KR Webzine Vol.136

1. Introduction

Global warming and the potential to exploit finite resources has led global oil companies and their partners to begin moving the offshore industry to Arctic regions. Therefore, appropriate material solutions for low-temperature Arctic applications are required to achieve the long-term durability of offshore structures. Currently, most investigations into materials for Arctic use have focused on the mechanical properties of these materials in a low-temperature environment[1, 2]. These studies have mostly investigated the risk of brittle fracture in structural steels and, regarding the tensile properties, fracture toughness, arrest toughness, and fatigue[2]. Unfortunately, the study of the corrosion characteristics under Arctic conditions for the prevention of structural corrosion is rare. Considering the difficulty of intermediate replacement or reinforcement of steel during offshore operations, the design of corrosion prevention must be done at an early stage.

In general, the corrosion of material in the Arctic region is generally expected to be less significant than that in temperate and tropical regions[3, 4]. In Arctic conditions, metallic corrosion is limited because the ice layer covering the metallic surface reduces oxygen access.

However, the monthly precipitation in winter and summer is low in the Arctic region, salts on the metallic surfaces from seawater are rarely washed away by rain; thus, corrosion progresses continuously. Other studies have reported that corrosion occurs even at -5 °C due to delayed freezing caused by the deposition of a saline layer on the surface[5, 6], and electrochemical corrosion activity is possible below the ice layers[7]. In addition, the splash zone suffers severe corrosion damage because of wave impact, the presence of sufficient oxygen, and the salt spray conditions that continually wets and dries on the structure. Moreover, Arctic weather is becoming increasingly variable and unpredictable, showing high daily and monthly temperature variances. Therefore, adequate information concerning the total corrosion impact in Arctic conditions is required.

Therefore, appropriate corrosion management should be considered based on the survey of the installation and operational environment for the safe design of offshore structures. However, it is very difficult to design an accurate electric anticorrosion system in an environment where there are repeated icing-thawing cycles. In the case of a protective coating, maintaining long-term durability is difficult because of low embrittlement of such coatings at low temperatures, desorption due to ice impact and thermal expansion due to temperature variations. In this research, the corrosion characteristics of Arctic offshore steel in a low temperature environment is investigated and the major corrosion factors are analyzed.  

 
2. Experimental

 
Experiments were conducted using two types of bare metal(BM)s of YS460 MPa grade(FH460, Steel delivery condition H460TM), 100 mm thickness, Arctic offshore structural low-carbon steels. These steels were produced by the TMCP(Thermo-Mechanical Controlled Process). Weld metals(WMs) were prepared by flux cored arc welding (FCAW) and submerged arc welding (SAW), which are the most commonly used welding process for the production of ships and offshore structures. WM specimens were sampled from the quarter thickness (t/4) location, and each specimen contained a fusion zone (FZ, weld metal), a heat affect zone (HAZ), and parent metal(unaffected base metal) regions. To investigate the corrosion properties, corrosion tests on BM and WM were conducted by immersion, electrochemical, salt spray tests (SST), and cyclic corrosion tests (CCT).

3. Results

Figure 1. Corrosion rates of two types of experimental specimens obtained

from immersion tests and electrochemical tests.

  
Immersion and electrochemical corrosion tests were performed on bare metal and weld metal specimens to evaluate the corrosion characteristics of the low-carbon steel in an Arctic offshore seawater environment. In particular, the corrosive difference between the bare metal and the weld metal was compared and the corrosion change of the steel was observed as the seawater temperature decreased.

Figure 1 shows the results of the immersion tests and electrochemical polarization tests on bare metals and weld metals using two types of Arctic offshore structural steels. In the immersion tests (seawater closed condition), the corrosion rate was lower than 0.035 mm/yr, and the difference in corrosion rate between the bare metal and the weld metal was small. It was confirmed that the corrosion rate reduced as the seawater temperature decreased. However, the electrochemical test showed no particular tendency of corrosion decrement according to seawater temperature. This phenomenon is predicted by the effect of dissolved oxygen. It is presumed that the corrosion rate has been maintained as the amount of dissolved oxygen is relatively high at low seawater temperature. This can be related to the flow rate, thus the concentration of oxygen can be increased as the flow rate increases. Therefore, it should be considered that, in the Arctic seawater environment where a constant flow rate exists, the corrosiveness of the Arctic offshore structure is not significantly lower than in a temperate or sub-tropical region environment.

Figure 2 shows SST and CCT test results for two types of steel. Through the experiment, we tried to investigate the corrosion change according to salinity by repeating the phenomenon of exposure to salt wetting and drying as experienced in the splash zone of an offshore structure.

As a result of the test, it was confirmed that abrupt corrosion occurred under the condition of a repeated wet, dry and temperature change cycle. Particularly, in the case of the sample A, rapid corrosion occurred in accordance with the temperature change of -40 °C in both the bare metal and weld metal specimens. In general, corrosion of the welded specimens was more progressed than the bare metal specimens, but an opposite phenomena was observed in Sample A. The phenomenon is thought to be due to the relatively high coefficient of thermal expansion of the base metal compared to the weld material[8].

4. Discussion

Corrosion phenomena is governed by various factors in the exposed environment. In the case of Arctic marine structures, corrosion occurs in various forms due to temperature changes, wet/dry conditions, salt residues, icing/thawing, and dissolved oxygen. In addition, corrosiveness varies depending on factors such as thermal expansion, microstructures, and the chemical composition of the material. Therefore, it is necessary to analyze the complicated relationship between the environmental conditions, the steel chemical component and the microstructure.

5. Conclusion

In this study, the corrosiveness of base metal and weld material was investigated by immersion corrosion tests, electrochemical polarization tests, SST and CCT for two types of low carbon steels for Arctic offshore structures. The following conclusions were derived.

(1) As a result of the Arctic seawater simulated corrosion test, the corrosion rate does not decrease remarkably in low temperature seawater. When the amount of dissolved oxygen is sufficient, the corrosion rate does not decrease significantly at low temperatures.

(2) A significant level of corrosion occurs in the splash zone. In general, the corrosion resistance at the weld zone was higher, but the corrosiveness of the bare metal is accelerated when there is a difference in the thermal expansion rate due to the change in temperature between the Arctic winter and summer.

Arctic offshore structures should be precisely designed for corrosion management considering the difficulty of mid-replacement and supplementation of structural steel after installation. Therefore, it is necessary to calculate the correct corrosion rate by examining the environmental conditions such as temperature change, flow rate, precipitation and icing effects.

6. Expected effect

The purpose of this study is to derive important test elements for survey from the view point of corrosion to ensure the long-term durability and safety of offshore structures and to establish appropriate KR classification rules by analyzing the corresponding corrosion test data.

7. References

1. Petroleum and natural gas industries-Arctic operations-Material requirements for arctic operations, ISO/TS 35105, 2018, 4.
2. A. M. Horn and M. Hauge, Twenty-first Int. Offshore Polar Eng. Conf., 2011.
3. Melchers R.E., Effect of temperature on the marine immersion corrosion of carbon steels, Corrosion, 2002, 58, p.768.
4. Mikhailov A.A., Strekalov P.V., Panchenko Y.M., Atmospheric corrosion of metals in regions of cold and extremely cold climate (a review), Prot. Met., 2008, 44, p.644.
5. Barton K., Bartonova S., Beranek E., Die Kinetik des Rostens von Eisen in der Atmosphäre, Werkstoffe und Korrosion, 1974, 25, p.659.
6. Brass G.W., Freezing point depression by common salts: implications for corrosion in cold climates. In: Perrigo L.D., Byars H.G., Divine, J.R., Cold climate corrosion: special topics. NACE International, 1999, p.29.
7. Gonza´lez J.A., Control de la corrosio´n. Estudio y medida porte´cnicas electroquı´micas. CSIC, Madrid, 1989.
8. Y. Y. Choi and M. H. Kim, Corrosion behaviour of welded low-carbon steel in the Arctic marine environment, RSC Adv., 2018, 8, 30155