NHML Resources - Cutting Through Metal Corrosion: Tips for Addressing Rust, Pitting and other forms of Metal Corrosion

The problem of metal corrosion is familiar to all of us who live in the Northeast. After a winter of snow and salt, who hasn't noticed a spot of rust starting or, worse yet, spreading? Corrosion becomes an even bigger problem for product manufacturers where product quality or production tools may be affected.

Below we've outlined some common types of metal corrosion, a little about their cause, and some tips for addressing them. These tips may help you understand and solve your corrosion problems.

Rust is the most common form of uniform corrosion and is routinely found on iron or steel. However, many metals (aluminum, copper, silver and brass included) may develop a protective "film" on their surfaces.

To solve the problem, try protecting the metal with a coating. Paints, oils and other types of coatings often work well. Exchange the material to one better suited for the application. Indeed, manufacturers are finding plastics increasingly valuable in places where metal was once the standard. Where there is no quality or safety concerns consider letting the material corrode.

The corrosion found on car battery terminals is a good example of galvanic corrosion. It is one of the most challenging types of corrosion to identify. It appears as corrosion on an "easily corroded" metal, like aluminum, galvanized steel, or low carbon steels.

This corrosion occurs when two conditions are met: (1) dissimilar metals must be in direct contact, or in indirect contact via a fluid path; and (2) an electrical potential must be present.

The amount of corrosion depends upon the distance between the two metals in the galvanic series and how easily the electrical current can flow between them. (The galvanic series is a list of metals and alloys that are arranged by their relative corrosive tendency.)

To minimize galvanic corrosion try to select metals close in the galvanic series (they are less aggressive to each other); use metals that are more resistant to corrosion like passivated 316 stainless steel, silver, graphite or titanium. When cost is a consideration, try to isolate dissimilar metals with plastics, greases, paint, or similar coatings. Try to increase the electrical resistance of the fluid. A chemical inhibitor may help.

Crevice corrosion results from a metal being exposed to a concentrated corrosive environment. Many weld joints suffer from this type of corrosion. Moisture and debris get trapped in the crevices and corrode the material.

Use butt weld joints in place of rivets or bolts. Seal lap joints with solder or caulk to fill-in the crevice. Be sure fluid vessels drain completely and are designed to eliminate corners and stagnant areas. Finally, use non-absorbent gasket materials (plastics or solid rubber) to keep debris and moisture out.

Like crevice corrosion, pitting is a localized corrosive attack. It makes the surface of the damaged metal look rough. Stainless steels and aluminum are highly susceptible to this form of corrosion.

The presence of chloride ions makes the problem worse. This is especially true for stainless steel. Look for signs of pitting in stagnant water. Follow the recommendations offered with crevice corrosion. These should help to reduce the problem.

Stress corrosion cracking is unique because the metal shows no visible corrosion except for a crack; it is often mistaken for a fatigue fracture. This failure is the result of corrosion combined with stress (either externally applied or residual stresses).

The microscopic corrosion weakens the surface. The stress causes it to crack. This crack will grow and may eventually cause the part to fracture. Or, it may stop growing once the stress is relieved.

Often stresses are introduced to the metal through welding or cold work. Try to eliminate activities which cause tensile stress like bending, crimping or shrink fitting. A thermal treatment may help to reduce these residual stresses.

See our Industry Definitions for further insight.

 

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