– Part 3 – Uniform, Galvanic, and De-alloying Corrosion
General attack, galvanic corrosion, and de-alloying (Fig. 1) are among the most common types of corrosion encountered in materials. General attack refers to the uniform deterioration of a material, often seen as surface tarnishing, roughening or rusting, typically due to chemical reactions with the environment. Galvanic corrosion occurs when two different metals are in electrical contact in the presence of an electrolyte, causing one metal to corrode faster than the other. De-alloying happens when one metal component of an alloy is preferentially corroded, leaving behind a weakened structure. These corrosion types can significantly affect the durability and integrity of materials in various applications.
General Attack or Uniform Corrosion
Fig. 1: The types of corrosion considered in this episode General or uniform corrosion attack occurs at a consistent rate across the entire exposed metal surface to the environment. In general attack, anodes and cathodes are not distinct, while in localized corrosion, they are separated, with the anode area being much smaller than the cathode area. This makes localized corrosion typically more severe than general corrosion. The rate of uniform corrosion is typically measured in IPY (inches of penetration per year) or MDD (milligrams per square decimetre per day), especially when dealing with chemical media. For instance, when zinc or steel are submerged in diluted sulfuric acid, they typically dissolve uniformly across their surface at a steady rate. Similarly, cast irons and steels commonly undergo uniform corrosion when exposed to open air, soil, and natural waters, leading to a rusted appearance.
Typically, this form of corrosion has a minimal impact on the material's performance [1-5]. The degree of corrosion is relatively predictable, can be accounted for in design calculations such as increasing the wall thickness. General corrosion often results from a poor material choice in a given corrosive medium. Strategies to mitigate general corrosion include: selecting appropriate materials, applying metallic and organic protective coatings, utilizing corrosion inhibitors, and implementing cathodic protection [6, 7].
Galvanic or Bimetallic Corrosion
Galvanic corrosion, also referred to as bimetallic corrosion or dissimilar metal corrosion. It occurs when dissimilar metallic materials are brought into contact in a common electrolyte. In this scenario, one metal – the more active one, known as the anode – corrodes preferentially, while the more noble metal, the cathode, corrodes to a lesser extent or at a later stage [6, 8]. Galvanic reactions are utilized in primary cells to generate a useful electrical voltage for powering portable devices, while galvanic corrosion is a detrimental effect that requires mitigation. Cathodic protection is commonly applied to buried or submerged structures and hot water storage tanks to prevent galvanic corrosion. The appropriate metal(s) for a particular application can be determined by referring to the galvanic series (Fig. 2). This series, also known as the electrochemical series, ranks metals according to their nobility. Noble metals are characterized by their resistance to corrosion and oxidation.
Fig. 2: Galvanic series of some metals and alloys
Fig. 3: Schematic representation of the galvanic corrosion Figure 3 depicts a schematic of galvanic corrosion in galvanized iron, where a zinc coating protects the underlying steel [9]. Even if the zinc layer is damaged, it corrodes first due to being less noble, keeping the steel safe until the zinc is entirely gone. In contrast, with a traditional tin can, the more noble tin allows the steel to corrode immediately once the tin coating is compromised.
The widely used method for cleaning tarnished silverware, or silver-plated items involves wrapping them with aluminium foil and immersing in a hot solution of baking soda is an instance of galvanic corrosion. Silver tarnishes and corrodes when exposed to sulphur compounds in the air. This corrosion layer can be effectively removed by the electrochemical reduction of silver sulphide molecules: aluminium’s presence in the sodium bicarbonate solution pulls the sulphur atoms from the silver sulphide, causing them to bond with and corrode the aluminium foil (a more active metal than silver or copper), thus leaving pure silver behind. The process does not result in any loss of silver.
There are various methods to mitigate galvanic corrosion [8]:
Cathodic protection: Sacrificial anodes, made from Zn, Mg, or Al, are commonly used to prevent galvanic corrosion. The white patches seen on a ship's hull indicate zinc block anodes (Fig. 4). Galvanic corrosion depends on the relative sizes of the anodes and cathodes, the types of metals, and environmental factors like temperature, humidity, and salinity. The effectiveness of the anode in managing the cathode's corrosion rate relies on the surface area ratio between them. Additionally, connecting a direct current power source can help neutralize the corrosive effects of galvanic current.
Fig. 4: Ship equipped with zinc block anodes on its hull
To electrically insulate two metals from each other, use non-conductive materials to separate metals with different electro potentials. Pipes can be isolated using a spool made from plastic.
- Choosing metals with similar electrochemical potentials minimizes potential differences and reduces galvanic current. While using the same metal for all components is the easiest solution, it may not always be practical.
To prevent contact with electrolytes, use water-resistant greases or non-permeable coatings like paint, varnish, or plastic on metals.
- Plating with noble metals like nickel, chromium, silver, and gold offers excellent corrosion resistance. Galvanizing steel with zinc protects the base metal through sacrificial anodic action.
- Applying antioxidant paste helps prevent corrosion in electrical connections between copper and aluminium. The paste is composed of a metal with lower nobility than both aluminium and copper.
Forms of Galvanic Corrosion
Galvanic corrosion can manifest in various forms. The primary forms of galvanic corrosion include:
Contact corrosion
The most familiar contact corrosion occurs at the points of contact between the different metals when they are directly coupled and exposed to an electrolytic medium like moisture or saline solutions.
This corrosion results from the distinct electrochemical characteristics of the metals involved [10, 11] as illustrated in Figure 3, the more active metal corrodes on the expense of less active metal. Affected areas are susceptible to damage through pitting, cracking, or other types of corrosion. The careful selection of materials, appropriate insulation of metals from one another, or the application of barrier coatings can minimize contact corrosion.
Deposition Corrosion
Deposition corrosion occurs from a displacement reaction when dissolved corrosion products deposit on a less noble metal in the solution. The process leads to the formation of intense local bimetallic corrosion cells on the surface. This phenomenon is often seen in circulating water systems containing copper, steel, galvanized steel, or aluminium. Even when these dissimilar metals are electrically isolated with flanges, galvanic corrosion is inevitable. Copper ions are prone to deposit on galvanized steel or aluminium, causing severe localized pitting on stainless steel or aluminium [10].
The process of deposition corrosion can be demonstrated by partially immersing an aluminium strip into a copper sulphate solution. Over weeks of exposure, metallic red-brown copper deposits on the aluminium, will result in considerable corrosion of the strip. Deposition corrosion can be often observed in locations where de-icing salt, such as sodium chloride is applied to roads and highways. Chloride-rich droplets or aerosols settle on copper surfaces. When water containing copper runoff comes in contact with more active metal parts such as aluminium or iron, metallic copper may deposit over them, leading to the dissolution (corrosion) of more active metal by a simple displacement reaction:
2 Al(s) + 3 Cu2+(aq.) → 2 Al3+(aq.) + 3 Cu(s)↓
Fe(s) + Cu2+(aq.) → Fe2+(aq.) + Cu(s)↓
Deposition corrosion can be mitigated by ensuring cleanliness, applying protective coatings, and using sealants. A thick, flawless anodized layer on aluminium can prevent this process. Stainless steel types like 316 or 316L, which contain molybdenum, offer improved resistance to deposition corrosion [12].
Thermogalvanic corrosion
Thermogalvanic corrosion occurs when a metal experiences a thermal gradient due to uneven heating or heat dissipation. This process similarly affects the metal as normal bimetallic galvanic corrosion does. It causes differential polarization of the metal, leading to the formation of anodic and cathodic regions 
Fig. 5: Illustration of thermogalvanic corrosion [10]. The higher temperature region acts as the anode, and the cooler as the cathode. The anodic region undergoes thermogalvanic corrosion (Fig. 5) [13]. Thermogalvanic corrosion is commonly encountered in heat exchangers due to temperature differences [14].
This form of corrosion can be mitigated through designing the component with minimal thermal gradient, introducing a coolant within the component to reduce temperature differences, checking insulation, and applying sacrificial coatings composed of steel alloys and iron. The latter approach is particularly preferred for bridges in coastal areas and wind turbines due to its extended protection compared to paints. The sacrificial coatings gradually corrode while simultaneously minimizing steel corrosion [13, 14].
De-alloying or Selective Leaching
Fig. 6: Illustration of selective leaching De-alloying, or parting corrosion is the selective leaching of active element from a solid alloy by corrosion processes. In many instances, corrosion may not be apparent to the naked eye, although perforation or fracture may occur due to the reduced strength. Figure 6 illustrates the process of selective leaching [15].
The most common example of selective leaching is the selective removal of zinc in brass alloys (dezincification). Copper-zinc alloys containing more than 15% zinc known as 'red brass’ are susceptible to dezincification. When brass comes into contact with mildly acidic, alkaline, or saline water, zinc preferentially dissolves from the brass. This is because zinc possesses weaker atomic bonds compared to copper, and additionally, copper is more noble than zinc, which renders it more resistant to corrosion.
Dezincification occurs in two forms: layer type and plug type [16, 17]. A microscopic image of dezincification is shown in Figure1.
Layer type: Progresses slowly and affects a wide area under mild conditions, such as in stagnant or slow-moving, slightly acidic water with high oxygen and carbon dioxide levels but low salt content. It uniformly reduces the wall thickness of valves and fittings.
Plug type: Progresses rapidly and is localized. It occurs in water with high chloride content, neutral or alkaline pH, and high salinity, often at above room temperature. It penetrates deeply into the sidewalls of valves and fittings.
Mild, uniform layer-type dezincification may cause only a cosmetic change, altering the surface colour from yellow to pink. However, severe localized plug-type dezincification can weaken the brass, potentially leading to perforation with conical pits filled with porous pure copper. Two mechanisms are proposed regarding the dezincification process: one involves the dissolution of zinc, leaving behind copper, which then reorganizes on the surface to form copper crystals. The other theory suggests that the entire alloy dissolves, after which zinc repels the Cu2+ ions from the solution, leading to their precipitation as porous copper [15, 16, 18].
Another example of selective leaching is graphitization, also known as spongiosis of cast iron. This process involves the dissolution of iron while the graphite remains, rendering the cast iron as soft as butter. Similar processes take place in various alloy systems where elements like aluminium, iron, cobalt, chromium, and others are removed. The general term for such processes is 'de-alloying', while specific instances include deironification or graphitization, dezincification, denickelification, decobaltification, dealuminization, and so on.
To prevent de-alloying, it is essential to select the appropriate alloy and avoid conditions that encourage corrosion. For example, copper alloys containing less than 15% zinc and alpha brasses treated with arsenic or antimony are resistant to dezincification in aquatic or terrestrial environments. Brass should be kept clean and dust free. Apply cathodic protection, and ensure effective conditioning (pH, chloride) of environment. Corrosion inhibitors, especially film-forming types like tolyltriazole, are notably effective in mitigating brass corrosion [10]. Some of the commercial polishes contain acids or alkaline solutions to speed up cleaning and remove dirt and grease, both can cause dezincification. An abrasive slurry could be prepared instead, based on precipitated calcium carbonate or other harder abrasives [19].
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