Corrosion and Corrosion Detection : focusing on the marine environment

Corrosion is defined as the destruction or degradation of a material (mostly metals) due to its interaction with the environment. This destruction takes place on the metal surface in the form of material dissolution or in some other forms, such as rust.

The Costs of Corrosion

According to SSPC, corrosion of metals costs about $276 billion annually. NACE estimates that unmitigated corrosion costs the U.S. economy roughly 3.1 percent of the country’s total GDP. The losses due to corrosion can be divided into 3 categories:

1 - Direct Losses

  • Waste of energy and materials

2 - Indirect Losses

  • Shutdown, loss of product

  • High maintenance costs

  • Loss of efficiency

3 - Environmental Impact

How and Why does Corrosion Occur?

Common structural metals(such as steels) are produced from minerals(iron ores), which require a large amount of energy to produce. So these metals are in a high energy state and have a tendency to corrode. Certain environments offer opportunities for these metals to recombine and revert to their lower energy states. Thus, corrosion can be described as “extractive metallurgy in reverse”.

The Electrochemical Principle of Corrosion

Corrosion involves both oxidation(anodic) and reduction(cathodic) reactions. The anodic reaction involves metal dissolution(“corrosion”). The cathodic reaction consumes the electrons(shown in Fig. 1).

Figure 1: Electrochemical reaction of corrosion

In order for corrosion to occur, the following are needed.
1) Anode or anodic sites on the surface of the metal (electrode on which             oxidation or corrosion occurs)
2) Cathode or cathodic sites on the surface of the metal (electrode on                  which reduction occurs)
3) Electrolyte in contact with both anode and cathode (provides a path for           ionic conduction)
4) Electrical connection between anode and cathode (provides path for                the  flow of electrons)

The Thermodynamics of Corrosion(deciding factor for corrosion to occur or not)

Free energy changes(ΔG) provide the driving force and control the spontaneous direction for an electrochemical reaction(corrosion). The more negative the value of ΔG the greater is the tendency for the reaction to proceed and the more is the possibility of corrosion. Based on this theory, we have 3 main types of  cells that can take part in corrosion reactions:
 1) Dissimilar Electrode Cells/Galvanic Cells (different metals)
 2) Concentration Cells(salt and oxygen)
        3) Differential Temperature Cells(the anode and cathode consist of the               same metal and differ only in temperature)
Corrosion contributors of The Marine Environments

Seawater is the most efficient electrolyte because of its chloride(Cl) content. The presence of oxygen in marine atmospheres and splash zones at the interface also increase the aggressiveness of salt attack on a ship. 
The corrosion rate for steel in a marine environment varies based on the differential aeration (presence of oxygen), temperature and chloride content. For instance, the differential concentration of oxygen dissolved at the interface aggravates the corrosion on the ship hull.
Corrosion of steel in the marine environments is mainly affected by the following factors:
1) Oxygen Diffusion 
   Corrosion rate is controlled by the rate of diffusion of oxygen through              the  water to the metal surface.
     Exposed bare steel corrodes faster than a steel covered with a layer of                 rust.
2)  Temperature
      Diffusion rates are controlled by temperature, so an increase in                             temperature will increase the corrosion rate.
3)  Type of ions
      Presence of chloride & sulphur containing ions increases corrosion.

Types of Corrosion in Marine Environments
1) Uniform Corrosion (general corrosion)
    *  The most common form of corrosion where the electrochemical                           reaction proceeds uniformly over the entire exposed surface or over                     a large area.
    *  Uniform or general corrosion represents the greatest destruction of                   metal on a tonnage basis. The metal becomes thinner and eventually                  fails.

2) Galvanic Corrosion
    *  Occurs when 2 different metals are electrically connected in the same                  electrolyte.
    *  Driving force for corrosion is the difference in the electrode potential –                 this potential difference produces electron flow between them.

3) Stray Current Corrosion
   *  Occurs on metal with an electrical ‘stray current’ passing from it to an                 electrolyte.
   *   Electricity from an outside source flows through ship components and                 out by the water to the ground.

4) Pitting Corrosion
  *   Pitting corrosion is an extremely localized attack that results in the                       development  of cavities or “pits” in the metal, which in most cases are                relatively small.
  *   Pitting is considered to be the most destructive and insidious form of                  corrosion because it is more difficult to detect, predict and design                        against.

5) Crevice Corrosion
   * An intense localized corrosion, which often occurs within crevices and                 other shielded areas on metal surfaces exposed to corrosive                                 environments.

6) Flow-induced Corrosion
    *  Erosion Corrosion
        Damage induced by corrosion reactions and mechanical effects from                 relative movement between a corrosive fluid (electrolyte) and the                         metal surface.
    *  Cavitation Corrosion
       Cavitation corrosion is a form of erosion-corrosion and is defined as                    the degradation of a metal surface by sudden formation and collapse                   of bubbles of vapour.

7) Biological Corrosion
 * Marine biological organisms accelerate corrosion by changing the                normal environment.

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Detection of Marine Corrosion
Some corrosion listed above, such as uniform corrosion, can be found by visual inspection. But, some localized corrosion can exist under the coatings and it is hard to be detected by human eyes. Thus, it has become increasingly critical to develop non-destructive testing(NDT) that is capable of detecting corrosion under the ship coating. 
NDT methods used in under coating corrosion detection:

1)  Eddy Current(EC)
EC is one of the most effective NDT methods. EC uses alternating currents applied to a conducting coil held in close proximity to the object under inspection. In accordance with Lenz’s Law, the inspected object generates eddy currents to oppose the alternating current in the coil, as illustrated in Figure 2. 
The eddy currents are then detected by the same coil, separate coils, or magnetic field-sensors. Changes in the induced eddy currents may be caused by changes in a material’s electromagnetic properties, variations in material thickness or the sharp discontinuities caused by the presence of corrosion. In order to implement this method, the surface of the material must be accessible. 

Figure 2: Schematic diagram of eddy current corrosion detection

        2)  Microwave NDT(MNDT)
Microwave frequency range is between 300MHz and 300 GHz. Coatings can be easily penetrated by microwave signals and then internal structures of materials can interact with these signals. These microwave signals would then totally reflect at the metal surface. Therefore, these signals travel twice through the areas of corrosion and defects,
which increases the possibility of detecting the corrosion under coatings. The setup of MNDT is shown in figure 3. Because microwaves cannot efficiently penetrate through conductive materials, it is difficult to detect corrosion under the steel surfaces. 

Figure 3: Microwave NDT

      3) Thermography Testing
Thermography testing is a real time and non-contact method, which can inspect a large area in a short time. An IR camera is integrated within the system, which can measure the real time temperature of the sample. After giving a stimulus to heat up the testing area, there is a difference in temperature between the corrosion area and the defect-free area, which can be captured by the thermography testing system. Then, the system provides the thermography of the testing area.(shown in Fig. 4) One concern is that the high temperature caused by the thermography might damage the microstructure of the steel substrate. 

Figure 4: The averaged magnitude of the reflection coefficient of corrosion

       4) Radiograph Methods
Radiograph is based on differences in the attenuation of penetrating radiation in materials. Corrosion area and defect-free area have different attenuation coefficients. Therefore, corrosion under the coatings can be detected by the radiograph method. Either gamma or X-rays is used to image the profile of a structure(shown in fig. 5). However, there are several notable limitations on the use of radiography. For instance, both X-rays and gamma rays can cause safety risks to the operator. Besides, it is also not possible to measure the  corrosion’s depth by the radiograph.

Figure 5: Radiography of corrosion under the resin coating

     5)  Capacitance Imaging
Capacitance imaging is a new invention in the field of non-destructive testing, which aims to overcome the shortcomings of existing NDT technologies. 
The basic configuration is to “open up” a standard parallel-plate capacitor so that both electrodes are now in the same plane i.e., co-planar capacitor, as shown in Fig. 6. An alternating current is then applied to the capacitor, and an electric field distribution is established. An object between the electrodes will affect this electric field pattern and any localised change in sample properties will change the field distribution pattern and modulate the output signal.  Correlating these changes in signal with localised surface information will therefore produce an image as the probe is moved over the surface of the object (shown in fig. 7). The major practical advantage of this approach is that it is non-invasive, non-contact and only requires single sided access to the object being examined. 
Allied Scientific Pro provides capacitance imagers for the detection of corrosion under coatings. 

Figure 6: Configuration of capacitance imager

Figure 7: (a) The photograph of a rusted galvanized steel plate.
 (b) Capacitive image taken in the air, highlighting the main areas of rusting. 
(c) Capacitive image taken through a 5 mm thick insulating polymer foam coating.

5.Ruikun W.; Hong Z.; Ruizhen Y.; Wenhui C.; Guotai C. Nondestructive Testing for Corrosion Evaluation of Metal under Coating. Journal of Sensors. Volume 2021, Article ID 6640406, 16 pages

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