What causes stainless steel pipe welds to rust easily

Stainless steel pipes rely on a very thin, strong, dense, and stable chromium-rich oxide film (protective film) formed on their surface to prevent further oxygen infiltration and oxidation, thus achieving their rust resistance. If this film is continuously damaged for some reason, oxygen atoms from the air or liquid will continue to penetrate, or iron atoms from the metal will continue to be released, forming loose iron oxide, which in turn causes the metal surface to rust. This surface film damage can occur in many ways, the most common of which are as follows:

1. Dust or foreign metal particles containing other metal elements accumulate on the surface of the stainless steel pipe. In humid air, condensation between the dust and the stainless steel creates a micro-battery, triggering an electrochemical reaction that damages the protective film. This is called electrochemical corrosion.

2. Organic juices (such as vegetable juices, noodle soup, and spit) adhere to the surface of the stainless steel pipe. In the presence of water and oxygen, these organic acids form, which corrode the metal surface over time. 3. Acids, alkalis, and salts may adhere to the surface of stainless steel pipes (such as lye water or limewash splashed from wall decoration), causing localized corrosion.

4. In polluted air (such as atmospheres containing high levels of sulfides, carbon dioxide, and nitrogen oxides), condensation may form sulfuric acid, nitric acid, and acetic acid droplets, causing chemical corrosion.

To ensure a permanently bright and rust-free surface for stainless steel pipes, we recommend the following:

1) Regularly clean and scrub the decorative stainless steel surface to remove any buildup and eliminate external factors that may cause rust.

2) In coastal areas, use 316 stainless steel, which is resistant to seawater corrosion.

3) The chemical composition of some stainless steel pipes on the market does not meet the relevant national standards and does not meet the requirements of 304. This can also cause rust, requiring users to carefully select products from reputable manufacturers.

Key Technical Points of Stainless Steel Pipe Welding Process

Stainless steel welded pipes are formed on a welded pipe forming machine from stainless steel sheets through several dies and then welded. Due to the high strength of stainless steel pipes and their face-centered cubic lattice structure, they are prone to work hardening. This results in: Firstly, during welded steel pipe forming, the molds are subject to significant friction, causing wear; secondly, the stainless steel sheet easily bonds (seizes) with the mold surface, causing strain on both the welded steel pipe and the mold surface. Therefore, high-quality stainless steel forming molds must possess extremely high wear and adhesion (seize) resistance. Our analysis of imported welded steel pipe molds indicates that these molds are typically surface-treated with superhard metal carbides or nitrides.

Compared to traditional fusion welding, laser welding and high-frequency welding offer faster welding speeds, higher energy density, and lower heat input. This results in a narrow heat-affected zone, minimal grain growth, minimal weld distortion, and excellent cold-working performance. This facilitates automated welding and single-pass penetration of thick plates. Most importantly, I-groove butt welds require no filler material.

Welding technology is primarily applied to metal-based materials. Commonly used methods include arc welding, argon arc welding, CO2 shielded welding, oxy-acetylene welding, laser welding, and electroslag pressure welding. Non-metallic materials such as plastics can also be welded. There are over 40 metal welding methods, primarily categorized as fusion welding, pressure welding, and brazing.

Fusion welding involves heating the interface of the workpieces to a molten state without applying pressure. During fusion welding, a heat source rapidly heats and melts the interface between the two workpieces, forming a molten pool. The molten pool moves forward with the heat source and cools to form a continuous weld, joining the two workpieces.

During the fusion welding process, if atmospheric air comes into direct contact with the high-temperature molten pool, the oxygen in the atmosphere will oxidize the metal and various alloying elements. Atmospheric nitrogen and water vapor, which enter the molten pool, can also cause defects such as porosity, slag inclusions, and cracks in the weld during the subsequent cooling process, deteriorating the weld quality and performance.

Pressure welding, also known as solid-state welding, achieves atomic bonding between the two workpieces under pressure. Resistance butt welding is a common pressure welding process. When current passes through the connecting ends of two workpieces, the high resistance causes the temperature to rise. Once heated to a plastic state, the workpieces are joined together under axial pressure.

A common feature of all pressure welding methods is that they apply pressure during the welding process without the addition of filler material. Most pressure welding methods, such as diffusion welding, high-frequency welding, and cold pressure welding, do not involve a melting process. Therefore, they avoid the problems of beneficial alloying element burnout and harmful element intrusion into the weld, as occurs with fusion welding. This simplifies the welding process and improves safety and hygiene. Furthermore, because the heating temperature is lower and the heating time is shorter than with fusion welding, the heat-affected zone is smaller. Many materials that are difficult to weld using fusion welding can often be produced using pressure welding with high-quality joints of equal strength to the parent metal.

Brazing uses a metal with a lower melting point than the workpieces as a brazing filler metal. The workpieces and filler metal are heated to a temperature above the filler metal's melting point but below it. The liquid filler metal wets the workpieces, filling the gaps at the interface and allowing atomic diffusion to occur between the workpieces, thus achieving the desired weld.

The seam between two joined parts formed during welding is called a weld. Both sides of the weld are subjected to welding heat, causing changes in their structure and properties. This area is called the heat-affected zone (HAZ). Due to differences in workpiece materials, welding materials, and welding current, the weld and HAZ may overheat, become embrittled, hardened, or softened after welding, degrading weld performance and impairing weldability. This requires adjusting welding conditions. Preheating the weldment interface before welding, holding the weld during welding, and performing post-weld heat treatment can improve weld quality.