Progress of Mechanism and Research Methods of

Marine Corrosion of Steels

Yang Jie 1,a, Xi Guangfeng 2,b and Fan Xiqiu 3,c 1,3 School of Electromechanical Engineering, Zhejiang Ocean University, Zhoushan, China

2 Shandong Special Equipment Inspection Institute, Jinan, China

akittyangj@yahoo.com.cn, bxiao940312@163.com, cxiqiufan@163.com

Keywords: Marine environment, influence factor, corrosion mechanism

Abstract. Steel constructions located in marine environments are exposed to more serious corrosion

than in other environments, which result in great loss in lives and economy. So it is very important to

carry out research work on the influence factor and mechanism of marine corrosion with necessary

methods. In this paper, influence factors in marine environment of steels were discussed, the

corrosion mechanism of five different zones in marine environment was summarized and the progress

of research methods for marine corrosion was reviewed.

Introduction

Exploitation and utilization of marine resources can not be separated from the construction of marine

infrastructure. As the marine environment is a highly corrosive environment, relative humidity in

marine atmosphere is always higher than its critical value and corrosive water film is very easy to

form on the surface of steel in the marine atmosphere; seawater containing high concentration of salt

is an easily conductive electrolyte solution and one of the most corrosive natural cauterants as well.

At the same time, wave, tide and current produce low-frequency and reciprocating stress and impact

to metallic components. In addition, marine micro-organisms, fouling organisms and their metabolic

products also have direct or indirect effects on acceleration of corrosion process. Therefore, steel and

other materials widely used in many engineering constructions are prone to corrosion damage, which

result in not only waste of material, but also more seriously disastrous accident, environmental

pollution, casualties and huge economic losses.

As industrial materials, for its toughness, high strength and low price, steel is widely used in

marine environment; but marine corrosion of steel structures is still inevitable for the severe

environment. Then corrosion and protection of steels in marine environment becomes a major

subject. Therefore, it is of great significance to carry out research work on steel corrosion and its

protection measures in marine environment, in order to extend the life of the marine steel facilities

and ensure the normal operation and safe use of offshore steel structures.

Influence factors of steel corrosion in marine environment

Sea water is not only a natural strong electrolyte solution with salinity of 32‰~37‰ and pH value

between 8~8.2, but also a complex system with suspended sediment, dissolved gases, biology and

organism. Marine corrosion of steel in marine environment results from integration of many factors,

such as dissolved oxygen, salinity, temperature, pH, current rate, marine biology and steel alloying

elements.

DO. One of the most important factors of marine corrosion is dissolved oxygen in sea water. It has

a very strong effect of cathodic depolarization and continuously reacts in micro-cathode region of the

steel corrosion, resulting in the dissolution of the metal, formation of ferrous hydroxide in anode

region and corrosion of the metal. On the other hand, formation of oxide film on some of the metal

surface inhibits the corrosion reaction to some extent.

Applied Mechanics and Materials Vols. 80-81 (2011) pp 3-6Online available since 2011/Jul/27 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.80-81.3

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Salinity. A large number of neutral salts, such as NaCl, KCl, Na2SO4, are dissolved in seawater

and 78% of them are NaCl. NaCl concentration in seawater is generally 3%, while around the

concentration the corrosion rate reaches its maximum value. When the salt concentration is lower, the

corrosion rate increases with the salt content rapidly, which is mainly due to the anode reaction

promoted by the increase of Cl-. In addition, with the increase in salt concentration the solubility of

oxygen reduces, and then the corrosion rate decreases significantly when the salinity of the solution

continues to increase.

Temperature. Generally, oxidation reaction of steel in seawater gets faster as the temperature

rises. The interrelationship is complex, that is, the corrosion rate is not increasing proportionally with

the increase of temperature but also connected with the oxygen diffusion and other factors. Corrosion

rate is dominated by oxygen diffusion, and the rise of temperature reduces the solubility of oxygen in

the solution, slowing the process of cathodic reaction.

PH value. In general, increase of the pH value in seawater is helpful to inhibit the steel corrosion.

However, pH value in seawater changes little, which has no significant effect on corrosion of steel

(far less than DO). Although pH values in the surface water are higher than that in the depth,

corrosion of steel in the surface water is more severe than that in the depths of water for the higher

dissolved oxygen content.

Current rate. The flow of liquid on metal surface can not only promote the cycle of corrosive

elements in the solution and accelerate the diffusion of oxygen, but also remove corrosion products

attached on the metal surface, thereby promoting the metal corrosion.

Biology. Influence of biology on marine corrosion of steel is very complex. Attachment of some

organisms can decrease the corrosion rate of steel, afterwards accelerate the corrosion and initiate

pitting corrosion or destruction of coating. On the one hand the coverage of organisms on the steel

surface can produce localized corrosion, or oxygen concentration cell corrosion; on the other hand,

biological activities change the component and nature of water thus accelerating the corrosion of

steel.

Alloying elements. The influence of alloying elements on corrosion is also connected with

environmental factors. Among alloying elements, molybdenum, copper are effective to improve

corrosion resistance of steel in splash zone, while chromium, molybdenum and phosphorus are

effective elements in immersed zone.

Corrosion mechanism of steel in marine environment

From the viewpoint of corrosion, the marine environment is divided into five zones: marine

atmospheric zone, the splash zone, tidal zone, seawater immersed zone and sea mud zone [1,2].

Corrosion in marine atmospheric zone. Marine atmospheric zone refers to the area above the

splash zone and coastal atmospheric area, which means the part beyond the seawater perennially for

marine steel structures. For the high humidity of oceanic atmosphere, water vapor easily attaches to

the steel surface and forms a layer of invisible water film dissolved CO2, SO2 and salt, then it becomes

a highly conductive electrolyte solution. Compared with the inland atmosphere, the relative high

humidity of marine atmosphere results in thicker water film, higher salinity and more powerful

electrolytic water film, in addition with wet and dry cycles, which greatly accelerate the corrosion

rate.

Corrosion in splash zone. Splash zone is the area of structure surface exposed to the splash of

seawater while beyond the immersion of seawater at high tide in the marine environment. For many

metallic materials, corrosion in the splash zone is the most severe of all. A variety of reason for the

most serious corrosion in splash zone has been discussed by researchers, while the large amount of

salt particles, long residence time of water film and the dry-wet alternation are the external factors on

the whole, and the internal factor is the increase of cathode current due to the role of oxidant layer

during the corrosion process.

4 Information Engineering for Mechanics and Materials

Corrosion in tidal zone. Tidal zone refers to the area between the average high tide and low tide

level. Steel corrosion in the tidal zone has two main types, one of which is the corrosion of separate

steel components in the tidal zone, such as the discharge gate in the tidal zone; the other is the

corrosion of steel piles. Experimental results show that corrosion rate of separate specimens is far

higher than electrically connected specimens in the tidal zone, which is mainly due to the formation

of macro cell between the immersed part and the part in the tidal zone of electrically connected

specimen, with tidal parts serve as cathode for sufficient supply of oxygen and underwater part serve

as anode to provide protective current to the cathode, resulting in the decrease of corrosion in tidal

zone.

Corrosion in immersed zone. Immersed zone is the area under the low tide level to the bottom of

the sea perennially. According to different depths of the sea the zone is divided into shallow water

area, continental immersion area and deep sea area. Corrosion in the shallow water is dominated by

MIC and electrochemical corrosion, physical and chemical effects as the second; corrosion in

continental immersion area is lighter than shallow water, which is mainly electrochemical corrosion

supplemented by physical and chemical corrosion; in the deep sea the electrochemical and stress

corrosion are the main types.

Corrosion in sea mud zone. Sea mud area is mainly composed of the bottom sediment under the

immersion zone. The physical, chemical and biological properties of sediments vary with the sea area

and water depth. Compared with terrestrial soil, sea mud is a good electrolyte and more corrosive

with high salinity and low resistivity. In addition, bacteria, primarily anaerobic sulfate-reducing

bacteria in the seabed soil also affect the corrosion of steel.

Research methods of marine steel corrosion

In order to assess the corrosion resistance and study the corrosion mechanism of different steels in

marine environment, seawater exposure test is usually needed. This method has advantages for the

test media and conditions accord with the natural situation thus the result is reliable. The initial work

in this area is done by Larrabee [3], who carried out the work on the accumulation of atmospheric

corrosion data and summary of the corrosion mechanism. Countries all over the world have

established many marine corrosion test sites, and a great deal of research work applying the method

of seawater exposure test has also been carried out in Qingdao, Zhoushan, Xiamen and Yulin of

China by national researchers since 1983 [4,5,6]. However, a national study of seawater corrosion test

in the confined environment usually can not fulfill the requirements of production and scientific

research, therefore, a number of international technical cooperation have been undertaken in recent

decades in terms of transcontinental and international joint testing.

Since seawater exposure test is a reliable method, its disadvantage is obvious. A lot of labor,

material and heavy workload are needed in the test and it is inconvenient for maintenance and

observation of the specimen. In addition, because of the severe marine environment, the specimen is

easy to lose, which cause difficulties to the work, or even failure. For convenience, researchers have

developed more test methods. A patent proposed the device and method for simulated marine

corrosion test, which has a good correspondence with the seawater exposure test result. With the rapid

development of modern science and technology, researchers have developed a number of indoor

corrosion test methods, such as electrochemical test and accelerated corrosion test, etc.

Electrochemical test can get instantaneous information of material corrosion, which is helpful to

analyze the corrosion state of materials and influence of environmental factors on the corrosion

behavior, thus obtaining in-situ, dynamic, continuous corrosion data, which is more important for

accumulation of environmental corrosion data.

Accelerated corrosion test is widely used in material or structural performance test, which

concentrate on accelerated environmental spectrum and accelerated corrosion factor (ACF). In terms

of the research of accelerated corrosion test, the basic requirements to accelerated environmental

spectrum are simulation, reproducibility and acceleration. Currently, a number of standards for

accelerated corrosion test of metal and non-metallic materials have been established in the field of

material science and widely used in engineering.

Applied Mechanics and Materials Vols. 80-81 5

Summary

Influence factors and the corrosion mechanism of steel corrosion in the marine environment are very

complex. Influence factors can be summarized as three aspects: chemistry, physics and biology,

which are closely related to the characteristics of the corrosion in five zones of marine environment.

With people's understanding for marine corrosion of steel, more and more advanced test devices and

corrosion monitoring technologies are developed, which is very important for the material selection

and anti-corrosion design in practical engineering.

Acknowledgements

This material is based upon work funded by Program for International S&T Cooperation Projects of

China (2010DFR50860) and Zhejiang Provincial Natural Science Foundation of China under Grant

No.Y4090065. The support from China Spark Program (No.2010GA700082) is also gratefully

acknowledged.

References

[1] Hou Baorong, Zhang Jinglei, Marine Science No.4(1980), p.16.

[2] Hou Baorong, Corrosion & Protection Vol. 28(2007), p.174.

[3] Larrabee C. P. , Corrosion Vol.14(1958), p.501.

[4] Zhu Xiangrong, Wang xiangrun, Huang guiqiao, Marine Science Vol. 3(1995), p.23.

[5] Dai Mingan, Zhang Chaoyu, Huang Guiqiao, Corrsion Science and Protection Technology Vol.

3(1995), p.249.

[6] Hou Baorong, Zhang Jinglei, Guo Gongyu, Marine Science No.4 (1995), p.74.

6 Information Engineering for Mechanics and Materials

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