Maintenance

Pitting Corrosion: An In-Depth Analysis

What is Pitting Corrosion?

Pitting corrosion is a specific type of corrosion that creates cavities or “holes” within a material. It is more perilous than uniform corrosion because it’s challenging to detect, predict, and safeguard against. Corrosion products frequently conceal these pits, and even a small, narrow pit with minimal metal loss can trigger failure in an entire engineering system. Pitting corrosion, a common factor in various localised corrosion attacks, can manifest in diverse shapes. These pits may appear open or covered with a semi-permeable membrane of corrosion products, assuming hemispherical or cup-shaped forms.

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Pitting Factor

This factor is a measure used to quantify the severity of pitting corrosion. It’s calculated by comparing the depth of the deepest pit caused by corrosion to the average penetration of the metal surface, which is determined by measuring the weight loss of the metal due to corrosion.

Pitting Resistance Equivalent Number (PREN)

PREN is an empirical formula used to predict the resistance of specific types of stainless steels—namely austenitic and duplex stainless steels—against pitting corrosion. The formula, expressed as PREN = Cr + 3.3(Mo + 0.5W) + 16N, combines the concentrations of various elements in the steel (chromium, molybdenum, tungsten, and nitrogen). The result of this formula provides an estimate of the steel’s ability to resist pitting corrosion. Higher PREN values generally indicate better resistance to pitting.

Types of Pitting Corrosion

Trough Pitting Corrosion

Trough pitting corrosion manifests in hemispherical, cup-shaped, or irregularly formed pits. It arises when the protective layer, known as the passive film, is compromised, allowing the metal wall to be eroded, resulting in the formation of narrow and deep troughs. The exposed flat walls reveal the crystal structure of the metal. Such corrosion can rapidly penetrate through the material’s thickness, impacting components like roof sheets, trusses, or gutter systems.

Sideway Pitting Corrosion

Sideway pitting corrosion involves the formation of pits covered by a semi-permeable rust layer, characterised by horizontal grain attack, undercutting, and subsurface shapes. These pits can swiftly penetrate the metal, proving challenging to detect as the metal surface seems unaffected and devoid of visible corrosion. Even with only scattered spots of rust, the damage might misleadingly appear minor.

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Causes of Pitting Corrosion

a. The breakdown of a protective oxide film due to localised chemical or mechanical damage can be influenced by various water chemistry factors. These factors include acidity, low levels of dissolved oxygen (which reduce the stability of the protective oxide film), and high chloride concentrations, such as in seawater.

b. Localised damage to a protective coating or its improper application can also lead to corrosion.

c. The presence of non-uniformities in the metal structure of a component, such as non-metallic inclusions, can contribute to corrosion initiation.

Physical Damage

Structural integrity suffers when protective layers are breached, enabling corrosive agents to harm materials. Even minor flaws like cracks, scratches, or chips in coatings can lead to pitting corrosion. This corrosion, initially subtle, gradually expands and weakens the structure. Thus, damage need not be overt to be detrimental.

Irregular Pressure or Variations in Fluid Movement

The disproportionate strain on protective coatings intensifies vulnerability in specific regions, accelerating their erosion and exposing underlying surfaces.

Unexpected vibrations or movements cause uneven stress, hastening wear in select areas, potentially escaping detection if other sites remain unaffected. Likewise, erratic fluid flow accelerates corrosion of protective coatings, particularly around pipelines, when the fluid’s nature or pipeline design isn’t suitably matched, deteriorating them at a faster rate than steady, compatible flows.

Impure Materials

Inferior or flawed metals are highly susceptible to pitting corrosion. Using high-quality metals ensures durability against corrosive elements. Rough surfaces from manufacturing or quality issues also promote pitting corrosion, especially in areas where coatings are unevenly applied.

Unequal or Irregular Protective Layer

Unevenly applied protective coatings are more prone to pitting corrosion. Areas with varying thicknesses or transitions in the coating collect fluids, breaking down protective compounds. A consistently thin, even layer generally offers superior protection compared to an uneven coating.

Chemical Exposure

Highly corrosive and ionised chemicals swiftly degrade solid protective coatings. Applying the suitable layer effectively counters anticipated chemical exposure. While unexpected, limiting exposure to corrosive elements prolongs the protective coating’s lifespan.

Other Factors Leading to Pitting Corrosion

Imperfections in protective coatings, such as cracks.
Surface damage like scratches, scuffs, and minor chips.
Uneven stress distribution.
Flaws in the metal substrate.
Turbulent fluid flow.
Irregular application of protective coatings.
Chemical degradation of protective coatings.

Pitting Corrosion Mechanism

Pitting corrosion, an electrochemical redox process, occurs within localised holes (cells) on metal surfaces coated with a passive film. It’s considerably more perilous than uniform corrosion due to its 10-100 times faster rate.

This type of corrosion accelerates significantly in the presence of chloride, sulphate, or bromide ions in the electrolyte solution. Metals like stainless steel, aluminium alloys, copper alloys, and chromium, which form passive oxide layers on their surfaces, are particularly vulnerable to pitting corrosion in both electrolytes and atmospheric conditions.

The mechanism of pitting corrosion resembles that of Crevice corrosion. It involves the dissolution of the passivating film and gradual acidification of the electrolyte due to inadequate aeration, limiting oxygen penetration.

Pitting corrosion progresses through several stages:

Pit Initiation

Initial pits may form due to mechanical damage to the passive film, such as scratches, causing anodic reactions on exposed metal surfaces while the surrounding passivated area acts as the cathode.

The presence of second-phase particles (non-metallic inclusions, intermetallic inclusions, metallic particles, or microsegregation) along grain boundaries serves as local anodes, leading to localised galvanic corrosion and pit formation.

Localised stresses in the form of surface dislocations may become anodes and trigger pit initiation.

Non-uniform environments dissolve the passive film at certain spots, leading to the formation of initial pits.

Pit Growth

Pits grow due to an autocatalytic mechanism in the presence of chloride ions, illustrating pitting corrosion in stainless steel.

Anodic reactions occur within the pit, leading to the dissolution of iron (Fe = Fe2+ + 2e-). Electrons released by the anode move to the cathode (passivated surface), where they are involved in the cathodic reaction (1/2O2 + H2O + 2e- = 2(OH-)).

As a result of these reactions, the electrolyte enclosed in the pit gains a positive electrical charge, attracting negative chlorine ions (Cl-), thereby increasing the acidity of the electrolyte according to the reaction (FeCl2 + 2H2O = Fe(OH)2 + 2HCl).

This decrease in pH inside the pit (increase in acidity from 6 to 2-3) accelerates the corrosion process further. A larger ratio between the anode and cathode areas increases the corrosion rate. Corrosion products (Fe(OH)3) form around the pit, leading to further isolation of its electrolyte.

How to Treat Pitting Corrosion

It is best to use recommended cleaning procedures to expose the pits fully and remove the corrosion products. Avoid using solutions that attack the base metal excessively. It is advisable to periodically probe the pits with a pointed tool during cleaning. This will enable you to determine the extent of any undercutting or subsurface corrosion. Vigorously scrubbing the pit with a wire brush will enlarge the openings sufficiently. Removing the corrosion products and undercut metal will help you to evaluate the extent of damage.

When the metal material is clean and free from debris, carry out a final inspection of the damage. Proceed with applying a base primer coating followed by one or two more top layers. If the damage is severe and the component is too weak, it must be replaced.

Zinc Phosphate

Zinc phosphate priming is one of the most popular coating methods to protect against pitting corrosion. Specially formulated primers such as zinc phosphate improve corrosion resistance.

Zinc Spray

Zinc spray metallising is a technique that is very effective against corrosion. It has a smooth finish, which is aesthetically appealing and popular. It doesn’t provide the same protection as hot dip galvanising, but because this is a cold process, there is no risk of distorting the metal. Zinc spray metallising is ideal for ornate metal items such as art exhibits, metal railings and fences.

Chemical Coating

Chemical coating uses electrostatic or compressed air to apply a specially formulated powdered material to a steel surface. It is then melted to form a smooth protective film. Steel treated in this way is not only protected against corrosion and UV damage, but it is also highly resistant to peeling, scratching, and cracking.

Hot Dipping

Hot dipping is a galvanisation method of coating that can be used on steel of all shapes and sizes. It involves immersing the steel in a bath of molten zinc at temperatures up to 450°C. Steel galvanised in this way is a particularly popular approach for pipe-related applications because it is highly protected against corrosion as well as extreme weather conditions.

Preventing Protecting Against Pitting Corrosion

Environmental Factors

Minimising the impact of environmental factors known to induce pitting corrosion is crucial. Control and reduction of humidity, temperature, chloride, pH acids, and salt levels should be prioritised whenever feasible.

Cathodic Protection

Implementing cathodic protection involves applying a reactive metal coating to susceptible metal, typically in the form of galvanised zinc or a similar material. This reactive metal serves as the sacrificial anode, corroding in place of the primary metal to prevent pitting in the substrate.

Corrosion-Resistant Metals

For defence against chemical attack, utilising corrosion-resistant metals such as titanium, nitrogen, chromium, and molybdenum-containing alloys is highly effective, especially in environments with elevated chloride ion levels.

Strategies to Control Pitting Corrosion Include

Application of coatings to shield metal surfaces from pitting
Utilisation of more corrosion-resistant materials
Regularly washing away fluids in contact with the material
Using cathodic protection techniques
Preventing stagnant zones
Implementing inhibitor use/fluid chemistry control
Maintaining the protective film of the material

ORAPI RECOMMENDS:

PROTOR BITUM

Medium Term Corrosion Protection

Anti-corrosion and sealing product providing a dry and flexible film for long-lasting protection

ORAPI RECOMMENDS:

IMMERKLEEN

Immersion Scale & Corrosion Remover

IMMERKLEEN is a concentrated acidic cleaner designed for the effective removal of rust from iron and steel and conditioning scale for subsequent removal treatment.

Conclusion: Pitting Corrosion

Pitting corrosion, a stealthy yet devastating form of material decay, poses a grave threat due to its hidden nature and profound impact on structural integrity. This type of corrosion, forming discrete holes within materials, presents significant challenges in detection and prevention. The article delves into its complexities, highlighting causes such as protective film breakdown and stress irregularities, and stresses the importance of appropriate coatings, material selection, and proactive maintenance. It underscores the significance of treating affected surfaces and employing preventive strategies like cathodic protection and corrosion-resistant metals. A holistic approach involving material science, maintenance, and timely intervention is crucial to combat this pervasive issue of pitting corrosion.

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Pitting Corrosion: An In-Depth Analysis