3.2.2. Corrosion Morphology

Steel in Seawater without Bacteria

After exposure for 7 days samples S8, S9, S14 and S15 were taken out for surface analysis. After cleaning, the surface of sample S15 is shown in Figure 11a. The upper part of the sample corroded less than the lower part, e.g., little corrosion started on the upper-left corner in the image, where the original polishing pattern can be recognized. This is attributable to the fact that the oxygen is more easily accessible near the water surface than in the lower part. Localized corrosion is visible on the bottom-left corner (Figure 11a), although a large area of the sample S15 showed general corrosion. Small pits are visible in the magnified image Figure 11b. These small pits were in the initial stage. Inside the pits corrosion involved metal hydrolysis and pH decrease. The surrounding area of pits acted as cathodes. The halos surrounding the small pits in the Figure 11b are evidence. The small pits may grow in depth and laterally, becoming big pits or connecting in surface area as general corrosion.

**Figure 11.** An image for the sample S15 exposed to seawater for 7 days (after cleaning the surface). Local corrosion attack was found near the bottom-left corner (**a**) and small pits near the upper-right corner zoomed in the image (**b**).

Figure 12 shows an image for the sample B3 exposed to seawater for 28 days after cleaning. The whole surface was corroded. Most areas of the steel surface showed general corrosion. The different colour on the surface is due to the rough surface after corrosion attack and possibly remaining deposits which were not completely removed in the standard cleaning procedure. Localized corrosion spots are visible at the sample surface. The corrosion products rolling down from the upper part affected the corrosion at the lower part of the sample (e.g., lower right corner in Figure 12).

**Figure 12.** An image for sample B3 exposed to seawater for 28 days (after cleaning). Localized corrosion attacks were found in the steel (red circle). The lower part of the steel sample was affected by corrosion products rolling down.

The maximum depth of the pits was measured as 46 μm in sample B3 (Figure 13). It corresponds to 0.6 mm/y, assuming that the local corrosion attack keeps occurring at the same speed.

**Figure 13.** Cross-sectional view at pits for samples B3 in SW for 28 days.

Steel in Seawater with Bacteria

For sample S9 (Figure 14) exposed to SW + bacteria for 7 days, many small pits and a few big pits (about φ 0.6 mm) were found. The maximum depth was about 13 μm. More than 80% of the sample surface area showed micro pits; only about 5% surface area showed uniform corrosion; about 10% of the area did not show corrosion.

**Figure 14.** An image for the sample S9 exposed to SW + bacteria for 7 days (after cleaning the surface). Many small pits (yellow, diameter < 0.1 mm, >80% area) and a few big pits (red, diameter > 0.5 mm, ~3% area) were in the steel. Uniform corrosion took place in a small area (black, ~5% area).

With the addition of bacteria in SW, typical localized corrosion due to MIC was observed (Figure 15). The corrosion boundary has a round shape. Approximately 40% surface area was corroded. Assuming one third of surface area was corroding, the real corrosion current density would be three times of the average corrosion current density, which means the corrosion rate would be three times of the measured average corrosion rate.

**Figure 15.** An image of sample S13 exposed to seawater with bacteria for 28 days (after cleaning). Half of the surface area was attacked by local corrosion.

Similar corrosion morphology was found for the samples B1 and B2 exposed to SW+bacteria (MIC) for 28 days. The maximum pit depth is 63 μm (Figure 16), which corresponds to 0.82 mm/y, assuming that the corrosion keeps occurring at the same speed. This corrosion rate is higher than that measured in field assessment (0.53 mm/y in diameter reduction in the worst case), which is attributable to those corrosive bacteria added in the seawater and the temperature in the lab test being higher than in North Sea waters.

**Figure 16.** Cross-sectional view at two pits for sample B2 after exposure to the SW + bacteria for 28 days. The maximum pit depth is 63 μm.

The relative percentage of the corroded area and maximum pit depth in samples B1-B4 were evaluated using photo microscopy and are given in Table 7. The areas of micro-pits and macro-pits were estimated by taking a few images and adjusting the black/white contrast. The samples (B1–B2) exposed to SW+bacteria showed a greater percentage of localized corrosion area, while the samples (B3–B4) exposed to SW showed a higher percentage of uniform corrosion and micro-pit area.


**Table 7.** Percentage of corrosion area (%) and maximum pit depth of samples B1–B4.

3.2.3. Microstructure and Inclusions in the Steel

Figure 17 shows the microstructure of the steel. It shows a typical fine grain microstructure, composed of tempered martensite and bainite.

**Figure 17.** Microstructure of the chain grade R4 steel, composed of tempered martensite and bainite.

Inclusions have been found in the steel sample. Figure 18 shows a backscatter image at a cross section (a) and EDS plots at a particle (b) and nearby area (c) for an exposed steel sample (S8). The composition analysed using EDS (see Table 8) indicates that the dark-grey particle in Figure 18a is an MnS inclusion. This inclusion is about 20 μm in length and 5 μm in width. The elements detected at position (3) are related to the composition of the steel matrix.

**Figure 18.** An inclusion (backscattering image (**a**), EDS plots in the position 1 (**b**) and at position 3 (**c**) for an exposed sample (S8) after exposure to seawater without bacteria for 7 days.



TiVCr-enriched particles were also found in this steel sample (see Figure 19 and Table 9). These particles are known for initiating local corrosion due to their potential difference with regard to that of the surrounding steel matrix.

**Figure 19.** An inclusion (backscattering image (**a**), EDS plots in the position 1 (**b**) and at position 3 (**c**) for an exposed sample (S8) after exposure to seawater without bacteria for 7 days.


**Table 9.** Element percentage (wt.%) measured at positions in Figure 19a.
