About Titanium Anodes
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Update time : 2023-07-20 05:07:36
About Titanium Anodes
Details about titanium anodes
What is titanium anode?
The full name of titanium anode is titanium-based mixed metal oxide coated titanium anode (MMO). It is also known as KSA anode. It is the anode with stable size and shape. With titanium as the base plate, precious metal coating is brushed on the plate, giving it good electrocatalytic activity and electrical conductivity.
Classification of Titanium Anodes
According to evolved gas by anodes during electrochemical reaction, titanium anodes could be classified as chlorine evolution anodes (e.g. ruthenium-coated titanium electrodes) and oxygen evolution anodes (e.g. iridium-coated titanium electrodes and platinum titanium meshes/plates).
Chlorine evolution anodes (ruthenium-coated titanium electrodes): electrolyte is high in chloridion, and the environment is usually hydrochloric acid, electrolyzed seawater and electrolyzed saline solution. Among our company’s products, chlorine evolution anodes are Ru-Ir-coated titanium anodes and Ru-Ir-Sn titanium anodes.
Oxygen evolution anodes (iridium-coated titanium electrodes): electrolyte is usually in sulfuric acid environment. Among our company’s products, oxygen evolution anodes are Ir-Ta-coated titanium anodes, Ir-Ta-Sn titanium anodes and high-Ir-coated titanium anodes.
Platinum plated anodes (platinum titanium meshes/platinum titanium plates): titanium is the base material. Platinum is plated on the surface and the coating thickness is usually 1~5um. Size of mesh hole for platinum titanium meshes is usually 12.7*4.5mm or 6*3.5mm.
Selection of anodes
If the customer does not know which anode should be used, we need the customer to let us know anodes’ working environment and medium.
1. Ampere density per square meter, which is the result after current intensity borne by single anode is divided by electrode’s surface area.
2. Temperature
3. PH value
4. Ingredients and their percentages
5. Manufactured products or purpose
What is the role of metal oxide coating on titanium anodes?
By covering different coatings, enhance conductivity and electrocatalytic activity, improve electrolytic reaction, prolong lifespan of anodes in different application environment, and achieve expected effect.
Issues about thickness of metal oxide coating on titanium anodes
It is mainly precious metal in coating that sets off electrochemical catalytic reaction. Only if content of precious metal meets requirements of use, can anode’s normal operation be guaranteed. Thickness is just presentational, which is mainly decided by number of times of coating brushing and by concentration solvent. It has no direct relation with content of precious metal. Excessive thickness is easy to fall off instead.
What is anodic passivation?
What is metal’s anodic passivation? When dip iron wire in dilute nitric acid, iron wire will dissolve quickly. But when put iron wire in concentrated nitric acid, iron wire almost does not dissolve; at this time, if put iron wire in dilute nitric acid again, iron wire does not dissolve any more. And there is no change on iron wire’s surface if observed with naked eyes or even microscope. At this point people think iron wire is in status of anodic passivation.
During operation in electric angle, anodes’ lifespan has time limit. When voltage increases high and actually no electric current passes, anodes don’t work. Such phenomenon is called anodic passivation.
Main factors influencing anode’s lifespan
1. Current density: current density is inversely proportional to anode’s lifespan. The larger the current density, the shorter the anode’s lifespan. We design anode’s current density within 2000A/㎡ and suggest current density should not exceed 2000A/㎡. And try to avoid unstable electric current.
2. Titanium base material: generally pure titanium is used to be anode’s base material and above TA1 pure titanium is required. If purity of base material is not enough, anti-corrosion will weaken and lifespan of anodes will be influenced greatly.
3. Electrolyte: fluorinion, cyanoion and sulfion are not allowed to exist in electrolyte. Otherwise anodes’ lifespan will be influenced greatly. Content of chloridion should be controlled within 200PPM.
4. Pole reverse: it is a common method to restore anodes, and it could make passivated coating fall off. Therefore it is forbidden to be used during use of anodes.
5. Frequent power failure: when no electric current passes, coating will be damaged a lot if it is immersed in solution for a long time. So when anodes need not work, it is better to give low current to maintain circuit. If anodes are not used for a long time, take anodes out from solution, wash them with clean water, let them dry and then store them.
6. Artificial damage: damage could not exist on anodes’ surface. If part of coating is damaged, even if it is smallest scratch, anode would be corroded or passivated quickly, thus causing anodes out of work. Therefore, during installation and use of anodes, anodes should be handled gently to avoid damage.
7. Short circuit: cathodes and anodes are forbidden to connect when power on. Short circuit causes large instantaneous current, thus leading to burnout of polar plate.
8. Temperature: temperature is also inversely proportional to anodes’ lifespan. We design anodes’ working temperature between 40~60°C and we suggest temperature should not exceed 60°C.
Advantages of titanium anodes (in contrast to lead anodes and lead-tin anodes)
1. Stable size (insoluble base material), unchanged anode-to-cathode distance. Cell voltage is ensured to be stable.
2. Low working voltage and low consumption of electric energy. Save 20% more electricity than lead anodes.
3. High purity of cathode products.
4. No anode mud and no contamination of cell body.
5. Light weight. Could reduce labor intensity.
6. Base material could be used repeatedly.
Application of titanium anodes in electroplating industry
Definition of electroplating: it is a process of using electrolysis principle to plate a thin layer of other metal or alloy on some metals’ surface. Generally, coated metal is used as anodes and is oxidized to be cationic to enter into electroplate liquid. To-be-plated metal products are used as cathodes. Coated metal’s cationic is reduced to form plating. Generally solution containing coated cationic is used as electroplate liquid, to keep concentration of coated metal's cationic unchanged.
Purpose of electroplating: plate a layer of metal coating on base material, to change base material’ surface property or size. With coated metal as corrosion-resistant metal mostly, eletroplating could increase metal's anti-corrosion, increase hardness, prevent abrasion, and improve electrical conductivity, unctuosity, heat resistance and beauty of surface. Trivalent chrome plating: in nature chrome exists mainly in forms of trivalent chrome and hexavalent chrome. Trivalent chrome is one of essential microelements in human body, while hexavalent chrome is definite harmful element. EPA determines hexavalent chrome as one of 17 highly hazardous toxic substances. Oral lethal dose of hexavalent chromic compound is around 1.5g. If content of hexavalent chrome in water exceeds 0.1mg/L, it will be toxic. In China in order to prevent danger of chrome, it is stipulated that the level of hexavalent chrome in drinking water should be less than 0.05mg/L. According to industrial wastewater emission standards, hexavalent chrome is class 1 pollutant and its level has to be less than 0.5mg/L. EU countries, Japan and North America carry out a deadline for use and replacement of hexavalent chrome plating. They also forbid hexavalent chrome plating products to enter into the market, to realize clean production and protect environment and human. And treatment fee for wastewater generated by trivalent chrome is only around 20% of treatment fee for wastewater generated by hexavalent chrome.
Trivalent chrome plating is white and bright, and has high hardness and good mechanical property. Its dispersing performance and covering power are far higher than hexavalent chrome. When current density is same, its deposition velocity doubles hexavalent chrome’s. But main advantage of trivalent chrome is that its toxicity is only 1% of hexavalent chrome’s. And it is easy to handle. It is one of most promising electroplating technique to replace hexavalent chrome which is widely used but pollute environment seriously. As previously mentioned, developed countries have decided to reduce use of hexavalent chrome plating until it is not used completely, just because there are environment-friendly new techniques, like trivalent chrome plating.
But, trivalent chrome plating also has its issues, mainly because it has low tolerance of impurities, thus leading to poor stability, sometimes influencing quality of plating and failing to produce qualified products.
And thickness of trivalent chrome is difficult to exceed 5μm and its hardness could not reach hardness of hexavalent chrome. Generally it is used as decorative plating. Technique of trivalent chrome mainly includes chloride system and sulphate system.
Chrome plating:
Types of chrome plating:
1. Protective decorative chrome plating. It could prevent base metal rusting, and it has decorative glossy appearance. It is widely applied to components of such items as vehicles, bicycles, sewing machine, timepieces, instruments and daily used hardwares.
2. Hard chrome (wear resistant chrome). Mechanical parts are plated with hard chrome, which could improve wear resistance and prolong lifespan.
3. Semi-opaque chrome plating. Its pore size is very small and it has better anti-corrosion than hard chrome. Extra layer of glossy wear-resistant chrome plating on the layer of semi-opaque chrome plating could improve anti-corrosion and wear resistance; it is called protective wear-resistant double-layer chrome plating, which is widely applied to aircraft parts, watercraft parts and guns’ chamber.
4. Spongy chrome plating
5. Black chrome plating
When plating, anode materials used include DSA titanium anode (precious metal oxide coating titanium anode), PBSN (lead-tin), graphite and titanium-based lead dioxide electrode. These materials could oxidize chromium ions into chromic acid ions. And when insoluble titanium anode is used, anode’s oxidation overpotential is around 450mv lower than lead electrode’s. Meanwhile titanium anode could work under large current density. It has long lifespan and glossy and clean surface. Its chrome plating is bonded with base material firmly.
Rhodanizing:
Rhodium plating is silver. For now, technique of rhodium plating mostly adopts rhodium sulfate plating solution. It is mainly applied to jewelry industry, slidable rotational contacts, printed circuit switches, communication equipment’s components and high-frequency switches’ components. For rhodium plating generated by using rhodium plating solution with selenic acid included and rhodium plating solution with magnesium sulfamate hydrate included, generally industrial requirement on thickness of rhodium plating is 0.1~0.2um. In technique of rhodium plating, platinum/titanium-based platinum plated and DSA-coated titanium anodes could be used.
Palladium plating:
Palladium plating is silver. In 1970s, electroplating technology of using palladium to replace gold developed quickly in electronic industry. The most commonly used palladium plating solution is ammoniacal system, in which palladium exists in form of chromium ammino complex. Palladium plating is usually easy to obtain, but also often get cracks easily, because within potential range of electrodeposited palladium, co-electrodeposition often happens with hydrogen; palladium plating absorbs hydrogen, and hydrogen releases gradually during storage period, thus forming cracks on plating. Pure platinum/titanium-based platinum plated/DSA titanium anodes could be used for palladium plating.
Gold plating:
Gold plating solution with or without cyanide is used currently. Anodes for gold plating originally used pure god/platinum/titanium-based platinum plated anodes, but platinum/titanium-based platinum plated electrodes are expensive, and meanwhile platinum-plated titanium anodes have shorter lifespan than DSA titanium anodes, which are cheap but good.
Ruthenium plating:
Ruthenium has high hardness and is wear resistant. Ruthenium plating is used for electric contacts. It is significant to use ruthenium plating to replace expensive rhodium plating. However, up to now it is still rarely applied in China. Platinum/titanium-based platinum-plated/DSA-coated titanium anodes could be used for ruthenium plating.
