Technical Case Study : SASSDA

Stainless steel is widely used in architecture for its durability, corrosion resistance, and modern aesthetic. Yet its visual appearance often causes confusion and frustration.

End users are frequently surprised by how different the same stainless steel surface can look under varying lighting conditions. The contrast between direct sunlight and indoor incandescent lighting, can significantly alter perceived colour, brightness, and uniformity. This becomes particularly problematic in large outdoor installations such as cladding, signage, and facades, where adjacent panels may appear mismatched despite being manufactured to the same specification.

As a result, Sassda regularly receives enquiries in this regard. To understand and manage these concerns, it is important to recognise the key factors that influence the visual appearance of stainless steel.

Key Factors That Determine the Visual Appearance of Stainless Steel

1. Surface Texture / Roughness (Ra, Rz, Rq)

Surface roughness is one of the primary determinants of appearance because it controls how light interacts with the metal. Smooth surfaces result in higher reflectivity and a brighter appearance. Rougher surfaces diffuse reflected light and create a matte effect.

The reflectivity and overall appearance of stainless steel are directly linked to surface texture. Roughness metrics such as Ra, Rz, and Rq describe the height and distribution of microscopic peaks and valleys. Even surfaces with the same Ra value can look different due to variations in the spacing and shape of surface features. Surface finish influences far more than just visual appeal. It significantly affects:

Corrosion resistance - Smoother surfaces are more resistant to corrosion due to fewer imperfections where contaminants and moisture can accumulate
Cleanability and sanitation - Smoother finishes are easier to clean and less likely to harbour bacteria, making them suitable for food processing, pharmaceutical, and medical applications.
Friction and wear - Surface roughness affects the coefficient of friction and therefore influences wear performance. Smoother finishes generally provide lower friction and improved wear resistance.
Adhesion of coatings - Surface roughness affects the adhesion and durability of coatings or paints. Although stainless steel rarely requires coating, it is worth noting that it has strong resistance to filiform corrosion, which can cause coatings to peel from other base materials.

2. Finish Type

Different processing routes can dramatically alter the appearance of stainless steel surfaces.

Mechanical Finishes

Mechanical processes such as grinding, brushing, and polishing use wet or dry abrasives, typically in belt or stone form, applied at relatively high speeds. These processes create directional grain marks or a smooth directional sheen.

Surface quality depends on abrasive type, grit size, application speed, and applied pressure. Many of these operations are manual, which makes consistency difficult to control. Nevertheless, reasonably high levels of smoothness can be achieved.

Directional finishes, such as brushed No.4, produce a visible linear grain that affects perceived brightness and texture. Grain direction strongly influences reflectivity and shading. Highly polished or Bright Annealed (BA) finishes have little or no visible directionality.
Bead blasting uses small glass beads projected at high velocity onto the surface. This produces a uniform matte, non-reflective finish. While visually consistent, bead blasted surfaces have relatively high surface roughness, which can create sites where moisture and contaminants
accumulate, potentially initiating corrosion under certain conditions.

Chemical Treatments

Chemical methods are primarily used to remove heat tint caused by welding and other fabrication processes. Heat discoloration often indicates chromium depletion in the near-surface layers. Chromium is essential for corrosion resistance, and heat-tinted material must be removed to
expose sound stainless steel. While mechanical removal is possible, chemical treatments are generally more effective. The process typically involves two steps: pickling and passivation. Passivation is not always required, as the passive layer reforms naturally in a clean environment.

Pickling removes heat tint and scale using acid, leaving a clean surface free from iron contamination and chromium-depleted material. The resulting appearance is generally uniform and dull.
Passivation is also acid-based and promotes the formation of a stable passive layer. It helps restore brightness and stabilise the surface appearance. Uniform passivation supports consistent sheen, while contamination or disruption of the passive film can result in discoloration and uneven appearance.

Electropolishing

Electropolishing is an electrochemical process that produces a bright, smooth, highly reflective surface. The smoothness achieved through electropolishing is unmatched and significantly enhances corrosion resistance and cleanability.

3. Alloy Composition

The stainless steel family includes more than 220 grades with variations in chemical composition. Individual grades influence colour tone, the ability to form a stable passive film, and the response to polishing.

Higher chromium content generally enhances passivity and contributes to uniform appearance. In austenitic grades, nickel influences colour tone and lustre.

Although stainless steels are typically metallic grey, subtle colour differences can arise from variations in oxidation behaviour, alloying elements, and surface finish.

4. Surface Cleanliness and Contamination

Fingerprints, oil residues, embedded iron particles, and fabrication marks can significantly alter appearance. Cleanliness directly affects perceived colour uniformity.

Contaminant build-up, particularly on rougher or partially corroded surfaces, can change appearance over time. Manufacturing steps may leave visible traces such as heat tint and scale from welding. In addition, grain direction can change in bent areas, distorting light reflection and affecting visual consistency.

5. Lighting Conditions and Viewing Angle

Since stainless steel reflects light, its appearance varies depending on illumination type and angle. Viewing angle also affects perception. This is especially critical in architectural applications and is discussed further below.

6. Environmental Effects Over Time

Even corrosion-resistant stainless steel can change in appearance due to atmospheric contaminants, chloride exposure (which may cause tea staining in coastal environments), and natural surface oxidation. Different grades and finishes respond differently to these environmental influences.

Practical Implications for Architecture

The visual appearance of stainless steel in buildings is not purely aesthetic. It directly affects performance, maintenance, user experience, and long-term value. The governing factors of appearance namely roughness, finish, reflectivity, directionality, and passivation; carry important consequences in real-world architectural applications.

1. Reflectivity, Gloss and Light Interaction

Surface texture determines how a facade reflects light and therefore how it appears throughout the day. Smooth finishes reflect more light and create a polished or mirrorlike appearance, while rougher textures scatter light and produce a softer matte effect.