Corrosion looks different depending on the metal involved, but the most familiar form is the flaky, orange-brown rust that appears on iron and steel. On other metals, corrosion can show up as white powder, green patina, black film, or tiny pinholes that are nearly invisible to the naked eye. Knowing what to look for on each type of metal helps you catch damage early, before it compromises the strength or function of the material.
Iron and Steel: Red-Orange Rust
Iron and steel produce the corrosion most people recognize instantly. It starts as small reddish-brown spots or streaks on the surface, often where water has pooled or where the metal stays damp. Over time, those spots spread into a rough, flaky layer that ranges from bright orange to deep reddish-brown. The texture becomes uneven and gritty. If you scrape it, the rust crumbles away, revealing more damaged metal underneath.
On structural steel or tools, you might also notice that the surface feels thinner in some spots. Rust doesn’t just sit on top of the metal. It actively eats into it, so heavily rusted areas can develop visible depressions or weak points. Paint blistering or bubbling on a steel surface is another early sign, since rust often forms underneath coatings before it becomes visible on the outside.
Aluminum: White Powder and Pitting
Aluminum doesn’t turn red or brown. Instead, its corrosion appears as a white, chalky powder or a dull, hazy film on the surface. This white layer is aluminum oxide, and in many cases it actually protects the metal underneath by forming a barrier against further damage. You’ll commonly see it on aluminum window frames, outdoor furniture, and engine parts.
When aluminum corrosion goes beyond that protective layer, it creates pitting: small, shallow craters scattered across the surface. These pits can be hard to spot at first because they’re tiny and sometimes hidden under the white powder. On bare aluminum, the corroded area looks rough and matte compared to the surrounding metal’s smoother finish.
Copper and Bronze: Green Patina
Copper starts out shiny and pinkish-orange, then gradually darkens to a pale brown, then a deeper brown-black as a thin oxide layer forms. Over months or years of outdoor exposure, it develops the famous blue-green patina seen on old rooftops and statues like the Statue of Liberty. This green layer forms when copper reacts with moisture, carbon dioxide, and sulfur compounds in the air.
A uniform, stable patina is generally considered protective and even desirable. The problem comes when corrosion is uneven or aggressive. Bronze objects exposed to salt or chloride-rich environments can develop what conservators call “malignant patina,” a rough, powdery, brownish-green or greenish-blue crust that actively damages the metal beneath it. Unlike stable patina, which is smooth and firmly attached, malignant corrosion looks bumpy and irregular, sometimes with dark brown patches mixed into green areas. If you see bright green, powdery spots on a bronze or copper object, that’s a sign of active, ongoing corrosion rather than harmless aging.
Silver: Yellow to Black Tarnish
Silver tarnish follows a predictable color progression. It begins as a faint yellow or golden discoloration, then shifts through reddish and bluish tones, and finally settles into the familiar dark gray or black film. The color depends entirely on thickness: a very thin tarnish layer looks yellow, while a thicker one appears black. The black layer is silver sulfide, formed when silver reacts with trace sulfur compounds in the air.
Tarnish on silver is often uneven, concentrated in crevices, engravings, or areas that don’t get touched or polished regularly. An object with patchy, uneven tarnish can look more disfigured than one with a thick but uniform dark layer. In rare cases, silver can also develop tiny three-dimensional whisker-like growths on its surface from prolonged sulfur exposure.
Stainless Steel: Tea Staining and Spots
Stainless steel resists corrosion far better than regular steel, but it’s not immune. The most common visible sign is called tea staining: a light brown or rust-colored discoloration that streaks across the surface. It looks exactly like a dried tea drip mark. Tea staining is primarily cosmetic and doesn’t necessarily mean the metal is structurally compromised, but it signals that the protective surface layer has been disrupted by salt air, poor cleaning, or an unsuitable grade of steel for that environment.
More serious corrosion on stainless steel shows up as small rust-colored spots or pits, often near welds, scratches, or areas where food, chemicals, or saltwater have sat on the surface for extended periods.
Pitting: The Hardest Corrosion to Spot
Pitting corrosion deserves special attention because it’s one of the most dangerous forms and one of the hardest to see. Instead of spreading visibly across a surface, pitting creates small holes that bore deep into the metal. The pits can be tiny, sometimes smaller than a pinhead, while extending far deeper than their width suggests. A surface can look nearly intact while being riddled with deep pits underneath.
What makes pitting especially tricky is that the holes are often covered by a cap of corrosion products. You might see a small raised bump or discolored spot on the surface with a pit hidden beneath it. Pitting doesn’t follow a regular pattern. One area of a metal sheet can be heavily pitted while an adjacent area remains completely clean. On steel, you’ll sometimes see small rings of rust particles accumulated around each pit opening, which is one of the more reliable visual clues.
Galvanic Corrosion at Metal Junctions
When two different metals are joined together and exposed to moisture, the more reactive metal corrodes faster than it normally would. The visual signs concentrate right at the junction where the two metals meet. You’ll see discoloration, roughening, or a buildup of corrosion products clustered around the contact point, while the metal farther from the junction may look perfectly fine.
In mild cases, the damage appears as surface fretting, a dulling or slight roughening of the metal near the joint. In more advanced cases, visible pitting and material loss develop around the connection. This is common where steel fasteners meet aluminum panels, or where copper pipes connect to steel fittings. The telltale sign is corrosion that’s clearly worse right at the boundary between the two metals and fades with distance.
Hidden Corrosion in Concrete
Some of the most consequential corrosion is invisible at first because it happens inside concrete, where steel reinforcing bars (rebar) are embedded. As the rebar rusts, it expands, and the signs eventually show up on the concrete surface. The earliest visible clue is rust-colored staining: brown or orange streaks bleeding through the concrete, often following the lines of the rebar beneath.
As corrosion progresses, the expanding rust cracks the concrete from the inside out. You’ll see hairline cracks first, then wider splits, and eventually spalling, where chunks of concrete break away and fall off, exposing the corroded rebar underneath. Persistent dampness or water seepage around a concrete structure is both a cause and a warning sign. If you notice brown staining plus cracking on a concrete wall, column, or foundation, internal rebar corrosion is the likely culprit.
Quick Visual Guide by Metal
- Iron and steel: Orange-brown flaky rust, blistering paint, surface thinning
- Aluminum: White chalky powder, dull haze, small surface pits
- Copper and bronze: Brown-to-black darkening, then blue-green patina; powdery green spots signal active damage
- Silver: Yellow, then reddish-blue, then black film, concentrated in crevices
- Stainless steel: Light brown tea-stain streaks, small rust spots near scratches or welds
- Concrete structures: Rust stains bleeding through, cracking, and chunks of concrete flaking off