Galvanic Corrosion

Delve into the critical world of Galvanic Corrosion, a predominant concern in the field of Materials Engineering. This comprehensive guide provides a deep dive into understanding Galvanic Corrosion, its implications, and its differentiation from electrolysis. You will discover how a Galvanic Corrosion Chart aids in quantifying the severity of this phenomenon and how real-world examples further portray its effects. Lastly, uncover what causes Galvanic Corrosion and the significance of the Galvanic Corrosion Index in practical applications. This is essential knowledge for anyone engaged in or studying materials, civil, or environmental engineering.

Understanding Galvanic Corrosion

An insight into Galvanic Corrosion

This phenomenon occurs due to the potential difference between the two materials. Thus, one metal acts as an anode (the metal that corrodes) and the other as a cathode. The potential difference causes the anodic metal to lose electrons (oxidation), while the cathodic metal gains those lost electrons (reduction). This exchange of electrons results in the corrosion or dissolution of the anode.

function galvanicCorrosion(anode, cathode) {
  const potentialDifference = getPotentialDifference(anode, cathode);
  return potentialDifference;
}

An example of Galvanic Corrosion would be:

Difference between Galvanic Corrosion and Electrolysis

While Galvanic Corrosion and Electrolysis may seem similar, they are inherently different processes.

Galvanic Corrosion	Electrolysis
Occurs naturally	Requires an external power source
Occurs between two different metals	Can occur in a single metal
Requires physical contact between metals	Does not require physical contact

By understanding these differences, you can better protect materials from both galvanic corrosion and electrolysis.

Implications of Galvanic Corrosion on Materials Engineering

Galvanic Corrosion has significant implications on Materials Engineering. In fact, preventing this form of corrosion is a major area of focus in the design and selection of materials to be used in various applications. Here are some important factors:

• Correct Material Pairing: To avoid galvanic corrosion, it is important to be aware of the galvanic series and select material pairings that minimise potential difference.
• Proper Insulation: In order to prevent contact between two different metals, insulation material can be used.
• Protective Coatings: Certain coatings can be applied to the materials in order to prevent or slow down the corrosion process.

Efforts to comprehend and prevent Galvanic Corrosion contribute to longer-lasting materials, reduced maintenance costs, and safer structures.

Galvanic Corrosion Chart

A Galvanic Corrosion Chart, also known as a Galvanic Series, provides essential information for distinguishing between the different reactivity rates of dissimilar metals in a particular electrolyte.

Components of a Galvanic Corrosion Chart

At its essence, the Galvanic Corrosion Chart is a list of metals and alloys arranged according to their standard electrode potentials. It consists of three key components:

1. Metals and Alloys: The chart lists a wide range of metals and alloys used across industries, from noble metals like gold and platinum to active metals like zinc and aluminium.
2. Standard Electrode Potentials: A crucial aspect of this chart is the "standard electrode potential". This potential refers to the measure of the individual potential of a reversible half cell at standard state, with all solutes at an effective concentration of 1M and gases at 1 atm. The standard electrode potential is given by the equation: \[ E^⦵ = E - \frac{RT}{nF} \ln Q \] where:
   • \(E^⦵\) is the standard electrode potential.
   • \(E\) is the cell electromotive force (EMF).
   • \(R\) is the universal gas constant.
   • \(T\) is the absolute temperature.
   • \(n\) is the number of moles of electrons (~\(\pm~1\)).
   • \(F\) is the Faraday constant (~96485 C·mol−1).
   • \(Q\) is the reaction quotient, which reflects the balance of concentration or pressure of each species in the redox reaction.
3. Anodic and Cathodic Ends: The chart is arranged in such a way that metals toward the top are more "noble" or "cathodic", meaning they are less likely to corrode compared to metals at the bottom ("active" or "anodic" metals), which are more likely to corrode. This distinction is crucial in determining the likelihood of galvanic corrosion occurring when two metals are in contact.

How to read a Galvanic Corrosion Chart

Reading and interpreting a Galvanic Corrosion Chart proves invaluable when selecting materials for a construction, manufacturing, or engineering project. Firstly, identify the metals in consideration. Find these metals on the galvanic series. The greater the distance, in terms of positions on the chart, between two metals, the greater the galvanic activity between them when they are in contact. Secondly, note the direction of galvanic action. The metal higher on the list (more cathodic or noble) will act as the cathode and will be protected. The metal lower on the list (the more anodic or less noble metal) will act as the anode and will corrode. Finally, use this information for material selection. If galvanic corrosion is likely to be a concern, designers should aim to select metals that are close together on the chart. If this is not possible, measures should be taken to insulate the metals from each other or to provide a coating or barrier to the less noble metal to prevent corrosion. It is important to note that additional factors, such as environmental conditions or the relative surface areas of the connected metals, can impact the rate and severity of galvanic corrosion. However, utilizing a Galvanic Corrosion Chart remains a fundamental part of the preliminary evaluation process for material selection in various industries.

function assessGalvanicAction(metal1, metal2) {
  const positionDifference = getPositionDifference(metal1, metal2);
  
  if (positionDifference > threshold) {
    return 'High Galvanic Activity';
  } else {
    return 'Low Galvanic Activity';
  }
}

By understanding the components and method of reading a Galvanic Corrosion Chart, you can make more informed decisions, consequently reducing the risk of premature failure or maintenance requirements in your projects.

Analysing Galvanic Corrosion Examples

Real-world Examples of Galvanic Corrosion

Galvanic corrosion, due to its nature, can occur wherever dissimilar metals are in contact in the presence of an electrolyte. Breaking this down to real-world examples presents concrete demonstrations of this phenomenon.

Marine Applications

One of the most commonplace occurrences of galvanic corrosion is in marine applications. Oceans, for example, are an incredibly conducive environment for galvanic corrosion due to their salty water acting as an effective electrolyte. A prime example is seen in boats or ships where different metals are often used together for their individual attributes. For instance, a propeller of a ship may be made of a bronze alloy due to its strength and malleability, whereas the hull of the ship could be constructed from steel for its durability. When these two different metals are in contact and immersed in seawater, galvanic corrosion can occur, causing significant damages to the structure over time.

function checkMarineCorrosion(boatMaterial, propellerMaterial) {
  const potentialDifference = getPotentialDifference(boatMaterial, propellerMaterial);
  const corrosionRisk = potentialDifference * SEA_WATER_CONDUCTANCE;
  
  if(corrosionRisk > THRESHOLD) {
    return 'High corrosion risk';
  }
  return 'Manageable corrosion risk';
}

Building Structures

Similarly, amalgamated metals used in building materials can also experience this phenomenon. For example, stainless-steel bolts may be used to secure aluminium sheets on a roofing structure. If the building is located in a rainy area, the water serves as an electrolyte, initiating the electrochemical reactions that cause the aluminium sheets to corrode.

Outcomes of Galvanic Corrosion in various examples

The consequences of galvanic corrosion can range from mild to severe depending on the materials involved, their relative surface area, and the environment.

Effect on Marine Structures

In the case of marine structures, unchecked and severe galvanic corrosion can lead to structural compromise. Corrosion near load-bearing areas can lead to a framework's instability. On a micro scale, it can cause:

• Increased friction on moving parts.
• Sealing failures, leading to possible fluid leakage.

Furthermore, excessive corrosion of the propellers can affect the operational efficiency, increase fuel consumption and eventually leading to an unexpected breakdown of propulsion systems.

function calculateFuelIncrease(corrosionRate) {
  const factor = analyseFrictionIncrease(corrosionRate);
  return BASE_FUEL_CONSUMPTION * factor;
}

Implications on Building Structures

For building structures, galvanic corrosion can compromise structural integrity, risking collapse. Particularly, in the case of roofing, the corrosion can lead to holes in the roof, causing leakages and further water damage to the building's interior. Treatment, repair, and replacement of these corroded components could lead to excessive unexpected costs. Consequently, prevention is always a better strategy than cure. Hence, understanding the concept of galvanic corrosion, its real-life examples, and impacts is critical in minimising associated risks.

Causes of Galvanic Corrosion

Galvanic corrosion is a complex process influenced by several factors. Understanding the cause of this prevalent form of corrosion can provide you the necessary tools to combat it.

Primary Causes of Galvanic Corrosion and their effects

Galvanic Corrosion is driven by two primary conditions: the contact between dissimilar metals and the presence of an electrolyte. Dissimilar Metals: Each metal or alloy has a characteristic known as electrode potential. When two different metals have significantly different potentials, and they come into direct contact, galvanic corrosion is more likely to happen. The metal with the lower potential (anodic) will corrode in favour of the metal with a higher potential (cathodic). However, the potential difference between the metals alone does not cause galvanic corrosion, it sets one up to be more likely corroded. It's important to note that it's not always the less noble (anodic) metal that corrodes but can depend on the environmental conditions and the nature of the electrolyte (anions or cations presence). Electrolyte: An electrolyte is a substance that produces an electrically conducting solution when dissolved in a polar solvent, such as water. It contains ions that are free to move in the solution, and they are the substances that allow for current flow. An example of galvanic corrosion can illuminate this point. Recall the example of a ship with a steel hull and a bronze propeller immersed in seawater, which acts as the electrolyte. Without the seawater, the two different metals (steel and bronze) could be in contact without much issue. But once submerged in the seawater, the electrically charged solution provides a pathway for ion movement, thereby initiating the galvanic corrosion process. Here's an example represented in code:

function calculateCorrosionRate(metal1, metal2, electrolyteConcentration){
  let potentialDifference = getElectrodePotential(metal1) - getElectrodePotential(metal2);
  let corrosionRate = Math.abs(potentialDifference) * electrolyteConcentration;
  return corrosionRate;
}

The Link between Galvanic Corrosion and Electrolysis

Galvanic corrosion and electrolysis are both electrochemical processes. How are they similar and how do they differ? Firstly, both processes involve the movement of electrons and ions, which facilitate the transfer of energy and matter in the system. Galvanic Corrosion: As explained, it stems from dissimilar metals in contact with each other where an electrolyte is present. The metal with the lower electrode potential (anodic) corrodes faster than it would if it were alone, while metal with the higher electrode potential (cathodic) corrodes slower, or may not corrode at all. Electrolysis: This is a process that uses an external source of electrical energy to drive a non-spontaneous chemical reaction. In other words, electrolysis needs an external energy source to occur, unlike galvanic corrosion, which is a spontaneous reaction.

function electrolysisReaction(electrolyte, electricCurrent){
  let reactionRate = electrolyte.ionConcentration * electricCurrent;
  return reactionRate;
}

However, some confusion can arise because the term 'electrolysis' is also often interchangeably used with 'galvanic corrosion' especially when it comes to plumbing systems. This usually refers to a situation where an external stray electrical current is imposed on a pipe and results in accelerated metal corrosion, more like an electrochemical reaction. Remember, while both galvanic corrosion and electrolysis revolve around the movement of electrons leading to chemical changes, their causes, behaviour, and control methods are distinct. Understanding these differences is essential in diagnosing and addressing issues related to each process accurately in your engineering tasks.

Galvanic Corrosion Index

The Galvanic Corrosion Index, also known as the Anodic Index, is an essential numerical representation that assist in predicting the likelihood of galvanic corrosion in a specific metallic combination. It provides a measure of the voltage difference between dissimilar metals when connected in an electrolyte. Understanding this index is key to making informed decisions in materials selection, especially in environments susceptible to galvanic corrosion.

Understanding the Galvanic Corrosion Index

The Galvanic Corrosion Index uses the concept of electrode potentials, a measure of how a metal will react when paired with another in an electrolyte. Metals with higher electrode potentials behave as cathodes and are less likely to corrode, while those with lower electrode potentials, the anodes, corrode more easily. The calculation of the Galvanic Corrosion Index involves finding the potential difference between the two metals. Mathematically, it can be represented as: \[ GCI = |E_1 - E_2| \] Where \(E_1\) and \(E_2\) are the electrode potentials of the two metals, and the absolute value ensures that the index value is non-negative. Absolute values are important in this context because the focus is on the magnitude of the potential difference, not the direction. A larger index value implies a greater likelihood of galvanic corrosion, given that the electrolyte conditions are favourable. However, this index is a relative measure. It does not provide actual corrosion rates, and other factors, such as surface area ratios and environment, may affect the actual corrosion rate.

function GalvanicCorrosionIndex(E1, E2){
  return Math.abs(E1 - E2);
}

Material	Electrode Potential (V)
Zinc	-0.76
Aluminium	-0.50
Stainless Steel	0.50

Application of Galvanic Corrosion Index in Materials Engineering

In Materials Engineering, understanding the Galvanic Corrosion Index is pivotal in the process of materials selection, especially in environments where moisture or another electrolyte is present. The application of this index can significantly minimise the risk of metallic structures suffering from galvanic corrosion damage, potentially saving both time and resources. Consider improving the corrosion resistance of offshore structures. Here, highly saline conditions and the prevalent use of mixed-metal assemblies make such structures susceptible to galvanic corrosion. The engineer would ideally select material combinations with a lower Galvanic Corrosion Index, indicating a lower risk of galvanic corrosion, all other factors being equal. Selection is not limited to the metal components alone, but also extends to protective coatings and finishes. Both the substrate material and the protective layering should ideally be close on the galvanic series to prevent galvanic corrosion if the integrity of the coating is damaged, exposing the substrate to the electrolyte. Sorting materials based on their galvanic compatibility can also be necessary, using their relative positions in the galvanic series or their electrode potentials. For instance, the selection of fasteners in construction is significant, as a poorly chosen fastener can corrode, leading to structural failure.

function selectFastener(materials){
  const potentialDifferences = calculatePotentialDifferences(materials);
  let selectedPairs = [];
  for(let i=0; i

However, the mere application of the Galvanic Corrosion Index can be overly simplistic. Other factors like relative surface area, position, and concentration of ions in the electrolyte also substantially contribute to corrosion risk. Hence, a comprehensive understanding of corrosion mechanisms and preventive measures is indispensable, with the Galvanic Corrosion Index acting as a vital tool in this context.

Galvanic Corrosion - Key takeaways

Galvanic Corrosion is the process where one metal corrodes when it comes into contact with another metal and an electrolyte.
Material pairing, proper insulation and protective coatings are some measures that can prevent galvanic corrosion, leading to longer-lasting materials.
A Galvanic Corrosion Chart, listing metals and alloys according to their standard electrode potentials, helps understand the likelihood of galvanic corrosion when different metals come into contact.
Real-world examples of galvanic corrosion include marine applications where different metals come into contact with seawater functioning as an electrolyte, resulting in structural damage over time.
Major causes of Galvanic Corrosion include contact between dissimilar metals and presence of an electrolyte. Factors such as electrode potentials of metals and properties of the electrolyte influence the rate of galvanic corrosion.