Aluminum alloy corrosion potential, corrosion inhibitors can become a “lifesaving straw”?
What is a corrosion inhibitor?
Aluminum corrosion inhibitor is an appropriate concentration and form exists in the environment (medium), can make aluminum and its alloys in the medium of the corrosion rate significantly reduced until zero chemical substances or a mixture of several chemical substances. It slows or prevents corrosion by forming a protective film on the aluminum surface and preventing the corrosive medium from coming into contact with the aluminum. According to the mechanism of action, aluminum corrosion inhibitors can be divided into anode-type, cathode-type and hybrid. Anodic corrosion inhibitors react with the aluminum surface to form a dense oxide film or precipitation film to inhibit the anodic reaction; cathodic corrosion inhibitors increase the over-potential of the cathodic reaction or absorption of dissolved oxygen, etc., to inhibit the cathodic reaction; mixed-type corrosion inhibitors inhibit the anodic and cathodic reactions at the same time. Common aluminum corrosion inhibitors include silicon corrosion inhibitors, phosphate ester corrosion inhibitors and so on.
In modern industry and daily life, aluminum alloy occupies an important position with its excellent performance. However, the corrosion of aluminum alloy often brings us a lot of trouble. Today, let us explore the causes of aluminum alloy corrosion and understand the magic role of corrosion inhibitors in dealing with this problem.
Reasons for aluminum alloy corrosion
Aluminum alloy is not a single metal, but by aluminum and other elements (such as copper, magnesium, silicon, etc.) alloy. This complex composition makes it prone to corrosion in certain environments.
(1) Electrochemical corrosion: when aluminum alloy is in contact with other metals or electrolyte solution, a microscopic primary battery will be formed due to the difference in electrode potential between different metals. In this primary cell, aluminum alloy as anode will gradually lose electrons and be corroded. For example, in humid air, electrochemical corrosion reaction may occur between aluminum alloy surface and trace copper impurities.
(2) Chemical corrosion: Aluminum alloy will be corroded by direct chemical reaction under the action of some strong acids and alkalis and other chemical substances. For example, when aluminum alloy comes into contact with highly acidic industrial wastewater or alkaline cleaning agents, the aluminum on its surface will react chemically with the acid or alkali, generating the corresponding aluminum salts or meta-aluminate, which will lead to the loss of materials.
(3) Stress corrosion cracking: While subjected to tensile stress, aluminum alloys that are in specific corrosive environments, even if the stress level is lower than the yield strength of the material, cracks may also occur in the emergence and expansion. For example, in the marine environment containing chloride ions, aluminum alloy structural parts subjected to certain stresses are prone to stress corrosion cracking phenomenon, which seriously affects their structural integrity and service life.
(4) Intergranular corrosion: Aluminum alloy in improper heat treatment or in some specific corrosive media, the formation of potential difference between the grain boundary and the internal grain, resulting in the priority of the grain boundary to be corroded. This kind of corrosion is often carried out quietly inside the material, and it is not easy to notice at the initial stage, but it will gradually weaken the strength of the material, so that the aluminum alloy products will suddenly fail in the process of use.
Corrosion Inhibitor - “Protective Guard” of Aluminum Alloy
Corrosion inhibitor is a chemical substance that can effectively slow down or inhibit the corrosion of aluminum alloy. It works by forming a protective film on the surface of aluminum alloy or changing the electrochemical process of corrosion reaction.
1.Mechanism of corrosion inhibitor
(1) Adsorption-type corrosion inhibition: These corrosion inhibitor molecules can be adsorbed on the surface of aluminum alloy to form a layer of compact protective film, blocking the contact between corrosive medium and aluminum alloy. For example, some organic amine corrosion inhibitors, their polar groups will be adsorbed on the surface of the aluminum alloy, non-polar groups toward the solution, thus forming a hydrophobic protective film, effectively inhibiting the corrosion reaction.
(2) Passivation corrosion inhibitors: can make the surface of aluminum alloy passivation, the formation of a layer of dense oxide film. Like chromate class corrosion inhibitor, under certain conditions can promote the aluminum alloy surface to generate a layer of stable aluminum oxide passivation film, greatly improving the corrosion resistance of aluminum alloy. However, due to chromate pollution of the environment, is now gradually replaced by other environmentally friendly corrosion inhibitors.
(3) precipitation type corrosion inhibition: on the surface of aluminum alloy and corrosion medium in certain ions react to generate insoluble precipitation film, so as to play a protective role. For example, phosphate corrosion inhibitors in the presence of calcium ions, can be formed on the surface of aluminum alloy calcium phosphate precipitation film to prevent the further development of corrosion.
2. Types and characteristics of corrosion inhibitors
(1) Inorganic corrosion inhibitors: such as chromate, phosphate, nitrite and so on. Chromate corrosion inhibition effect is good, but toxicity, pollution of the environment; phosphate corrosion inhibition performance is relatively weak, but more environmentally friendly; nitrite corrosion inhibition efficiency is higher, but under certain conditions may produce carcinogenic substances, the use of caution.
(2) Organic corrosion inhibitors: there are many kinds of inhibitors, including organic amines, imidazolines, benzotriazoles, silicone ketones and so on. Organic amines have good inhibition effect on the corrosion of aluminum alloy in acidic environment; imidazolines show good corrosion inhibition performance in a variety of corrosive mediums and have certain environmental protection; benzotriazoles have special corrosion inhibition effect on the system composed of copper alloy and aluminum alloy, and can protect two metals from corrosion at the same time; silicone ketone-type aluminum corrosion inhibitor CSUM® 8150 is a complex product made of many kinds of special silane oligomerization by hydrolysis and condensation. CSUM® 8150 is a complex product of siloxane oligomerization by hydrolysis of a variety of special silanes, is a water-based aluminum corrosion inhibitor with excellent performance and low cost, which can be completely dissolved in water, soluble in alcohols, acetone, most of the aliphatic and esters, mainly used for corrosion inhibition of aluminum in aqueous environments, and at the same time has a very good corrosion inhibition of solder to reduce the corrosion of the solder in the aqueous environment.
Note:UNPChemicals CSUM® 8150 is a complex product of siloxane oligomer, also known as siloxane ketone, formed by hydrolysis and condensation of various special silanes. Mainly used for corrosion inhibition of aluminum in water-based environments, CSUM® 8150 also offers outstanding corrosion inhibition for soldering tin, reducing its corrosion in water environments.
3. The use of corrosion inhibitors and precautions
(1) Usage:
Corrosion inhibitor can be directly added to the corrosion medium, such as adding appropriate amount of corrosion inhibitor in the coolant and cleaning solution during aluminum alloy processing; it can also be applied by brushing, spraying and other ways to coat the corrosion inhibitor on the surface of aluminum alloy to form a pre-protective film. For some large aluminum alloy structural parts, corrosion inhibitor ions can also be introduced into the surface layer of aluminum alloy by electrochemical methods to enhance its corrosion resistance.
(2) Precautions:
First of all, the appropriate corrosion inhibitor should be selected according to the material of aluminum alloy, the use environment and the type of corrosion. The corrosion inhibition effect of different corrosion inhibitors on different systems varies greatly, and the expected protective effect may not be achieved by choosing the wrong corrosion inhibitor.
Secondly, the amount of corrosion inhibitor should be properly controlled. Add too little, the corrosion inhibition effect is not obvious; add too much, not only will increase the cost, but also may produce side effects, such as affecting the surface quality of aluminum alloy or lead to the accumulation of corrosion inhibitor pollution in the environment.
In addition, when using corrosion inhibitors, pay attention to its compatibility with other additives or treatment processes to avoid adverse reactions.
Understanding the causes of aluminum alloy corrosion and the reasonable use of corrosion inhibitors are of great importance for improving the quality of aluminum alloy products, prolonging their service life as well as guaranteeing the safety and stability of related industrial production. In the field of materials science and engineering in the future, with the deepening of the research on the corrosion mechanism of aluminum alloy and the research and development of new corrosion inhibitors, aluminum alloy will certainly play a greater role in more fields. Let's look forward to the new breakthroughs and new development in this field, which will add bricks and mortar to the progress of modern industry.