Can stainless steel flanges and carbon steel pipes be welded together directly?
Today, a customer asked me when installing the pipeline system on site: stainless steel flanges and carbon steel pipes, why not install them directly? Separate the protective pad (Teflon) in the middle to isolate it! The author below will share with you the reasons based on years of installation experience!
A carbon steel pipe can be identified with Grade A or B, a stainless-steel pipe with Grade TP304 or Grade TP321, a carbon steel fitting with Grade WPB etc..
Below you will find as an example 3 tables with chemical requirements for:
- Stainless Steel Flanges according to ASTM A182 Grade F304, F304L F316L
- Stainless Steel Pipes according to ASTM A312 Grade TP304, TP304L, TP3016L
- Stainless Steel Fittings according to ASTM A403 Grade WP304, WP304L, WP316L
ASTM A105 has no Grade. Sometimes ASTM A105N is described;
N stands not for Grade, but for normalized. Normalizing is a type of heat treatment, applicable to ferrous metals only. The purpose of normalizing is to remove the internal stresses induced by heat treating, casting, forming etc..
- A106 = This specification covers carbon steel pipe for high-temperature service.
- A335 = This specification covers seamless ferritic alloy-steel pipe for high-temperature service.
- A333 = This specification covers wall seamless and welded carbon and alloy steel pipe intended for use at low temperatures.
- A312 = Standard specification for seamless, straight-seam welded, and cold worked welded austenitic stainless steel pipe intended for high-temperature and general corrosive service.
- A105 = This specification covers standards for forged carbon steel piping components, that is, flanges, fittings, Valves, and similar parts, for use in pressure systems at ambient and higher-temperature service conditions.
- A182 = This specification covers forged or rolled alloy and stainless steel pipe flanges, forged fittings, and Valves and parts for high-temperature service.
- A350 = This specification covers several grades of carbon and low alloy steel forged or ring-rolled flanges, forged fittings and Valves for low-temperature service.
Direct welding is possible, and it is necessary to select a suitable electrode (dissimilar steel welding), but both will have galvanic corrosion in corrosive media, and carbon steel pipes and welds will corrode quickly. Unless the pipe medium is corrosive or non-corrosive.
The contact between stainless steel and carbon steel, the iron ions of carbon steel will pollute the stainless steel, and electrochemical corrosion will occur due to the difference in potential energy between the two. Therefore, when storing, transporting, processing and installing stainless steel, it must be strictly separated from carbon steel. In actual life, I have seen: 304L stainless steel and carbon steel are put together, no isolation measures are taken, and 304L stainless steel is “rusty”, which is why isolation measures must be taken;
If the material is different, it is easy to cause electrochemical corrosion. I encountered a similar situation when I encountered the tube and the tube rack last time. Reasons why carbon steel and stainless steel cannot be directly connected:
- 2.1 Electrochemical corrosion
- 2.2 Carbon steel pollution: Scratches and corrosive media caused by contact with carbon steel parts form galvanic cells and cause electrochemical corrosion.
- 2.3 Cutting: The adhesion and corrosion of rust-prone substances such as cutting slag and splashing form a primary battery to produce electrochemical corrosion.
- 2.4 Roasting: The composition of the flame heating zone changes and the metallographic structure is uneven, and the primary battery is formed with the corrosive medium to cause electrochemical corrosion.
- 2.5 Welding: Physical defects (biting, pores, cracks, unfused, incomplete penetration, etc.) and chemical defects (grain coarse, grain boundary chromium-depleted, segregation, etc.) and corrosive medium forming galvanic cells to produce electrochemical corrosion.
- 2.6 Material: Chemical defects in stainless steel (uneven composition, S, P impurities, etc.) and physical defects on the surface (loose, blisters, cracks, etc.) are conducive to electrochemical corrosion caused by the formation of galvanic cells with corrosive media.
- 2.7 Passivation: The acid passivation passivation effect is not good, resulting in uneven or thin passivation film on stainless steel surface, which is easy to form electrochemical corrosion.
- 2.8 Cleaning: The acid-washed passivation residue remaining and the chemically corroded product of stainless steel form electrochemical corrosion with the stainless steel piece.
- 2.9 Stress concentration is prone to stress corrosion.
In short, due to its special metallographic structure and surface passivation film, stainless steel is generally difficult to be corroded by chemical reaction with the medium under normal conditions, but it cannot be corroded under any conditions. In the presence of corrosive media and incentives (such as scratches, splashes, slag, etc.), stainless steel can also be corroded by slow chemical and electrochemical reactions with corrosive media, and the corrosion rate is relatively fast under certain conditions. Corrosion, especially pitting and crevice corrosion. The corrosion mechanism of stainless steel parts is mainly electrochemical corrosion.
In the GB/T50235-98 industrial metal pipeline engineering construction and acceptance specifications, 6.3.23; “Stainless steel pipes and brackets should be padded with stainless steel or non-metallic gaskets with chloride ion content not exceeding 50ppm.” Stainless steel valves Of course, no exception, your teacher is doing it right.
Carburizing occurs in direct contact between stainless steel pipe and carbon steel, causing electrochemical corrosion of stainless steel. It is no problem to insert non-metallic gaskets. However, the chloride ion content of gaskets is required. The specific inspection standards can be seen!
If a pipe and flange is of carbon steel, then orifice plate (to be installed in that pipe/flange) will be of carbon steel or any other material (Stainless steel etc) can be used ?
Corrosion products from the CS orifice plate may choke and hence distort the readings of the instruments to which the orifice flange is connected. So an SS plate is generally used.
Galvanic corrosion between the CS piping and SS orifice plate is negligible. However, carbon pick-up by the SS plate could be an issue. Always better to use a low carbon SS.
Source: China Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.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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