Do both the inner and outer surfaces of a pressure vessel made of stainless steel S30408 need to be pickled and passivated?

The inner surface is determined based on the operating environment; the use of stainless steel serves to prevent corrosion or to avoid contamination by iron ions, among other purposes. Whether the outer surface needs to be treated depends on the customer’s requirements and your company’s standards. This serves the purpose of surface treatment. It just looks better.

First, we need to understand what the purpose of pickling and passivation is. The goal of pickling and passivation is to treat the surface of stainless steel by removing various types of oils, rust, oxide scales, weld spatter, and other contaminants; as a result, the surface becomes a uniform silver-white color, which **improves the corrosion resistance of stainless steel**. The surface of stainless steel sheets is already passivated upon leaving the factory. It is understandable that stainless steel containers require pickling and passivation because, during the production process, damage to the surface of the stainless steel occurs due to storage and manufacturing, necessitating re-passivation. So I believe that as long as it is manufactured in strict accordance with the specifications for stainless steel containers (using protective measures such as cladding roll formers and rubber mats in the work area), it is sufficient to carry out pickling and passivation of the inner and outer surfaces around the welds.

Well, the original poster answered in quite some detail. It’s learnable; in other words, it can be used or not used :) – there are no mandatory rules or regulations

Generally speaking, if high corrosion resistance is required for stainless steel containers – such as in food-grade industries where drinking water is stored, or in applications where iron ions are a concern, like in machine pump lubricants – it is necessary to carry out acid washing and passivation of the inner surface of these containers. Otherwise, the unpassivated surface of the stainless steel container will react upon first contact with the medium, forming a dense oxide layer on the surface that serves a similar function to that of acid washing and passivation; it is through this process that high corrosion resistance can be achieved. However, this process inevitably leads to the loss of iron ions from the stainless steel surface layer, thereby contaminating the medium. Therefore, in this case, the user will request pickling and passivation of the inner surface. For the outer surface of stainless steel containers, in marine climates or environments with salt spray, or when insulation is required for the outer surface but the chloride content in the insulating material cannot be controlled, it is recommended to carry out acid washing and passivation on the outer surface of the stainless steel container.

I’ve learned it, thanks to the friend from above. :)

There are a few additions to what was said above. First of all, the original poster is correct; pickling and passivation basically depend on the requirements – if the buyer requests pickling, then it is carried out. There are already many manufacturers in the industry that carry out acid washing and passivation on all stainless steel equipment, mainly to make the surface look better. After pickling and passivation, the surface of stainless steel appears attractive, usually in a silver-white color. Pickling: Removes dirt and harmful substances embedded in the surface of the material that could affect its corrosion resistance. Passivation: Forms an oxide layer on the surface of the material, preventing direct contact between the material itself and harmful substances, thereby enhancing its corrosion resistance. Stainless steel is generally subjected to solution treatment upon leaving the factory; pickling and passivation are not performed. Except as required by special orders.