Why is the stainless steel pipe weld prone to rust and how to deal with the rust on the stainless steel pipe weld

Stainless steel pipes rely on a very thin, strong, fine, and stable chromium-rich oxide film (protective film) formed on their surface to prevent the continued infiltration and oxidation of oxygen atoms, thereby obtaining the ability to resist rust. Once for some reason, this film is constantly damaged, oxygen atoms in the air or liquid will continue to infiltrate or iron atoms in the metal will continue to separate out, forming loose iron oxide, and the metal surface will continue to rust. There are many forms of damage to this surface film, and the following are the most common in daily life:

1. The surface of the stainless steel pipe is deposited with dust containing other metal elements or foreign metal particles. In humid air, the condensed water between the attachment and the stainless steel pipe connects the two into a micro-battery, triggering an electrochemical reaction, and the protective film is damaged, which is called electrochemical corrosion.

2. Organic juice (such as vegetables, noodle soup, sputum, etc.) adheres to the surface of the stainless steel pipe. In the presence of water and oxygen, it forms organic acid, which corrodes the metal surface for a long time.

3. The surface of the stainless steel pipe adheres to substances containing acid, alkali, and salt (such as alkaline water and lime water splashing from the wall decoration), causing local corrosion.

4. In polluted air (such as an atmosphere containing a large amount of sulfide, carbon oxide, and nitrogen oxide), it encounters condensed water to form sulfuric acid, nitric acid, and acetic acid liquid points, causing chemical corrosion.

To ensure that the surface of the metal steel pipe is permanently bright and not corroded, we recommend:

1) The surface of the decorative stainless steel pipe must be cleaned and scrubbed frequently to remove attachments and eliminate external factors that cause rust.

2) 316 stainless steel pipes should be used in coastal areas. 316 materials can resist seawater corrosion.

3) The chemical composition of some stainless steel pipes on the market cannot meet the corresponding national standards and cannot meet the requirements of 304 materials. Therefore, it will also cause rust, which requires users to carefully choose products from reputable manufacturers.

What should be paid attention to when welding stainless steel pipes?

Key points of stainless steel pipe welding process technology

Stainless steel welded steel pipes are formed by rolling and welding stainless steel plates through several molds on a welding pipe forming machine. Since stainless steel pipes have high strength and their structure is a face-centered cubic lattice, they are easy to formwork hardening. When forming welded pipes, on the one hand, the mold has to bear greater friction, which makes the mold easy to wear; on the other hand, the stainless steel sheet is easy to forms adhesion (bite) with the mold surface, causing the welded pipe and the mold surface to be strained. Therefore, a good stainless steel pipe forming mold must have extremely high wear resistance and anti-adhesion (bite) performance. Our analysis of imported welded pipe molds shows that the surface treatment of such molds is treated with superhard metal carbides or nitride coatings.

Compared with traditional melting welding, laser welding and high-frequency welding have the characteristics of fast welding speed, high energy density, and low heat input. Therefore, the heat-affected zone is narrow, the grain growth is small, the welding deformation is small, the cold working forming performance is good, and it is easy to realize automated welding and single-pass one-time penetration of thick plates. The most important feature is that I-shaped groove butt welding does not require filler materials.

Welding technology is mainly used on metal-based materials. Commonly used methods include arc welding, argon arc welding, CO2 shielded welding, oxygen-acetylene welding, laser welding, electroslag pressure welding, etc. Non-metallic materials such as plastics can also be welded. There are more than 40 metal welding methods, which are mainly divided into three categories: fusion welding, pressure welding, and brazing.

Fusion welding is a method of heating the interface of the workpiece to a molten state during the welding process and completing the welding without pressure. During fusion welding, the heat source quickly heats and melts the interface of the two workpieces to be welded to form a molten pool. The molten pool moves forward with the heat source, and after cooling, a continuous weld is formed to connect the two workpieces into one.

During the fusion welding process, if the atmosphere is in direct contact with the high-temperature molten pool, the oxygen in the atmosphere will oxidize the metal and various alloy elements. Nitrogen, water vapor, etc. in the atmosphere enter the molten pool, and defects such as pores, slag inclusions, and cracks will be formed in the weld during the subsequent cooling process, which will deteriorate the quality and performance of the weld.

Pressure welding is to achieve atomic bonding between two workpieces in a solid state under pressure conditions, also known as solid-state welding. The commonly used pressure welding process is resistance butt welding. When the current passes through the connection end of the two workpieces, the temperature rises due to the large resistance. When heated to the plastic state, they are connected into one under the action of axial pressure.

The common feature of various pressure welding methods is that pressure is applied during the welding process without adding filler materials. Most pressure welding methods such as diffusion welding, high-frequency welding, cold pressure welding, etc. do not have a melting process, so there is no problem of beneficial alloy element burning and harmful elements invading the weld like fusion welding, which simplifies the welding process and improves the safety and sanitation conditions of welding. At the same time, because the heating temperature is lower than that of fusion welding and the heating time is shorter, the heat-affected zone is small. Many materials that are difficult to weld with fusion welding can often be welded into high-quality joints with the same strength as the parent material by pressure welding.

Brazing is a method of using a metal material with a lower melting point than the workpiece as a brazing material, heating the workpiece and the brazing material to a temperature higher than the melting point of the brazing material and lower than the melting point of the workpiece, using liquid brazing material to wet the workpiece, fill the interface gap and achieve mutual diffusion between atoms with the workpiece, thereby achieving welding.

The seam connecting two connected bodies formed during welding is called a weld. Both sides of the weld will be affected by welding heat during welding, and the structure and performance will change. This area is called the heat-affected zone. Due to the differences in workpiece materials, welding materials, welding current, etc. during welding, overheating, embrittlement, hardening or softening may occur in the weld and heat-affected zone after welding, which will also reduce the performance of the weldment and worsen the weldability. This requires adjusting the welding conditions. Preheating the weldment interface before welding, heat preservation during welding, and heat treatment after welding can improve the welding quality of the weldment.