The effect of lithium bromide on corrosion of metals

Introduction

The main objective of the present study is to investigate the correctness of the selection of suitable materials in the chiller device for those chiller equipment that are in direct contact with the lithium bromide solution.

Lithium bromide (LiBr) is a salt that absorbs moisture and water vapor. The lithium ion in the lithium bromide solution forms a strong bond with water molecules, which creates the necessary adsorption effect for the chiller to operate, and in fact, this solution is the main factor in reducing the pressure and taking heat from the chilled water.

Concentrated solutions of lithium bromide are widely used in absorption refrigeration and heating systems. However, LiBr solutions can cause serious corrosion problems in structural materials (copper, steels and other metals) in an absorption plant.

Lithium bromide solutions can cause corrosion in metal components, especially carbon steels, used in refrigeration systems and heat pumps.

Effects of LiBr Concentration on Corrosion

The concentration of LiBr in solution can significantly affect the rate and severity of corrosion in metals. Here are some of the effects associated with different concentrations:

Increased corrosion rate:Higher concentrations of LiBr can lead to increased corrosion rates in sensitive metals, especially carbon steels. Increasing the ionic strength of the solution can enhance the electrochemical reactions that lead to corrosion.

pH levels:LiBr concentration can affect the pH of the solution. Lower pH values ​​(more acidic conditions) can exacerbate the effects of corrosion. Maintaining an optimal pH is crucial in reducing corrosion.

Formation of corrosion products:At higher concentrations, the solubility of corrosion products may be affected. This can lead to the formation of protective layers or, conversely, the deposition of unwanted by-products or corrosion that can further galvanically couple with the base metal.

Oxygen solubility:Lithium bromide concentration can also affect the solubility of dissolved oxygen in the solution, which can accelerate the rusting process in metals, especially in stagnant conditions.

Temperature effects:High concentrations of LiBr solutions are often used in high temperature applications (such as absorption chillers). Increased temperature can also intensify corrosion processes, potentially leading to more rapid degradation of metals.

Specific metal reactions:Some metals or alloys may exhibit more significant corrosive effects at certain concentrations of LiBr. For example, stainless steels may resist corrosion better than carbon steels at certain concentrations, but may face challenges in harsh conditions.

Corrosion Mechanisms in LiBr Solutions

Corrosion mechanisms in lithium bromide solutions mainly involve electrochemical processes. Here are some key mechanisms:

Electrochemical Corrosion

In lithium bromide solutions, metals can undergo electrochemical reactions in the presence of moisture and ionic solutions, leading to the formation of corrosion cells. The specific reactions depend on the metal involved but typically involve anodic oxidation and cathodic reduction.

Galvanic corrosion

If dissimilar metals are in contact in a LiBr solution, galvanic corrosion can occur. One of the metals acts as an anode (corrodes more quickly), while the other acts as a cathode (corrosion occurs more slowly).

Pitting corrosion

The presence of bromide ions can lead to localized corrosion called pitting. This is characterized by the formation of small, deep pits on the metal surface. The pits can be caused by the breakdown of protective oxide layers, which may be compromised by aggressive ionic species such as bromides.

Stress corrosion cracking (SCC)

Certain alloys, particularly stainless steels, may be susceptible to stress corrosion cracking in the presence of LiBr solutions due to the combination of tensile stress and the aggressive nature of bromide ions.

Uniform Corrosion

Although not common in controlled systems, uniform corrosion can also occur, resulting in a relatively uniform thinning of the metal surface due to the electrolytic action of the LiBr solution.

Factors Affecting Corrosion

Other factors that affect the rate and severity of corrosion in LiBr solutions include:

• LiBr concentration:Higher concentrations generally increase the corrosive nature of the solution.
• Temperature:High temperatures can intensify the corrosion rate.
• Oxygen content:The presence of dissolved oxygen can accelerate corrosion processes.
• Metal composition:Different metals and alloys exhibit different levels of susceptibility to corrosion in LiBr solutions.

How can corrosion be prevented in LiBr systems?

Corrosion prevention in lithium bromide (LiBr) systems can be achieved through several methods:

Material Selection:

Use corrosion-resistant materials such as stainless steel or alloys specifically designed for use in LiBr environments.

Coatings:

Apply protective coatings to protect vulnerable metal surfaces from direct contact with the LiBr solution. This can include the use of paint, epoxies, or specialized corrosion-resistant coatings.

Inhibitors:

Add corrosion inhibitors to the LiBr solution to reduce its corrosive effects. These chemicals can form a protective layer on the metal surface.

Maintaining Proper Concentration:

Ensure that the concentration of the lithium bromide solution is within the optimal range. High concentrations can exacerbate corrosion.

System Design:

Design the system to minimize stagnant areas where concentrated solutions can collect, as these areas are most susceptible to corrosion.

Regular Maintenance:

Perform regular system inspections and maintenance to identify and address early signs of corrosion.

Temperature and pH Control:

Maintain proper operating temperatures and pH levels, as these factors can affect the corrosivity of the solution.

Water Treatment:

If water is used to dilute LiBr, ensure it is treated to remove impurities that can contribute to corrosion.

What metals are resistant to lithium bromide solutions?

Special corrosion-resistant materials, such as stainless steel and corrosion-resistant alloys, are used to handle and contain lithium bromide solutions.

Lithium bromide solutions are known to be corrosive to many metals. Some metals that have shown resistance to corrosion in lithium bromide solutions include:

• Copper-nickel alloys (Cu-xNi alloys)
• Copper (Cu)
• Nickel (Ni)

It is important to note that the corrosion resistance of these metals can vary depending on factors such as the concentration of the lithium bromide solution, temperature, and the presence of other contaminants.

When selecting materials for use in lithium bromide (LiBr) refrigeration systems, it is necessary to consider both chemical compatibility with the lithium bromide solution and the operating conditions of the system. Here are some best practices for preventing corrosion.

Corrosion Resistance:

Choose materials that have good corrosion resistance in lithium bromide solutions. Copper-nickel alloys and special stainless steels (such as 316L) are commonly used due to their corrosion resistance.

Temperature Tolerance:

Consider the operating temperature range of the system. The materials must withstand the low and high temperatures typically experienced in refrigeration without losing their structural integrity.

Mechanical Properties:

Ensure that the selected materials have the appropriate mechanical properties (tensile strength, yield strength, etc.) to withstand the stresses and loads encountered in the system.

Thermal Conductivity:

To ensure efficient heat transfer, use materials with good thermal conductivity for components such as heat exchangers.

Weld ability and Fabric ability:

Choose materials that are easy to weld or fabricate to meet design requirements. This is especially relevant for custom parts or repairs.

Preventing Galvanic Corrosion:

Use materials that are compatible and avoid combinations that can lead to galvanic corrosion, such as pairing dissimilar metals in the presence of electrolyte.

Regular Maintenance and Inspection:

Even corrosion-resistant materials may require regular maintenance and inspection. Conduct a routine inspection to monitor for signs of wear and corrosion.

Conclusion:

In terms of material selection, since the tubes used in the absorber and generator sections of the chiller, which have the highest contact surface with the LiBr solution, are made of copper-based material, the alloy chosen provides the highest corrosion resistance.

Since parameters such as lithium bromide concentration and temperature greatly affect the corrosion resistance of metals in contact with this solution, it is recommended to monitor these parameters on a daily basis.

In addition to selecting the material and temperature and concentration parameters of lithium bromide, adding and injecting corrosion inhibitors such as lithium molybdate and lithium chromate and maintaining and controlling their percentage according to the instructions provided has a significant effect on controlling the corrosion of metals in contact with LiBr solution.