Solution to galvanic corrosion welding flange of offshore oil and gas pipelines

Combined with the project’s actual situation, the surfacing flange solution is proposed for the galvanic corrosion problem of offshore oil and gas platform pipeline systems, and the specific technical requirements are clarified. Compared with the traditional method, this scheme has the advantages of economy, reliability, easy access, and good application value.
Galvanic corrosion refers to the phenomenon that when dissimilar metals come into contact with each other in the same medium due to potential differences between different metal materials, the galvanic cell is corroded. The galvanic current flow is generated, which increases the dissolution rate of metals with lower potential and decreases the dissolution rate of metals with higher potential. It is also called contact corrosion or bimetallic corrosion. Three elements of galvanic corrosion: two metals with a potential difference; there is a conductive path between the two metals, a corrosive environment.
There are many grades of pipeline materials for offshore oil and gas platforms, dissimilar metal connections are widely used, and flanges mostly connect pipelines of different materials. Suppose the dissimilar metal flange joint needs to be handled properly. In that case, it is prone to galvanic corrosion, which accelerates the corrosion damage of the flange sealing surface and causes sealing failure (see Fig.1), resulting in major disasters such as oil and gas leakage.

Fig.1 Severely corroded flange sealing surface
According to Section 8.7 of ISO 21457 (Selection of materials for oil and gas production systems and corrosion control), potential corrosion can be avoided by the following measures: 

• 1) Install insulation pipe section between dissimilar metal flanges; non-metallic or metal pipe sections lined with insulation layer can be used.
• 2) Increase the corrosion allowance of low-potential metals.
• 3) Built-in cathodic protection system.
• 4) Electrical insulation is used between dissimilar metals.
• 5) Reduce the area of high potential cathode.
• 6) The key parts of low-potential metal were welded.

This project is located in the South China Sea. It is a gas field center processing platform. The pipeline system is mainly carbon steel, stainless steel, duplex stainless steel, copper-nickel alloy, etc. The original contract requires heterogeneous metal connections in the wet gas, oil, and water systems.

1. Technical scheme

1.1 Surfacing position and material

We overlay the corrosion-resistant alloy on the flange sealing surface, as shown in Figure 2. The surfacing layer must resist the corrosion of the conveying medium and the marine environment and not couple with the high-potential metal. For example, 316L stainless steel or UNS N06625 alloy can be deposited on the sealing surface of the A182 F316L stainless steel flange and A105 carbon steel flange.

Fig.2 Surfacing position

1.2 Surfacing requirements

MIG welding, TIG welding, electron beam welding, and laser beam welding are usually used in the welding method, and electroslag welding is not used. The following points should be paid attention to when surfacing : 

• 1) The surfacing surface should be clean, with no oil, dust, rust, oxide layer, or other impurities, and should be sandblasted to white metal according to NACE No.1 SSPC-SP5.
• 2) Tools such as backing weld bead and intermediate weld bead cleaning should not contain iron, sulfur, and chloride ions, and only an austenitic stainless steel brush or high nickel alloy tool can be used.
• 3) There are at least two layers of weld bead in the surfacing layer, and the maximum thickness of the first weld bead cannot exceed 2. 5mm. The surfacing layer should usually be greater than 4.5mm, and the thickness after machining should not be less than 3mm (should take at least three different points of ultrasonic thickness).

1.3 Chemical composition analysis

• 1) Before surfacing welding, the same melting furnace flange material sample should be taken for chemical composition re-examination according to ASTM A751 requirements to ensure that the flange meets the requirements before the next surfacing welding.
• 2) The chemical composition of each batch of welding wire was retested.
• 3) The flange of the same batch of welding wire was extracted, and the chemical composition was tested at 2mm from the fusion line to the surfacing layer. For the flange surfacing with UNS N06625 alloy, the iron content in this part should not exceed 10 % ( mass fraction).

1.4 Hardness requirements

Each furnace/heat treatment furnace should perform a hardness test on the surfacing layer, and the heat-affected zone (the maximum distance from the fusion line is 0.5mm), and the maximum hardness should not exceed 350Hv10. The maximum hardness for acidic environments containing hydrogen sulfide should be at most 250Hv10.

1.5 Macroscopic test

Sampling was performed on the substrate + fusion line + surfacing layer, and macroscopic inspection was performed according to ASME IX. According to ISO5817 Level C, no linear development was required, and the substrate and the surfacing layer were completely fused.

1.6 Micro/metallographic examination 

Substrate + surfacing layer sampling, according to ASTM E407 inspection, no crack, no linear display, substrate and surfacing layer completely fused.

1.7 Penetration test

After the surfacing layer is machined, a 100% penetration test should be carried out concerning ASME VIIIDiv.1 and Appendix 8, and the sealing surface is not developed.

1.8 Pitting & crevice corrosion test

The pitting & crevice corrosion test of austenitic steel and nickel-based alloy corrosion alloy surfacing layer should be carried out according to ASTMG48. There is no pitting on the sample’s surface under a low magnification magnifying glass, and the maximum weight loss of 24h is 4.0g/m² (40 ± 2 °C).

1.9 Intergranular corrosion experiment

The intergranular corrosion test of austenitic steel and nickel base alloy corrosion alloy surfacing layer should be carried out according to ASTM A262 experiment B.

2. Comparison of surfacing flange with the conventional method

Compared with conventional insulating gaskets, surfacing flanges have the following advantages, as detailed in Table 1.
Table.1 Surfacing flange and insulation gasket

Project	Common flange + insulation gasket (conventional practice)	Overlaying welding flange + ordinary gasket
1. Solutions	An insulating gasket and an insulating sleeve are added between the dissimilar metal flanges to block the conductive path between the two metals, thereby avoiding galvanic corrosion.	The corrosion resistant alloy is welded on the surface of the easy corrosion flange, and the corrosion is transferred to an acceptable position to ensure that the sealing surface is not damaged by corrosion.
2. Reliability	1) The whole offshore oil and gas platform is a steel structure. If the insulation layer at the pipe support is aged and damaged after poor on-site construction management or long-term use in the later period, it is easy for the dissimilar metal pipelines to achieve conductivity through the pipe support, which leads to insulation failure and galvanic corrosion.	1) The surfacing layer has good bonding between metal and substrate, fine structure, good mechanical strength and corrosion resistance.
	2) There is a risk of aging after long-term use of insulation gaskets, resulting in a decrease in insulation resistance or even failure.	2) It can be repeatedly disassembled and reused.
	3) Insulation gasket sealing ring is vulnerable, and new gaskets need to be replaced during maintenance and disassembly.
3. Economic	Taking the 2″ CL900 flange joint as an example, the surfacing flange solution is about 1/27 of the insulation gasket solution. In this project, surfacing flange is used instead of fire insulation gasket, which saves more than 1.5 million yuan in direct material cost.
4. Availability	For insulation gaskets, because the specification requires fire prevention, most of the manufacturers that can meet this requirement are abroad, and the procurement cycle is long and the price is expensive.	For the sealing surface surfacing flange, the availability is strong, the domestic surfacing process is mature, the quality is reliable, and the supply cycle is short.

3. Conclusion

There are many kinds of pipeline materials for offshore oil and gas platforms. The galvanic corrosion probability on dissimilar metals’ contact surfaces is high. Once the oil and gas leakage caused by flange corrosion is harmful, the design of offshore platform pipelines should be focused on.
Compared with the conventional practice, using surfacing flanges at the dissimilar metal joints of the pipeline system has the advantages of economic reliability and easy access to solve the above problems. It is suitable for promotion and application on offshore oil and gas platforms. This scheme has been applied to many oil and gas platforms at home and abroad and achieved good economic benefits.
Author: Wang Guofu