A stainless steel structure with dark welds and rainbow-coloured heat tint should not arrive on site as a finished product. Removing post-weld oxides and discolouration is a surface preparation step that affects both appearance and corrosion resistance.
Why is weld heat tint a problem?
Welding temperatures produce surface oxides and can leave a chromium-depleted layer immediately beneath them. As a result, corrosion resistance around the weld is lower than that of correctly treated material. Residual heat tint can increase susceptibility to localised corrosion, particularly in environments containing chlorides.
The term “stainless steel” does not mean resistance to every environment or fabrication practice. Material grade, service conditions, welding quality and final surface treatment all matter.
Delivered without pickling — an example from site
Fabricators unfamiliar with stainless steel sometimes stop at welding. The two photos below show a structure delivered to site without pickling: dark bands around welds, heat tint and an uneven surface are clearly visible.
This finish looks unattractive and incomplete. Reduced corrosion resistance can lead to complaints and remedial work. Cleaning becomes more difficult after installation, when access to some surfaces may be restricted.
Chemical pickling and electrochemical cleaning
Chemical pickling removes oxides and a thin layer of metal, including the chromium-depleted surface layer. The treatment and its extent must suit the steel grade, surface condition, component geometry and service requirements. Conventional pickling is not the only possible method, but visible post-weld oxidation needs effective removal.
The TIG Brush photo shows electrochemical weld cleaning. The operator moves a brush wetted with electrolyte over the surface. Electric current and the cleaning fluid remove heat tint and oxides; a correctly selected and controlled process also passivates the treated surface. Final cleaning and rinsing must follow the manufacturer’s instructions.
Pickling, cleaning and passivation are not interchangeable terms. Passivation alone does not replace the removal of heavy oxides or the chromium-depleted layer. Surface colour alone should not determine whether the treatment is effective.
Before and after — a visible difference
The comparison shows a component before surface treatment at the top and after treatment at the bottom. A clean, more uniform surface improves appearance. The required surface properties should nevertheless be verified through inspection appropriate to the project and service conditions.
What should be checked before delivery and acceptance?
Project documentation should define the required finish and surface inspection criteria. Before delivery, check that heat tint and oxides have been removed, that the component has been cleaned after treatment, and that it is protected against recontamination during transport and installation. Dimensions and weld quality matter, but they are not the entire acceptance check.
How does MESTIL support structural quality?
At MESTIL, we prepare structural designs, detailed drawings and shop drawings. Clear requirements for materials, connections and surface finish help reduce misunderstandings between design, fabrication and installation. Our technical consultancy, opinions and assessments support the resolution of problems in existing structures and the planning of repairs.
Need a design or advice for an industrial structure? Talk to the MESTIL team. See also our article on weld symbols in drawings.