Technical FAQ

There is no such thing as a bad question - only opportunities for deeper understanding. In Sassda’s training sessions, participants from all levels of the industry regularly raise insightful queries, some of which may seem simple at first but often lead to a clearer grasp of complex, technical  concepts. These questions not only benefit those in the immediate training environment but also provide valuable learning for students and professionals across the stainless steel value chain. Over time, we’ve noticed that certain questions come up repeatedly during and after our courses. In this new series we aim to explore some of the most common and thought-provoking questions we receive…

The question? Why do the shiny cutting edges of laser-cut parts corrode preferentially?
The answer...

This is not only a frequently asked question during our training courses but a common topic when assisting the industry with technical issues. In this particular instance, the question arose from repeated instances where stainless steel signage for outdoor applications in coastal areas ordered from reputable laser-cutting service providers - began to show localised corrosion near and around laser-cut surfaces.

In all cases, the material used was the austenitic Grade 304, which generally has high corrosion resistance. While Grade 304 might not be the best choice for coastal environments, the fact that corrosion occurs preferentially near the cutting surfaces, is an interesting phenomenon.

The returned pieces were tested, and the material grade was confirmed. A photo sent later, by the student who first raised the question, showed a smooth (2B) finish, eliminating surface finish as a contributing factor to corrosion. This is not an isolated problem; it occurs because the cutting surfaces are not passivated, making them more prone to corrosion. The student countered that the surfaces must have been passivated because they appeared shiny upon receipt of the finished product.

When stainless steel is laser-cut, it undergoes a thermal process. The laser beam heats the material to its melting point, although in a highly localised area. This creates a narrow Heat-Affected Zone (HAZ), which is difficult to detect with the naked eye but follows the same principles as broader heat-affected zones - such as those seen in welding.

Typically, areas that exhibit discoloration have been heat affected and covered by a heat oxide layer. These oxide layers reflect light differently, creating the colourful bands sometimes seen on welds.

The heat oxides are depleted in chromium, which can reduce the chromium content in affected areas below the critical 10.5% threshold required for stainless steel to remain corrosion-resistant. As a result, these areas behave more like carbon steel and become susceptible to rust. While welding
typically makes heat-affected zones more visible, the rapid movement of a laser beam makes this effect less noticeable.

In addition, the laser-cutting process employs pressurised cutting gasses to assist with dross removal and enhance cutting quality. When an inert gas, such as nitrogen, is used, the cutting takes place in a protected environment, preventing oxidation. This explains why no visible colour band appears. However, not all material escapes chromium depletion, making the affected areas vulnerable to corrosion. As corrosion sets in, iron oxide
deposits on the surface, causing discoloration.

Dross formation and its role in corrosion

Another factor influencing corrosion in laser-cut stainless steel is dross formation. Cutting gasses help to minimise dross formation, but some dross is always present, even if it is not visible to the naked eye.

Dross refers to the oxidized metal residue that forms on the cut edge due to the heat generated by the laser beam interacting with the atmosphere. It consists of partially melted metal debris that solidifies on the cut edge, often appearing as a rough, oxidized layer.

During laser cutting, intense heat melts the metal along the cutting line. If the molten material is not fully ejected, it resolidifies as dross. Similar to the HAZ, dross formation can contribute to corrosion through chromium depletion. The dross layer can become enriched in chromium oxides, reducing the chromium content in the underlying stainless steel and compromising its corrosion resistance.

Dross also creates surface irregularities, increasing the exposed area for moisture and corrosive agents. Some of the dross expelled from the cutting surface may adhere to  adjacent areas while still in a liquid state. This penetration of the passive layer can create galvanic corrosion cells, accelerating corrosion.

Optimising laser-cutting parameters - such as cutting speed, laser power, and assist gas pressure - can help to reduce dross formation. Proper mechanical removal of dross after cutting further minimizes the risk of corrosion.

Key takeaways

• Not all shiny surfaces are truly stainless. Do not assume that any heat-treated surface is fully passivated, if it has not received the proper  treatment.
• Minimise dross formation and remove any dross present to eliminate chromium-depleted areas that could trigger corrosion.
• Prevent unnecessary costs associated with removing, recutting, or treating affected products by taking proactive measures immediately after cutting.
• The moral of the story…this question was not ridiculous - it was Simply Brilliant!

Did you know?

The Southern Africa Stainless Steel Development Association (Sassda) supports industry growth by providing expert technical advice on fabrication, material selection, corrosion resistance, and failure analysis. Members have access to a comprehensive reference library, technical seminars, and training courses. This service is free for queries requiring less than half a day, with additional support available if needed. Sassda also maintains strong links with national and international technical institutions to stay at the forefront of global stainless steel advancements.

Click here to make use of this service https://sassda.co.za/about-stainless/technical-enquiry/ OR email mankabe@sassda.co.za to find out more about our training courses.