Discussion on galvanic corrosion control measures for dissimilar metal flange connection of offshore oil and gas platform pipeline
When the dissimilar metal flange connection of offshore oil and gas platform pipeline, the transmission of conductive and corrosive fluid will produce galvanic corrosion, which will lead to flange joint failure and leakage and other safety accidents. According to the standard of ISO 21457, the advantages and disadvantages of several control measures to reduce galvanic corrosion hazards and their applicability to offshore oil and gas platform topside pipelines are compared. Combined with the actual application of the project, two applicable measures, i.e., the insulation gasket assembly and the carbon steel flange sealing surface overlaying with corrosion-resistant alloy, are mainly discussed, and the comprehensive performance comparison of the two schemes is carried out. Compared with insulating gasket components, carbon steel flange sealing surface surfacing corrosion resistant alloy should be the first choice to control galvanic corrosion of dissimilar metal flange connection of pipeline due to its low cost, simple manufacturing and easy installation. In the specific project, detailed surfacing technical requirements should be formulated, and reasonable surfacing process should be adopted to ensure the quality of surfacing welding of flange sealing surface.
Galvanic corrosion of dissimilar metal connection
Galvanic corrosion (also known as contact corrosion or bimetallic corrosion) refers to that dissimilar metals contact in the same medium. Due to different corrosion potentials, the dissolution rate of metals with lower potential increases, resulting in local corrosion at the contact, while the dissolution rate of metals with higher potential decreases. Galvanic corrosion widely exists in shipbuilding, petrochemical and other industries, as well as in the connection of dissimilar metal pipelines on offshore oil and gas platforms. Different process processing modules of offshore oil and gas platforms will select different pipeline metal materials according to the corrosion environment and material economic evaluation. The common metal materials mainly include carbon steel, stainless steel and duplex stainless steel. Nickel based alloy materials will also be used for H2S containing oil and gas fields. There will be dissimilar material connection between pipes, that is, pipe material grade change, such as stainless steel and carbon steel connection, stainless steel and duplex stainless steel connection, etc. At this time, flange connection should be adopted, and direct welding of dissimilar metals should be avoided. After welding, the microstructure of dissimilar metal joint changes complex, and it is easy to form defects such as coarse grains. The mechanical properties, high temperature properties and corrosion properties of the joint are lower than those of the base metal, and there are complex stress distribution notes. When transporting conductive medium, due to the existence of the corrosion potential difference of dissimilar metals, the dissimilar metal flange connection has the galvanic corrosion condition, and will aggravate other corrosion such as crevice corrosion and pitting corrosion. Due to galvanic corrosion in the flange sealing surface of the most serious damage, under the combined effect of various corrosion will eventually lead to flange sealing surface failure leakage.
For the dissimilar metal flange connection of offshore oil and gas platform pipelines, the following principles are usually adopted to control galvanic corrosion:
- (1) It is necessary to consider galvanic corrosion when transporting conductive corrosive media, such as moisture, closed drainage, production water, etc.
- (2) Because the larger the corrosion potential difference between different materials, the more serious the corrosion is, the galvanic corrosion between carbon steel and corrosion resistant alloy should be controlled, such as carbon steel and stainless steel, duplex stainless steel, nickel base alloy, etc. Generally, galvanic corrosion between corrosion resistant alloys is not considered.
- (3) Galvanic corrosion is not considered in flange connection of small cathode and large anode. Such as carbon steel pipe temperature, liquid level meter and other stainless steel parts connection.
Galvanic corrosion control measures
The galvanic corrosion risk exists in the connection of dissimilar metal materials, and certain measures must be taken to prevent the occurrence of safety accidents. Some galvanic corrosion control measures are given in section 87 of ISO 21457, and their advantages and disadvantages are shown in Table 1.
Table.1 Comparison of galvanic corrosion control measures
|1||Insulation pipe section can be used for installation of transition pipe section, such as glass fiber reinforced plastic||Easy to replace||Due to the limited bearing capacity, this measure is only applicable to low-pressure pipelines|
|2||Increase corrosion allowance of low potential metals||Effectively control the corrosion of pipeline inner wall||The larger corrosion allowance increases the original investment and cannot effectively protect the sealing surface of flange|
|3||Sacrificial anode is built in the vicinity of material grade change||Uniform polarization can protect the potential of the parts and control the corrosion within the time limit||It is difficult to repair and replace, and the anode may fall and block the pipeline|
|4||Electrical insulation, such as insulating joint or insulating gasket, is used for electrical isolation||The electric insulation effect is obvious||The material cost is high, the installation requirements are high, and the insulation gasket seal ring is vulnerable, so strict protection must be given to ensure the sealing reliability|
|5||Internal coating reduces cathode area||The construction is simple||The sealing surface of the flange cannot be effectively protected, and the inner coating may fall off and the service life is short|
|6||Surfacing treatment of low potential metal flange||The material cost is relatively low, which can effectively protect the sealing surface and ensure the effectiveness of the joint||Electrical insulation cannot be achieved|
The galvanic corrosion control measures recommended in ISO 21457 are compared and analyzed. Meanwhile, considering the characteristics of tight installation and limited space of offshore oil and gas platform pipelines, the following measures are taken into account
- Measure.1 is limited in pressure bearing capacity and installation space;
- Measure.2 can not control the corrosion of flange sealing surface, and there is still the possibility of flange joint failure;
- Measure.3 requires accurate calculation of anode weight, and installation space is limited, so it is difficult to repair and replace;
- The coating of measure.5 may fall off and its service life is short, which can not effectively protect the flange sealing surface;
- In view of the above four measures are not applicable to the offshore oil and gas platform topside pipeline, while the measures.4 and 6 do not occupy space, and the construction is relatively convenient, so it is more feasible for the pipeline dissimilar metal flange connection.
Insulating joint (see Fig.1) is usually used for insulation and isolation of long distance pipeline with cathodic protection. Sometimes, insulated joints are used in offshore oil and gas platforms to isolate the submarine pipeline from the topside. Considering the strict manufacturing and inspection requirements, high construction requirements and space requirements of insulated joints, it is not suitable for large-scale application in offshore oil and gas platform topside pipeline.
Figure 1 insulated joint
Insulating gasket assembly
Composition of insulating gasket assembly
The composition of the insulating gasket assembly is shown in Fig.2, including insulating gasket, insulating sleeve, insulating washer and metal washer.
Figure 2 insulating gasket assembly
According to the requirements of section 18.104.22.168 of safety rules for offshore fixed platforms, “gaskets for piping accessories on oil pipes and sea water fire fighting piping shall be made of incombustible materials.” Therefore, the insulating gasket of offshore oil and gas platform process pipeline should meet the fire resistance requirements and pass the api6fb fire resistance type test. The commonly used fire insulation gasket is composed of glass fiber reinforced resin laminated material + metal core plate + non-metallic main seal + fire seal. Different insulation gasket manufacturers have their own approved products, which can be selected according to the service conditions and the manufacturer’s recommendation.
Application of insulation gasket
Insulation gasket can effectively isolate the electrical insulation of dissimilar metal pipes. Although there is still conductive connection through pipe support, deck, grounding wire, etc., which can not completely avoid galvanic corrosion, it can effectively control the impact of galvanic corrosion on the pipeline, especially the sealing surface of flange joint.
The insulating gasket is suitable for the standard flange with flat face, raised face and ring joint surface. It has been widely used in the flange of ASME B16.5 class 2500 and below. However, considering the flat type of insulating gasket, the manufacturer should be consulted for the application of flange joint under ultra-high pressure condition (such as API 6A 69 MPa flange joint), cyclic vibration condition and low temperature condition.
It is widely used in offshore oil platforms to control galvanic corrosion by using insulating gasket components. It is widely used in gas fields in East China Sea and South China Sea, and the application effect is good. G10 glass fiber reinforced epoxy resin (GRE) insulation material which meets the requirements of NEMA is usually used as the fireproof insulation gasket assembly of offshore oil and gas platform. The use cost of insulating gasket has always been the focus of the project. Taking the central processing platform of a project in East China Sea as an example, more than 60 sets of insulation gasket components are shared, and the contract price is more than 2 million. Higher material costs significantly increase the construction investment of the project.
Carbon steel flange sealing surface is overlaid with corrosion resistant alloy
Control galvanic corrosion principle
The corrosion rate is the highest at the joint of dissimilar materials, and the farther away from the joint, the lower the corrosion rate. The two sealing faces of dissimilar metal flange are most easily affected by galvanic corrosion through gasket. With the extension of pipeline, the influence of galvanic corrosion on the whole pipeline is weaker and weaker. Therefore, the key to control galvanic corrosion hazard of pipeline is to protect the sealing surface of flange joint.
By overlaying the carbon steel flange sealing surface with corrosion-resistant alloy, the flange sealing surface is effectively protected, and galvanic corrosion is transferred to the whole pipeline. With the injection of corrosion inhibitor and the corrosion allowance of the pipeline itself, the galvanic corrosion can be effectively controlled and the flange joint failure caused by galvanic corrosion can be avoided.
For carbon steel flange sealing surface surfacing, the welding material consistent with or higher than that of the matched corrosion-resistant alloy flange shall be selected. The commonly used surfacing materials include stainless steel, duplex stainless steel, nickel base alloy, etc., and the specific brand shall be selected according to ASME BPVC volume 1I, section C.
The surfacing position of carbon steel flange sealing surface is determined by the form of flange sealing surface. According to the characteristics of ASME B16.5 raised face RF and ring connection RTJ flange sealing surface commonly used on offshore oil and gas platforms, the surfacing position is shown in Fig. 3. The overall dimension of the flange after surfacing welding should still be the standard flange.
Figure 3 welding position of flange sealing surface
According to API 5ld, the thickness of corrosion resistant alloy layer of composite steel pipe should not be less than 2.5 mm. In order to control the dilution rate of the surfacing metal and ensure the chemical composition and corrosion resistance of the surfacing layer material of the flange sealing surface, the thickness of the corrosion resistant alloy layer on the flange sealing surface should be at least 3 mm after processing.
The raised face flange of ASME B16.5, Class 150 and Class 300 pressure is the common flange of offshore oil and gas platform, and its boss height is 2 However, the boss height of ring joint face flange of all pressure levels is the ring groove depth. When surfacing welding, it must be machined to below the boss thickness to ensure that the standard flange size can still be maintained after surfacing, which will consume the minimum thickness of flange and affect the overall strength of flange. Therefore, for the above cases, the surfacing material should be higher than or consistent with the corrosion resistant alloy flange.
For example, when the ASTM A105 carbon steel flange of class 300 is connected with the duplex stainless steel flange of ASTM A182 F51, if the stainless steel is selected as the surfacing material, the strength of the surfacing layer will be lower than that of the base material, and the temperature and pressure grade of the flange may not meet the process requirements.
Welding procedure qualification and reasonable welding procedure specification should be made for surfacing corrosion resistant alloy on carbon steel flange sealing surface. Firstly, the surfacing methods, such as TIG, GMAW and plasma arc welding, are selected; then the surfacing procedures are formulated, including the number and thickness of surfacing layers; secondly, the surfacing materials, transition layer materials and surface materials are selected; finally, the surfacing layer surface is inspected, including non-destructive inspection, which is usually penetration inspection or ultrasonic inspection, metallographic test, mechanical property test, etc Determination of chemical components. Through reasonable welding and product inspection, it is ensured that the surfacing layer is complete and corrosion resistance is reliable.
Combined with the above discussion and analysis, in order to ensure the reliability of the strength and corrosion resistance of the surfacing flange, the recommended material table of corrosion resistant alloy for carbon steel flange surfacing on offshore oil and gas platform is given (see Table 2). For example, er316l, er2209 or ERNiCrMo-3 can be selected for surfacing welding when class 15O carbon steel raised face flange is connected with A182 f316l stainless steel flange. However, considering the economy of er316l material, it should be the first choice. For the same project, several corrosion-resistant alloy pipeline materials exist at the same time. In order to avoid mixed use on site, a kind of surfacing material can be specified, and a higher level material should be selected. For example, if there is stainless steel or nickel base alloy, unified surfacing of ERNiCrMo-3 can be selected.
Table.2 suggestions on Application of corrosion resistant alloy materials for carbon steel flange surfacing welding on offshore oil and gas platform
|Flange type||Matching corrosion resistant alloy flange material||
Carbon steel flange surfacing
Corrosion resistant alloy materials
|ASME B16.5, Class 150 and class 300, raised face (RF)||
ASTM A182 F316L
(austenitic stainless steel)
ASTM A182 F51
(duplex stainless steel)
AWS ER 2209
ASTM 8564 N06625
(nickel base alloy)
|ASME 816.5, class 600, raised face (RF)||
ASTM A182 F316L
ASTM A182 F51
ASTM B564 N06625
|ASME B16.5 class 600 and above, ring joint surface (RTJ)||ASTM A182 F316L||
ASTM B564 N06625
Note: the transition layer material, such as er309lmo, should be selected according to the actual welding procedure qualification results.
Application of surfacing flange
Attention should be paid to the following points in the application of welding flange elbow:
- (1) The sealing gasket should be made of corrosion-resistant alloy. The material of gasket should be selected reasonably to ensure that the hardness of surfacing layer is higher than that of gasket, so as to ensure the integrity of the whole flange joint.
- (2) The fasteners matched with corrosion resistant alloy flange should be selected for the connection of metal flange joint of different scales.
- (3) For the key dissimilar metal connection position, the surfacing layer can be extended to the LRS in the flange, and the harm of electric corrosion to carbon steel flange can be strengthened. In the Runyang platform of a gas field in Myanmar, the wet gas pipe of LRS is connected with class 600 carbon steel raised face flange, which is lack of duplex stainless steel connection. 316L stainless steel is overlaid with carbon steel flange, 316L gold corrosion ring gasket and ASTM A453 gr 660B (terolon coating) fasteners are used for flange joint connection to control couple corrosion. Since the platform was put into operation in April 2016, the application effect is good.
Comparative analysis of comprehensive performance of two control measures
See Table 3 for the comprehensive performance comparison between the measures for controlling galvanic corrosion of fire-proof insulating gasket components and those for carbon steel flange sealing surface overlaying with corrosion-resistant alloy.
Table 3 Comparison of comprehensive performance of two measures
|Performance||Fire proof insulation gasket assembly||
Carbon steel flange sealing surface
Surfacing corrosion resistant alloy
|Scope of application||Limited scope of application, consult manufacturer for extra high voltage and special occasions||Wide range of application, can be used for standard flange and special flange|
|Convenience of construction||The construction technology requirements are high: 1. There are many insulation gasket components, which requires the assembly to be separated; 2. The pre tightening of fasteners must refer to the torque value recommended by the manufacturer to ensure the performance and service life of the manufacturer; 3. The insulation layer and sealing ring of the insulating gasket are non-metallic, which are easy to be damaged by the outside, which puts forward higher protection requirements for the materials used on site.||Convenient construction, according to the common flange construction|
|Availability||Poor availability, fireproof insulation gasket manufacturers are abroad, long procurement cycle||Good availability, mature domestic surfacing process, reliable production quality and short procurement cycle|
|Economy||Poor economy and high procurement cost||Good economy and low purchase cost|
According to the comprehensive performance comparison in Table 3, the carbon steel flange sealing surface overlaying corrosion-resistant alloy has the advantages of simple processing, low cost, easy construction and satisfactory control of galvanic corrosion. The carbon steel sealing surface surfacing corrosion resistant alloy control measures are more suitable for offshore oil and gas platform construction projects.
For dissimilar metal flange connection of offshore oil and gas platform, two measures can be adopted to control galvanic corrosion, i.e. insulation gasket assembly and surfacing corrosion-resistant alloy flange on sealing surface. Under normal conditions, the surfacing flange with sealing surface should be the first choice because of its relatively simple manufacturing, easy to obtain, relatively low cost and easy construction.
All insulating gaskets are products approved by manufacturers, and can be selected and designed according to actual needs. However, there is no corresponding standard and specification reference for the surfacing flange, so the detailed technical requirements should be formulated in the specific project, and the reasonable surfacing process should be adopted to ensure the quality of surfacing welding of flange sealing surface.
Authors: Shan Shichao, Hu Xiaoming, Liu Jifei, Cheng Jiuhuan, Hao Mengjiang
Source: China Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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