2-Theta - Scale2-T heta - Scale
File: Hong-Vien
KHVL-WS-PR-7d.raw
- Type:
2Th- Start:
alone
- Start:
End:
70.010
° time:
- Step:
- Step
time:- Anode:
1.0 s Cu
- Temp.:

25.0
°C 16:49:57
(Room) - Anode: Cu - Creation: 08/02/12 16:49:57
File: Hong-Vien
KHVL-WS-PR-7d.raw - Type:
2Th alone
5.000
° - End:5.000
70.010 °° --Step:
0.030
° - Step
1.0 s0.030
- Temp.:°25.0
°C (Room)
- Creation:
08/02/12
06-069606-0696
(*) - Iron,
synsyn
- Fe
by:1.000
1.000
1.54056
(*) - Iron,
- Fe- -dd x by:
- WL:- WL:
1.54056
(I) - Goethite
- FeO(OH) --ddx by:
1.0001.000

- WL: 1.54056
29-071329-0713
(I) - Goethite
- FeO(OH)
x by:
- WL: 1.54056
(N) - Akaganeite-M, syn
syn - -FeO(OH)
- d x by:
- WL:
1.54056
34-126634-1266
(N) - Akaganeite-M,
FeO(OH)
- d1.000
x by:
1.000
- WL: 1.54056
(N) - Lepidocrocite--FeO(OH)
FeO(OH) - d x- d
by:x1.000
WL: 1.54056
08-009808-0098
(N) - Lepidocrocite
by: -1.000
- WL: 1.54056
(D) - Copper
Chromium Oxide
- CuCr2O4
- d x by:-1.000

- WL:1.000
1.54056- WL: 1.54056
26-050826-0508
(D) - Copper
Chromium
Oxide
- CuCr2O4
d x by:
04-0755 (D) - Maghemite, syn - Fe2O3 - d x by: 1.000 - WL: 1.54056
04-0755 (D) - Maghemite, syn - Fe2O3 - d x by: 1.000 - WL: 1.54056

Fig.4.9. The X-ray spectrum of WS sample
after 3 and 7 day exposed in Phan Rang

The X-ray analysis results of WS samples in Phan Rang (fig.4.9) found FeOOH and -FeOOH phase after 7 day-exposure, however only Fe2O3 phase
was detected on the sample exposed for 1 and 3 days . It can be explained that
the high temperature and reduction of w during three first days in Phan Rang
promoted the reaction (4.3) to form Fe2O3. From 4th to 7th days of exposure, the
TOW is more longer (w increased from 16% up to 58%), thus CP includes
Fe2O3 and -FeOOH.
4.1.3. The appearance of alloying elements in CP at the early exposed stage
 Cu and Cr elements were presented after the first day exposure in all test sites,
except in PR, Cu only was found on the samples after 3 day- exposure. The
content of Cu and Cr in CP’s at HN and DH are higher than that at PR. It seems
to be high humidity or long TOW in HN and DH promoted Cu and Cr
dissolution. The closer to steel substrate the positions are, the higher Cr and Cu
contents were detected.
Elm
O
Si

Cl
Cr
Fe
Cu

Elm %mass
O
23,59
Si
0,36
S
0,59
Cl
0,63
Cr
0,54
Fe 74,12
Cu
0,16

a)

% mass
28,04
0,73
0,57
2,12
67,65
0,89

Elm
O
Si
Cl
Cr
Fe
Cu

% mass
29,19
0,26
0,18
0,33
70,03
< 0,01

c)

b)

Fig.4.11. The chemical composition of rust at steel substrate after the first
exposed day in Hanoi (a), Dong Hoi (b) and Phan Rang (c)
15

60


 Cu and Cr elements were found at the inner layer of rust and play role to
inhibit corrosion of WS; Cr formed the α-(Fe1-xCrx)OOH phase with
nanometer size. The X-ray analysis results (shown on 4.1.2) also found the

compounds of copper sulphate (HN), copper chlorua (DH) and chrom oxide
in CP of 3 test sites. Because in early exposed stage the wetting duration
was not long enough, only copper oxide was found in CP formed at PR.
Tab.4.2. The concentration of Cr and Cu at inner side rust layer in early
exposed stage, mass % .
Site

Hanoi

Dong Hoi

Phan Rang

Element

Cr

Cu

Cr

Cu

Cr

Cu

1 day

0.54

0.16

2.12

0.89

0.33

< 0.01

3 days

1.87

1.34

1.73

1.10

0.60

0.32

7 days

3.26

2.33

4.45

3.04

0.69

0.37

14 days

3.33

3.61

3.35

1.82

1.28

1.14

4.2. Characteristics and corrosion protectiveness of CP in long-term
exposure
In the early stage of testing, the rust layer formed on WS at each atmospheric
region contained typical phases. Especially, the -FeOOH phase was formed
very early and the rust layer closed to the steel substrate contains the
compounds of the Cu and Cr alloying elements. The formation of CP in the
early stages affect the structure and properties of the rust formed after long-term

exposure, and this impact the corrosion protectiveness of the CP layer over
time. The process of formation the CP layer in the long-term exposure was
studied, evaluated and discussed below.
4.2.1. The morphology and structure at cross section of CP
 The morphology of rust formed on WS samples showed flowery/sandy
crystals of γ-FeOOH phase and cotton ball crystal of α-FeOOH phase [35,
78, 84, 129-131].
 The CP in Phan Rang consists of spherical crystals packed with less cracks,
their size is smaller than that of crystals formed in the atmosphere of Hanoi
and Dong Hoi. This can be explained by the high temperature and the long
drying cycle in Phan Rang, which reduced the size of CP and compacted the
rust.
 The cracks of rust are filled with the new CP (like cracks are sewn). This
can be explain that the cracks are the channels through that the corrosive
agents attacks the steel substrate, there the corrosion reactions occur
16


continuously consequently the voids of rust layer are filled with new CP and
improved.
The cross section of CP showed that:
 The CP formed on the WS consist of two layers: (1) the inner layer close to
the steel substrate and has a dense structure, less cracks and adhere to the
steel base; (2) the outer layer is porous with many cracks, poor bonding to
the inner layer (fig.4.16 &4.19).
Surface of sample

-FeOOH crystal

α-FeOOH crystal

HN - 6
month

DH – 6
months

PR – 6
months

Fig.4.13. The morphology of WS samples after 6 months exposed

Hanoi
Dong Hoi
Phan Rang
Fig.4.14. The morphology of WS samples after 24 months exposed
 Time by time, the inner rust layer is thicker and becomes more completed,
denser and less porous/crack.
17




In the humid climate of Hanoi and Dong Hoi, the evaporation of water was
impeded, thus the formed rust layer are porous with the cracks. In contrast,
the long drying time favored the formation of rust on WS at Phan Rang,
which has the compact structure, less pore and crack.

 The outer rust layers formed at Hanoi and Dong Hoi have many cracks
which reduces the bond between the inner and outer layers, so they are easy

to peel off.
 The fresh CP created (dark brown) fills the cracks in the rust.
 The cross section of rust formed on CS shows that the rust has no clear
layered structure, it is porous with many cracks. This explains why there is a
big difference between ML of CS and WS at the test stations (see 3.1).
Outer layer
Inner layer
Steel substrate

Steel substrate

Hanoi

Phan Rang
Outer layer
Inner layer
Steel substrate
Dong Hoi

Fig.4.16. The cross section of rust formed on WS after 12 months exposed, 500x

Inner layer
Steel substrate
Steel substrate

Hanoi

Dong Hoi
Inner layer

Steel substrate

Phan Rang

18

Fig.4.18. The cross section of
rust formed on WS after 36
months exposed, 500x


Hanoi
Dong Hoi

Fig.4.20. The cross section of
rust formed on CS after 24
months tested, 500x
Phan Rang

4.2.2. The chemical compound of rust
The X-ray results of long-term exposure detected maghemite -Fe2O3,
goethite α-FeOOH, lepidocrocite -FeOOH, akaganeite -FeOOH and amorphous
compounds in the rust, those are typical phases formed on WS in the atmosphere.
Tab.4.3. The chemical compounds of CP formed on WS, mass %
Phan Rang
Hanoi
Dong Hoi
12 m 24 m 36 m 12 m 24 m 36 m 12 m 24 m 36 m
β-FeOOH
0

0 0,49
0 4,22 2,68 2,82 5,12 4,28
γ-FeOOH 31,84 29,74 24,32 32,84 36,95 30,87 33,61 68,68 50,44
α-FeOOH 11,99 19,85 19,38 11,26 15,42 19,15 7,37 13,52 13,88
Magnetite 1,64 1,25 1,10 0,80 7,90 0,92 0,56 1,15 1,06
amorphous 54,53 49,16 54,71 55,09 35,51 46,38 55,65 11,53 30,34
The relationship between content of phase with w and d/w shows that: the FeOOH phase content is low at the sites where the w is high (Hanoi and Dong
Hoi); the amorphous phase content is low at the site with high ratio d/w (Phan
Rang). According to Misawa [57, 99-101], the solubility and precipitation of FeOOH in the wet cycle formed the amorphous FeOx(OH)3-2x. Accordingly, the
long TOW in Hanoi and Dong Hoi promoted the phase transition of -FeOOH
to amorphous, thus the rust in Hanoi and Dong Hoi consists of lower -FeOOH
and higher amorphous phase than that of Phan Rang.
Phase

The analysis results showed that the content of α-FeOOH in CP and ratio of
goethite/lepidocrocite phases (α/) increased over time, which indicates that the
19


phase transition of -FeOOH to α-FeOOH takes time: the more exposure time is
long , the more amount of α-FeOOH is produced, thus the inner layer is more
improved and protectiveness of CP is better . The higher α/ ratio is the better
rust protectiveness [111] and this decreases CR. The ratio of α/ in CP
decreases in the order of Hanoi > Dong Hoi> Phan Rang. It is possible that the
wetting time in Hanoi (52.7%) and Dong Hoi (65.1%) are longer than those in
Phan Rang (42.4%), which accelerates the formation of -FeOOH phase and
transformation into intermediate amorphous phases before conversion into αFeOOH. However, the d/w (0.5) in Dong Hoi was much smaller than in Hanoi
(0.9), so the α-FeOOH phase was not as much supported as in Hanoi.
In Hanoi and Dong Hoi, α/> 0.3, so the CR decreases when this ratio increases
- fig.4.26, however, in Phan Rang, these relationship is no clear. It is because

the weather in Phan Rang is too dry, the transition rate of lepidocrocite to
goethite very slow, thus the amount of goethite is not enough to contribute to
the protection of rust.
Tab.4.4. The relationship of Lepidocrocite, amorphous phases and w, d/w ratio
with the exposed time.
Period

Site
Hanoi
12
Dong Hoi
months
Phan Rang
Hanoi
24
Dong Hoi
months
Phan Rang
Hanoi
36
Dong Hoi
months
Phan Rang

-FeOOH, %
31,84
32,84
33,61
29,74
36,95

68,68
24,32
30,87
50,44

Amorphous, %
54,53
55,09
55,65
49,16
35,51
11,53
54,71
46,38
30,34

w, %
49,4
64,9
42,8
51,3
66,9
42,9
52,7
65,1
41,4

d/w
1,02
0,5

1,34
0,95
0,5
1,33
0,9
0,5
1,41

Fig.4.25. The α/ ratio of CP
formed after 3 years exposed

20


Fig.4.26. The relationship
between / ratio with CR
of WS

4.2.3. The distribution of Cu and Cr elements in the CP
The results of the above analysis showed that the compact internal layer which
is rich of stable α-FeOOH phase was formed on the WS surface. Meanwhile,
under the same test conditions, such layer has not formed on CP of CS. This
indicate that Cu and Cr alloying elements are needed to promote the formation
of protectiveness rust layer.
The line scan results of rust cross-section showed that:
 At all test sites, Cu and Cr are concentrated in the rust with distance of about
20 ÷ 40 m (Hanoi and Phan Rang) or 60 m (Dong Hoi) far from WS
substrate, this distance is corresponding to the thickness of the internal rust
layer, this proves the richness of Cu and Cr elements in the inner layer of CP.
 The concentration of Cr element in the rust layer very closed to the steel

substrate demonstrates the ability to form a Cr-FG compound closed to the
steel surface; the thickness of Cr-FG rust increases over time.

Fig.4.31. The distribution of Cu and Cr in the rust formed on WS after 3 months
exposure

21