Corrosion and Protection of Submarine Metal Components in Seawater

Yuan GU School of Jet Propulsion

Beihang University Beijing, 100083, China

Ning LI, Fugang ZHANG Navy submarine academy Qingdao, 266071, China

E-mail: lining1957@ yahoo.com.cn

Lu LI BST-transportation LTD Qingdao, 266111, China

Abstract—Serious corrosion problems, corrosion effect factors and corrosion protection technologies of submarine metal components in seawater were detailed discussed based on deeply investigation. Then, the anticorrosion guarantee technologies to solve the serious corrosion damage mental components were raised.

Keywords- submarine metal components; corrosion damage effect factors; protection technologies

Corrosion occurs on naval equipments, airplanes, surface vessels and submarines, especially submarines, corrosion that occurs on metal components of submarines is an important subject that influences performance of submarines in the battlefield. Thus, that is meaningful to research the features effect factors, and protection methods of submarine metal components corrosion in seawater.

I. PHYSICAL AND CHEMICAL CHARACTERS OF SEAWATER THAT SUBMARINES HAVE BEEN OPERATED

Most of the time metal part of submarines is surrounded by seawater, so physical and chemical characters of seawater have to be studied before the research of corrosion damage and protection method.

A. Concentration of Main Kinds of Salts in Seawater There are many kinds of salts exist in seawater, and salinity

of surface seawater is about 3.2%~3.75%. The salinity increases along with the sea going deeper. But there is little difference on salinity among different oceans. The salinity Pacific Ocean is 3.49%, comparing to 3.54% of Atlantic Ocean and 3.48% of Indian Ocean. Most of the salts in seawater are chloride which account for 88.7% of the gross. Table 1 shows the content of kinds of salts in detail.

TABLE I. CONTENT OF MAIN KINDS OF SALTS IN SEAWATER

Element Content in 100g of seawater Percentage NaCl 2.7123 77.8

MgCl2 0.3807 10.9 MgSO4 0.1658 4.7 CuSO4 0.1260 3.6 K2SO4 0.0863 2.5 CuCl2 0.0123 0.3 MgBr2 0.0076 0.2

The conductivity of seawater is high as a result of the high salinity. The conductivity of seawater is about 4×10-2s.cm-1, which is much higher than river water 2×10-4s.cm-1 of and 1×10-3s.cm-1 of rainwater.

B. Average of Seawater Factors in Some Coastal Cities Through investigation, Average of seawater factors in

QingDao, XiaMen and YuLin is shown as below.

TABLE II. AVERAGE RATE OF SEAWATER STATUS IN THREE CITIES

Qingdao Month Tem

10°C Salinity

10% Dissolved

Oxygen1(ml/l) pH

1 4.2 3.181 6.82 8.82 2 3.0 3.188 6.94 8.05 3 4.3 3.190 6.54 8.26 4 8.6 3.180 6.52 8.27 5 13.0 3.178 5.76 8.27 6 17.7 3.1723 5.27 8.43 7 21.6 3.149 4.81 8.63 8 25.0 3.109 4.59 8.48 9 23.8 3.118 5.06 8.33

10 20.0 3.123 5.19 8.38 11 14.3 3.140 5.99 8.40 12 8.0 3.146 6.86 8.22

Average rate ─ 3.156 5.86 8.32

Xiamen

Month Tem 10°C

Salinity 10%

Dissolved Oxygen1(ml/l) pH

1 14.3 2.852 5.97 8.18 2 13.9 2.711 6.11 8.18 3 14.4 2.621 6.12 8.13 4 17.0 2.511 5.79 8.13 5 22.0 2.366 5.33 8.15 6 26.5 2.520 4.75 8.15 7 28.6 2.816 4.49 8.13 8 28.8 2.931 4.41 8.12 9 27.6 2.629 4.42 8.08

10 25.3 2.729 4.82 8.12 11 21.3 2.731 5.15 8.17 12 17.0 2.678 5.80 8.22

Average rate 21.4 2.658 5.27 8.15

978-1-4244-4905-7/09/$25.00©2009 IEEE 1226

Yulin Month Tem

10°C Salinity

10% Dissolved

Oxygen1(ml/l) pH

1 22.8 3.443 4.72 8.23 2 23.5 3.435 4.63 8.10 3 25.1 3.414 4.58 8.03 4 27.5 3.434 4.51 8.24 5 30.1 3.455 4.14 8.23 6 30.2 3.292 4.28 8.22 7 29.8 3.376 4.33 8.14 8 28.8 3.382 4.21 8.20 9 29.0 3.299 4.24 8.24

10 27.0 3.327 4.37 8.24 11 25.2 3.343 4.37 8.12 12 22.9 3.433 4.59 8.10

Average rate 26.8 3.386 4.41 8.17

From Table 2, pH of seawater is about 8.1~8.2, it can be

different when seawater goes deeper. If there are large number of sea plants, CO2 decreases, and dissolved oxygen rate goes up, pH can be close to 10; under the condition of anaerobic bacteria with high reproductivity, dissolved oxygen rate goes down, and water has H2S, then the pH rate can be lower than 7.

II. PROCESS AND CHARACTER OF SUBMARINE METAL COMPONENTS CORROSION IN SEAWATER

Seawater is nearly neutral electrolyte with large number of salts and certain amount of dissolved oxygen which determine the electrochemical feature of corrosion of submarine metal components in seawater.

A. Micro-cell Corrosion of Submarine Metal Components Metals and alloy in seawater have got micro-inequality of

electric-potential distribution due to the unevenness of the microcosmic physicochemical of the properties of surface, such as the unevenness of the components, the distribution of the phase and the surface stress. So it forms a lot of corrosion microcells. The area which has a low electrical potential such as ferrite of submarine carbon steel components become anode on which the following oxidation reaction occurs:

And the area with a higher electrical potential such as cementite of carbon steel become cathode on which the following reduction reaction occurs:

Anode releases electrons, cathode consumes electron. So metal corrosion occurs. The corrosion caused by electrochemical reaction of micro-cell is called micro-cell corrosion. Most of the corrosion of submarine metal components in seawater is micro-cells corrosion.

B. Contact Corrosion of Submarine Metal Components Submarines are made from different metals. When two

kinds of metal or alloy contact in seawater. Metal with lower electrical potential will be corroded and metal with higher electrical potential will be protected, which is called contact corrosion. If copper components and iron components of submarines contact to each other which were all immerged in seawater iron components and bronze components will have

Following electrochemical reaction will occur on iron part and copper respectively:

Fe components: 2 2Fe Fe +→ + e

Cu components: 1

2 22 4 4O H O e OH −+ + →

Experiments showed that natural corrosive electrical potential of iron and copper in seawater are -0.65v and -0.32v respectively. When connect these two different materials with wire, and then macroscopical galvanic corrosion cell is formed. (The oxidation reaction of copper was inhibited) So iron components are corroded faster and bronze components are protected.

C. Corrosion Characteristics of Submarine Metal Components

1) In neutral seawater, the corrosion of most of submarine metal components expect magnesium and its alloys are cathode process which are under the control of oxygen depolarization reaction due to high concentration of dissolved oxygen. It could be concluded from previous experiments that the diffusion rate of oxygen was limited through the surface seawater where usually saturated by oxygen. Thus, all conditions such as wave, seawater splashing and increased velocity of seawater that are beneficial to supply oxygen can promote cathode depolarization process, then the result is speed-up of the corrosion rate.

2) High conductance of seawater and low resistivity of seawater corrosion result in that microcells and macr-cells on the surface submarine metal components is flexible. Components that are made from different materials are easy to induce galvanic corrosion. Experiments showed that Galvanic corrosion occurred when there is electrical potential difference between the two kinds electric connected metals even though two metal pieces are away from each other for ten meters.

3) The concentration of chlorine ions in seawater is high, so most of metal components in seawater are not easy to be passive state. The anode polarity is small in the corrosion process of surface submarine metal components, so the corrosion rate is high. Experiment showed that it was limitted to prevent the corrosion of surface submarine metal components through increasing andode retardarce. The passivation film could be easily destroyed because of the exist of chlorine ions. Even stainless steel could not be provented from being corroded. But, addind molybdenum element into stainless steel could reduce the devastating effects of chlorine ions on passivation film.

III. EFFECT FACTORS THE CORROSION OF SUBMARINE METAL COMPONENTS IN SEAWATER

Seawater contains many kinds of salts, dissolved gases, suspending sand and rotten organism, which together with the movement of seawater and change of temperature have created an extremely complex environment.

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A. Effect of Salts Content of Seawater on the Corrosion of Submarine Metal Components.

Seawater contents many kinds of salts but mainly sodium chloride. Experiments showed that there was no distinct change on salinity and percentages in different seas. The salinity was 3.2%~3.75% in surface seawater of mare liberum, which made no distinct difference of the corrosion of submarine metal components. But change of salinity directly affect the conductivity of seawater, and conductivity of seawater is an important factor to corrosion of submarine metal components. Also, the chlorine ions in seawater could destroy the passivation film of the metal, so most of the submarine metal components would more or less suffer from being corroded.

B. Effect of Solutes of Seawater on the Corrosion of Submarine Metal Components

Most of the corrosion of submarine metal components in seawater are oxygen depolarization corrosion. Thus, concentration of oxygen is the most influential factor on the corrosion of submarine metal components. Especially for components made of steel and copper, corrosive rate can be reduced by decreasing the concentration of oxygen.

Solubility of oxygen in seawater mainly depends on salinity and humidity of seawater. When salinity and humidity increases, solubility of oxygen decreases. Table 3 gives more information about this issue.

TABLE III. SOLUBILITY OF OXYGEN UNDER NORMAL PRESSURE IN SEAWATER

Salinity % C Tem 0.0 1.0 2.0 3.0 3.5 4.0

0 10.30 9.65 9.00 8.36 8.04 7.72 10 8.02 7.56 7.09 6.63 6.41 6.58 20 6.57 6.22 5.88 5.52 5.35 5.17 30 5.57 5.27 4.95 4.65 4.50 4.34

The table shows that salinity remains the same, solubility

of oxygen decreases when humidity increases.

Oxygen is the depolarizer in seawater corrosion, so submarine metal components components would not be corroded if all oxygen were eliminated from seawater. Experiments showed that, for different submarine metal components such as carbon steel, low-alloy steel and cast iron, when concentration of oxygen increased, the cathode process were accelerated, so as to the corrosion were. But for aluminum alloy and stainless steel, the increase of oxygen was benefical to form and repair the passivation film and thus reduce the trend of pitting and crevice corrosion.

Apart from oxygen and nitrogen, seawater also contains CO2, which affect the pH of seawater. The change of pH rate can influence corrosion of submarine metal components

Experiments showed that carbonate was one of the key factors on metal corrosion. Besides the hydration of CO2, Carbonate could be produced by the metabolism of marinebiology and the decomposing of the corpses of animal and plants. Carbonate deposited on the metal surface and

formed a insoluble protective layer and thus inhibited the corrosion process.

C. Effect of Seawater pH on the Corrosion of Submarine Metal Components

The pH of seawater is around 7.8~8.6, s, surface seawater higher pH around 8.1~8.3 because of the photosynthesis of the marine plants. High pH is helpful to improve corrosive-resistance of submarine metal components.

D. Effect of Temperature on the Corrosion of Submarine Metal Components

The effect of temperature on the corrosion of submarine metal component is complex. From dynamic aspect, corrosion rate will increase when temperature goes up. But in another hand, when temperature of seawater goes up, dissolved oxygen will reduce and protective carbonate layer will be formed more easily, which will restrain the corrosion of submarine metal components. Experiments showed that, when seawater has normal content of oxygen, temperature played a key effect on the corrosion of submarine metal components. When content of oxygen was over 5ml/l, the cathode reaction was under the control of oxygen-diffusing rate instead of the content of oxygen. Especially under constant temperature, when oxygen content increased from 4.5ml/l to 7.4ml/l, corrosion rate of steel increased from 0.25mm/a to 0.3mm/a. when temperature went up from 00C to 250C, corrosion speed of steel increased from 0.15mm/a to 0.89mm/a. The influence of temperature was about 15 times more than influence of oxygen.

E. Effect of Seawater Flow Velocity to Submarine Metal Components Corrosion

The corrosion of submarine metal components caused by seawater is a cathode controlled process having the aid of oxygen depolarization, which is under the control of the oxygen diffusion rate. The flow velocity of seawater and the waves have changed the conditions of oxygen supply which certainly has the impact on corrosion of submarine metal components. The impact of seawater’s flow velocity on submarine metal components is shown as Fig. 1.

腐蚀速

度

Figure 1. Relation between flow velocity and Corrosion speed

In part a, when seawater velocity increases, oxygen diffusing speeds up, so corrosive process speeds up. In part b, velocity of seawater increases still, oxygen supply becomes more sufficient, cathode process is no longer controlled by diffusing, but velocity of seawater has little impact on cathode reaction of main redox. In part c, when velocity is over critical velocity Vc, rotten organism layer is washed off, so corrosive

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process speeds up. To mild steel components, Vc is 7m/s~8m/s,steel is 1m/s, albata is 4.5m/s.

F. The Effect of Marine Bbiology on the Corrosion of Submarine Metal Components in Seawater

There are large number of sea animals, plants and micro-organisms in the sea. Many marine biology and microorganisms could grow up even breed in the adsorption of the submarine’s hull and propeller’s components especially in warm waters during spring and summer. This reefs in the sea which is the hazard to submarine’s components is known as fouling organisms. Tests show that peeling off of the anti-corrosive coating on submarine’s metal components is caused by the absorption of fouling organisms so that all these metal components can be corroded seriously. Marine fouling-submarine attached to the metal components caused by corrosion damage has following the situations as below:

1) When the attachment is incomplete and uneven, the corrosion process will be carried out locally. Then the internal and external adhesion may have the oxygen concentration cell corrosion, especially the resultant crevice corrosion from submarines shell cracks happens on the surface.

2) The biological life cycle changes the composition of the local seawater. As the result of Photosynthesis of seaweeds plants after attachments increase concentrations of oxygen in local seawater and accelerate the corrosion. In particular CO2 of bio-breathing emissions as well as H2S with the formation of biological decomposition of the bodies also speed up submarine metal components corrosion.

3) Due to some marine biology could pass through the paints and other protective coatings while growing, which has the direct damage to protective layer of the submarine metal components and, thereby speed up the corrosion.

IV. SEAWATER CORROSION PROTECTION TECHNOLOGY OF SUBMARINE’S METAL PART

A. Reasonable Selection of Submarine’s Metal Components Reasonable selection of the materials is known as one of

the most effective method to prevent the submarine metal components from corrosion. Not only the mechanical properties and manufacturing process should be concerned, but also the corrosion resistance in a particular media should be focused on as far as possible to reduce the cost at the same time. Especially for the harsh corrosive environment, when only small amount of materials are need, the corrosion-resistance materials should be selected. For example, the corrosion-resistant copper alloy is used on submarine propeller.

B. Reasonable Design of Submarine’s Metallic Frames Submarine’s metal components should be kept in simple

shape without affecting the function so that stress corrosion

can be avoided. When two kinds of metal material with large difference on electrical potential have to be used, the insulating pads made of organic materials should be used to separate the two alloys. Meanwhile, to avoid the crevice should be paid attention when submarine’s metal components are designed as the cracks have the corrosive damage to those components. Therefore, it’s better to use welding instead of riveting bolts to connect; especially prone to corrosive site should avoid the use of intermittent welding. Gap should be filled with padding.

C. Anti-corrosion Measures on the Surface of Submarine’s Metallic Components

Main Anti-corrosion measures on the surface of Submarine’s metal components are summarized as below:

1) Protection technology of Organic coating

Protection technology of Organic coating has been widely used on Submarine’s metal components. There are a lot of varieties of special paint, which can basically meet the requirements of anti-corrosion, used for Submarine’s metal components. What should pay special attentions to is the construction’s quality while painting, exception of rust, oil or water must be allowed strictly and also add the attention of preser vatives such as cuprous oxide or organic tin compounds.

2) Protection technology of Metal spraying coating

The anodic coating is formed by the method of thermal spraying, which spraying zinc, aluminum and zinc- aluminum alloy over the surface of Submarine’s metal parts to avoid corrosion. For the porous layer on thermal spray coating, it’s quite necessary to be sealed with the organic coating such as polyurethane and aluminum power paint.

3) Protection technology of metal cladding layer

It is difficult to adopt cathodic protection in marine splash zone with the poor anti-erosion ability of Organic coating. As the result, steel hull of submarines can be coated by metal layer such as stainless steel, titanium, copper-nickel alloy, etc.

4) Protection technology of Lining

Lining materials include metallic materials and nonmetallic materials one such as FRP, rubber, enamel and metal.

REFERENCES [1] M.Q. Zhao, “Corrosion And Protection Of Metal,” Beijing: National

Defence Industry Publishing House, 2004. [2] Z.G. Jiang, “Corrosion Fatigue Of Airplane Structure,” Beijing:

Airplane Industry Publishing House, 1992. [3] M.Y. Yin, “Guide Of Controlling Of Structural Corrosion Fatigue For

Navy Equipments,” Beijing:Airplane Industry Publishing House, 2005. [4] Z.G. Wang, “Fatigue Of Materials,” Beijing: National Defence

Industry Publishing House, 1992. [5] Y.L. Liu, “Corrosion Protection Coating Of Polyurethane,” Beijing:

Chemical Industry Publishing House, 2006.

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