Atmospheric corrosion monitoring and modeling of a low alloy steel under an electrolyte film in cyclic wet–dry condition Prof. Dr. Junhua Dong Environmental

Atmospheric corrosion monitoring and modeling of a low alloy steel under an electrolyte film

in cyclic wet–dry condition

Prof. Dr. Junhua DongEnvironmental Corrosion Center

Institute of Metal Research, Chinese Academy of Sciences

Shenyang, China

Sept. 26, 2014

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1. Thermodynamics: boundary condition2. Kinetics: reaction way3. Corrosion rate: how fast? Need monitoring

The aim of corrosion study is to know:

Introduction

Introduction

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Initial stage: The adsorption and absorption of water

Middle stage: The deposition of gaseous phase,

The chemical changes in the liquid phase, and the dissolution of steel.

Final stage: The aggregation, growth,

and thickening of the corrosion product.

Atmospheric corrosion evolution of Low Alloy Steel (LAS)

The initial stage and the middle stage experienced very short time, even within

1 wet/dry cycle. The final stage occupied most of the service time.

Fe （ M ） = Fe2+sol ） + 2e （ M ）

4Fe(OH)2 +O2 + 2H2O = 4Fe(OH)3

solMg OHeOHO 442 22

Introduction

1. Industrial (SO2): - or - FeOOH

2. Coastal (Cl-): -FeOOH

3. Industrial – coastal (SO2+ Cl-) correlate with each content

1. Weathering steel: Cu, P, Cr, Ni, Mo, Si,

Cu: Fe3O4; Ni: Fe3O4,- FeOOH;

Cr: - FeOOH; Mn-Cu: Fe3O4, -FeOOH

Atmospheric corrosion occurs under thin electrolyte film with Wet-dry cyclic change

Humidity and temperature codetermine the electrolyte film exists or not and the time of wetness. Moreover, temperature increase promotes corrosion rate

Atmospheric pollutants such as SO2, Cl-, etc. impacts on the corrosion mechanism.

Alloy elements make effects on the corrosion process by improving the rust structure and composition

The rust can also make effects on the corrosion process

8FeOOH + Fe2+ + 2e = 3Fe3O4 + 4H2O

Misawa, T., Asami, K., Hashimoto, K., and Shimodaira, S., Corr. Sci., vol. 14, pp. 131-149, 1974.

Kamimura, T., Hara, S., Miyuki, H., Yamashita, M., and Uchida, H., Corr. Sci., vol. 48, pp. 2799-2812, 2006.

Introduction

• Wet/dry cyclic test• 25C, 60 (RH), 12hrs• Solution: 0.005% - 5% NaCl

Atmospheric corrosion evaluation of Low Alloy Steel (LAS)

• Wet/dry cyclic corrosion test• 25C, 60 (RH), 12hrs• Solution: 0.005% - 5% NaCl

• Traditional data sampling in exposure site: 0.5, 1, 2, 4, 8, 16, , in year• Cyclic Wet/dry corrosion test in laboratory: wet/dry cycle number

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• No data sampling within a year or 6 months.• monotonic increasing function for corrosion loss but decreasing function for corrosion rate

• Data sampling start from the first wet/dry cycle.• increasing function for corrosion loss.• first increase then decrease functions for corrosion rate

Introduction

Atmospheric corrosion of steel involves

1.Electrolyte properties : Air pollutants , e.g. SO2 and Cl-

2.Cycle of wetting and drying , TOW : Temperature and Humidity

3.The rust evolution

For Weathering steel , CR initially increases , but when the protective rust is formed , CR decrease.

Therefore, the rust evolution plays the important role to determine the remaining life time of this steel.

Characteristics of Atmospheric Corrosion

Atmospheric corrosion monitoring

-Categorized by the nature and pollutions content, the corrosive atmosphere can be divided into Rural, Urban, coastal, industrial, and coastal-industrial atmosphere.

Atmospheric environment classification

Most severe corrosive atmospheres

1. Industrial atmosphere: In this atmosphere , sulfur dioxide plays the major role in atmospheric corrosion process.

2. Coastal atmosphere : Chlorides from the sea play an important role.

3. Coastal – Industrial atmosphere

-The expansion of the industry along the coastline.

-In this atmosphere, the combined effect of sulfur dioxide and chloride plays the decisive role


www.mti-global.orgCorrosion Monitoring in the this thin layer provides us with direct information about the transient change in the corrosion behavior.

Effect of electrolyte film thickness on Atmospheric Corrosion

Prof. Tomashov’s model


- In 1990 , Stratmann et al

Technique : Polarization technique

The too high solution resistance during the evaporation process can cause error.

- 1995 , Tsuru et al

Technique : Needle probe + Electrochemical Impedance Spectrography (EIS )

Electrode : Chip shaped bi-electrode The Sensitivity is 10 um.

- 2007 , Dong et al

Technique : thickness measurement + EIS

Electrode : Chip shaped bi-electrode

Almost the end of evaporation process ,

Electrolyte tends to accumulate along the electrodes.

Needle probe techniques

Chip shaped bi-electrode

Electrolyte thickness measurement , Dong et al


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In 2009 , Dong et al Technique : Electrolyte thickness measurement + EISElectrode : Comb shaped bi-electrode , aiming to improve accuracy of data.The Sensitivity is 1 um.

- All of works are valuable. -These works did not study the rust evolution of steel in different polluted atmospheres and the effect of temperature on the corrosion kinetics of steel.-Besides , all of works were carried out at the

low humid conditions.

Comb shaped bi-electrode


A. Material : Weathering steel was used as the electrode material Element Cr Cu Ni C Mn Si P Fe

Composition 0.38 0.24 0.16 0.06 1.16 0.0135 0.019 Balance

Simulated Atmospheres

Electrolyte

Coastal 0.05 mol/l of Nacl

Industrial 0.005 mol/L of Na2SO3 pH 4

Coastal – Industrial 0.05 mol/l of Nacl + 0.005 mol/L of Na2SO3 pH 4

B. The simulated typical atmospheric corrosion electrolytes


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• To study the effect of the electrolyte thickness on the atmospheric corrosion behaviors taking place on the substrate of weathering steel under the simulated atmospheres.

• To obtain the corrosion kinetics of this weathering steel as a function of the cyclic wet – dry corrosion test (CCT) under the simulated atmosphere.

• To study the effect of temperature on the

atmospheric corrosion of this weathering steel.


*During EIS measurement, one part with the area of 0.5 cm2 worked as working electrode, and the other part worked as reference electrode and auxiliary electrode as indicated in Fig. d.

C. Electrode preparation


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D. Schematic Diagram of the apparatus of the experiment

The analytic balance with accuracy d = 0.1 mg.

Temperature and humidity were maintained with desired Temperature ± 1 oC and 60 ± 2 % RHThe humidity control system contains water-glycerol mixture, the ratio of which was prepared according to ASTM D 5032


E. Cyclic wet-dry Corrosion Test ( CCT )

Drying

Electrolyte

Drying

Water

1st CCT

Other CCTs

Electrolyte

Steel

Evaporation process. Electrolyte

Steel

X

F. The thickness of electrolyte film (X ) :

SWX

d

where and S are the density and surface area of the electrolyte film


G.Electrochemical Parameter Monitoring

Cdl

RsolRp

current

Metalsolution

the electrical double layer ( edl)

HF

LF

Rp−1 = ( ZLF - ZHF ) -1

Rrust = ZHF


Atmospheric corrosion monitoring study

The influence of temperature

H.Temperature conditions


N

N

b

a

AdNxf1

)(

1 N

a b

Trapezoidal rule

The mathematical approach


- Electrolyte decreases linearly.- Concentration-build up during the evaporation process.- Accelerating corrosion process.- Minimum corrosion resistance appears at 13 um, maximum corrosion rate.

Thickness measurement EIS resultsSubstrate ( 1st CCT)


- TOW becomes longer due to the formation of the thick rust layers.

- The corrosion process is also accelerated.- The frequency characteristics shifts to the lower frequency side, indicating the increased capacitance of the substrate.

Thickness measurement EIS resultsRusted Steel ( 56 th CCT )


A. During Evaporation Process,

In all CCTs, CR gradually increases with evaporation of electrolyte .

B. During CCT,

1. Initial Stage : CR increase

2. Barrier effect = the corrosion effect , corrosion reaches a max

3. Second Stage :

Barrier effect > the corrosion

effect , corrosion decreases

4. Finally , Stationary corrosion

Process.

- Evolution in the charge transfer resistance


A. During Evaporation Process,

Rrust gradually decreases with the evaporation of the electrolyte and then abruptly increases.

B. During CCT,

Initial Stage :

The rust layer is non-protective.

As the corrosion process proceeds ,the rust layer becomes compact and adhesive to the steel substrate.

Hence, the improved barrier effect of the rust layer can be attributed to an increase in the CCT number.

- Evolution in the rust resistance


Substrate Substrate

RustedRusted

The influence of the rust and temperature on TOW


The effect of temperature on substrate


The effect of temperature on rusted steel


The effect of temperature on the whole EIS data


• Corrosion process of steel in all condition showed 2 stages

• Transition point ( TP) indicates the formation of the protective rust

• 30 C : The earliest formation of the protective rust.

• 15 C : Rusting rate of 15 C is too slow to form the protective rust.

• Increasing T accelerates the formation of the protective rust and enhance the protectiveness of the rust

The effect of temperature on the corrosion rate and corrosion mass loss


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Under the higher temperature (30 and 25 C) Corrosion process is composed of two stages.- Steel shows the higher corrosion mass loss - The later appearance of the transition point

Under the lower temperature (20 and 15 C) Corrosion process is composed of four stages - Steel shows the lower corrosion mass loss -Decreasing T can prolong TOW, leading to 1.the longer period for corrosion 2.an increase in oxygen solubility in an electrolyte layer



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- Corrosion process of steel in all condition showed 2 stages

- TP represents the formation of the protective rust. - The higher temperature can lead to the earlier formation of the protective rust.

- Increasing T accelerates the formation of the protective rust and enhance the protectiveness of the rust.



- The higher the exposure temperature is, the earlier the transition point appears.

- The lower exposure temperature brings simultaneously with the longer TOW.

- In suitable conditions, T and TOW will not act in the opposite way ,

But both jointly accelerate the rusting process of steel

100 120 140 160 180 200

120

160

200

W X

10

^-

4 g

TOW /Hrs

Corrosion Mass loss , and TOW for whole CCT test

15C20C

30C

25C

0.05 molL-1 NaCl + 0.005 molL-1 Na2SO

3

pH = 4.0

(b)

Too low Temp

Under temp and TOW

The effect of TOW and temperature on corrosion mass loss


10-2 10-1 100 101 102 103 104 105-10

0

10

20

30

40

50

60

Ph

ase

an

gle

(d

eg

)

Frequency, Hz

3rd, 404 m

1st, 494 m

6th, 264 m

8th, 172 m

9th,124m

10th,76 m

11st, 32 m

13rd ,0m

12nd,0m

10-2 10-1 100 101 102 103 104 105-10

0

10

20

30

40

50

Ph

ase

(D

eg

)

Frequency (Hz)

3rd, 398 m1st, 491 m

5th, 309 m

7th, 218 m10th,79 m11 st, 36 m

10-2 10-1 100 101 102 103 104 105

-20

0

20

40

60

80

Phase a

ngle

(deg)

Frequency, Hz

15th,0m

3rd, 423m

5th,349m9th,206m

1st,496m

14th,22m

11st,133m

13rd,62m

16th,0m

10-2 10-1 100 101 102 103 104 105-10

0

10

20

30

40

50

Ph

ase

an

gle

(d

eg

)

Frequency, Hz

13rd,76 m

14th,41m

3rd, 421m

7th,283 m

1st,491 m

11st,145 m

Coastal atmosphere Industrial atmosphere Coastal-Industrial atmosphere


GraphicGoesHere

This research provided the systematic study of weathering steel

under Simulated coastal atmospheres, industrial atmospheres, Coastal-industrial atmospheres, The role of temperature on corrosion kinetics, and Mathematical approach based on the numerical integration method.

1. During the evaporation process of the electrolyte

- The peak of phase angle moves to the higher frequency side.

This is due to the increase in Cl concentration

- The peak of phase angle moves to the lower frequency side

because of the increase in sulphate concentration

2. During the evaporation process of the electrolyte,

The corrosion rate increases due to

- an increase in the concentration of chloride or sulphate,

- an enhancement of oxygen diffusion

which are influenced by the electrolyte thickness reduction.

Conclusions

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3. During wet-dry corrosion process ,

In the initial stage : the corrosion rate increases.

When the protective effect of the rust layer is in balance with the corrosion effect, the corrosion rate reaches a maximum. Then, as the rust layer grows in thickness and compactness , the corrosion rate decrease.

4.The application of the numerical method is successfully demonstrated in the study. This method would be an alternative to the traditional corrosion assessment and it would be useful for the development of the simulated indoor atmospheric corrosion tests.

5.The results on the influence of the temperature indicated that

-The corrosion process of steel for all temperature conditions was composed of more than one corrosion stages.

-An increase in temperature accelerated the formation of the protective rust and improved the protectiveness of the rust.

-Under the appropriate weathering conditions,

TOW and temperature did not act in the opposite way, but both jointly enhanced the rusting process of steel.

Conclusions

Thank you for your attention!

Documents

Atmospheric corrosion monitoring and modeling of a low alloy steel under an electrolyte film in cyclic wet–dry condition Prof. Dr. Junhua Dong Environmental