Pitting Corrosion

Pitting corrosion affects metals and alloys such as steel, iron, aluminum, and others. and is usually confined to specific areas.

It penetrates and attacks quickly, but is difficult to detect. It most commonly occurs where the passive coating is physically damaged or chemically attacked. This creates a weak point where water or caustic solutions attack the substrate.

Pitting Corrosion Explained

Pitting corrosion is a cavity, hole, or pit that forms in a small area or spot. The pits or holes are concealed by a small amount of corrosion products (rust) on the surface. When a cathodic reaction in a large area (coating) sustains an anodic reaction in a small area (exposed metal), a pit, cavity, or small hole is formed. Oxidation occurs in metal even without the presence of oxygen.

The large cathode's high electron demand is transferred to the small anode, resulting in intense pitting corrosion. This corrosion is gradual, occurs quickly, and has very damaging effects. Only a small patch of rust is visible on the surface, while the damage develops deep in the underlying metal structure.

What causes pitting corrosion?

Pitting corrosion occurs when the cathode (damaged coating) is large and the anode (exposed metal) is small. Typically, the surface protective layer or film becomes the cathode when it becomes damaged and cracked. A small area of metal is exposed and becomes the anode.

Pitting corrosion is particularly severe when the solution on the metal surface contains chloride, hypochlorite, or bromide ions. Other harmful solutions include fluoride and iodide solutions. Sulfides and water also promote pitting corrosion.

Most common causes of pitting corrosion are.. Cracks in protective coating Scratches, scuffs & small chips Non-uniform stress Defective metal substrate Turbulent fluid flow Non-uniform protective coating Chemical attack on protective coating

Metals prone to pitting corrosion are.. Stainless steel Chromium Passive iron Mercury Cobalt Aluminum Copper Associated alloys

Another example of pitting corrosion occurs when a metal is poorly maintained and exposed to water droplets and dust particles. The area beneath the droplet lacks sufficient oxygen, while the surrounding areas are well supplied with oxygen. This results in differential ventilation corrosion, in which the surrounding areas are cathodic and the small area beneath the droplets and dust particles becomes anodic. Electrons flow through the metal and encounter water and oxygen. Ions are formed, which discharge together to form rust. The formation of rust creates holes, cracks, and crevices in the metal.

Types of Pitting Corrosion

Pitting corrosion can occur in various forms. The shape of the pits depends largely on the affected material and the grain direction. Passive metals and alloys, including stainless steel and aluminum, are most commonly affected. However, almost any metal or material susceptible to corrosion can be affected.

What is a Pitting Resistance Equivalent Number?

How is pitting corrosion tested?

The first and most basic method is a visual inspection of the corroded metal surface. Count the number of pits through a microscope eyepiece on a defined surface to determine their approximate size and distribution. The number of pits is not necessarily the most important factor. The pit depth is by far the most dangerous. A narrow, deep pit can be more dangerous than many shallow pits. A metallographic cross-section to measure the pit depth provides information about the intensity of the attack.

Other options are.. Sonics testing Electromagnetic testing Electrochemical testing Immersion testing Weight loss testing

How do you treat pitting corrosion?

It is recommended to use specific cleaning methods to completely expose the holes and remove the corrosion products. Avoid solutions that excessively attack the base metal. It is recommended to regularly inspect the holes with a pointed tool during cleaning. This allows you to determine the extent of undermining or subsurface corrosion. Vigorously scrubbing the holes with a wire brush will sufficiently enlarge the openings. Removing the corrosion products and the undermining metal helps to assess the extent of the damage.

Once the metal is clean and free of deposits, perform a final damage inspection. If you're confident the corrosion was detected early, you can proceed with applying a primer, followed by one or two more topcoats. If the damage is severe and the component is too weak, it will need to be replaced.

Popular coating methods are.. Zinc phosphate priming Zinc spray metallising Chemical coating Hot dip galvanising

How can pitting corrosion be prevented?

Environmental factors that cause pitting corrosion should be minimized wherever possible. Humidity, temperature, chloride and pH levels, acidity, and salt content should be controlled and minimized.

For cathodic protection, vulnerable metal should be coated with a reactive, corrosive metal. This can be achieved, for example, by electroplating zinc or similar. The reactive metal acts as an anode and corrodes first, preventing pitting in the substrate.

For protection against chemical attack, corrosion-resistant metals such as alloys with titanium, nitrogen, chromium, and molybdenum are very effective in environments with high chloride ions content.

Reference(s)..
www.nickel-systems.com
www.heartlandrepaircoatings.com
hintegratedglobal.com