Iron powder adds to the rate of metal deposition, thus giving a leg-up to the productivity.<\/li>\n<\/ol>\n\n\n\nTypes Of Flux Coatings On Carbon Steel Electrodes<\/h2>\n\n\n\n The covering gets its\u2019 identity from one or two of its\u2019 principal ingredients. The types of coatings found on carbon steel electrodes are discussed in brief below:<\/p>\n\n\n\n
Cellulose-Sodium (EXX10)<\/h3>\n\n\n\n Coverings of this type contain cellulose content, which is added in the form of wood flour. Cellulose is an organic compound composed of carbon, hydrogen, and oxygen. The gases evolved when this covering burns up are rich in carbon dioxide and hydrogen, which are reducing agencies. These products shield the molten metal from oxidation. These gases also help in producing an arc that provides deep penetration. A small amount of iron powder is added for stabilizing the arc.<\/p>\n\n\n\n
These electrodes end with a \u201810\u2019 in their designation, and were one of the earliest developed varieties of SMAW electrode. These are still in wide usage today, and are used in cross-country pipelines with direct current reverse polarity. The mechanical properties are good enough.<\/p>\n\n\n\n
Cellulose-Potassium (EXX11)<\/h3>\n\n\n\n This coating is pretty similar to the one described above, except that more potassium is present. The presence of potassium makes the electrode suitable for welding with alternating current. These electrodes end with a \u201811\u2019 in their designation. Other characteristics such as depth of penetration, mechanical properties are similar to those of EXX10 electrodes. A small amount of iron powder is added to stabilize the arc. This also helps in increasing the rate of deposition slightly.<\/p>\n\n\n\n
Rutile-Sodium (EXX12)<\/h3>\n\n\n\n When the coating is made rich with rutile powder or titanium dioxide, the arc operation is very smooth, and very welder friendly. The arc does not make much noise, can be easily controlled, the amount of spatter is low, and the bead has a uniform smooth appearance. However, these electrodes produce a lower depth of penetration in comparison to the cellulose-rich coatings. The rate of deposition is high, however the properties are a tad inferior to those obtained with cellulose coating.<\/p>\n\n\n\n
These electrodes end with a \u201912\u2019 in their designation, and can be used with direct current straight polarity.<\/p>\n\n\n\n
Rutile-Potassium (EXX13)<\/h3>\n\n\n\n This coating is similar to the rutile-sodium coating, however the potassium content is increased. Addition of potassium makes the electrode amenable to welding with alternating current. This electrode can be used with DCRP or DCSP. The arc is a quiet, and makes for a smooth operation. These electrodes end with a \u201813\u2019 in their designation.<\/p>\n\n\n\n
Rutile-Iron Powder (EXX14)<\/h3>\n\n\n\n Rutile-iron powder coating is also similar to the two rutile coatings described above, except that iron powder is an extra addition. Adding iron powder gives some boost to the rate of metal deposition. The amount of iron powder determines the increase in rate of deposition.<\/p>\n\n\n\n
Based on the percentage of the iron content, these coatings give origin to two different electrode designations. Coatings in which the iron powder is present between 25 to 40%, have a \u201814\u2019 at the end in their electrode designation. These electrodes can be used in all positions.<\/p>\n\n\n\n
Coatings that have iron powder in more than 50% percentage are have a \u201824\u2019 at the end in their designation. Due to higher percentage of iron powder, it is difficult to use these electrode in overhead and vertical positions. So these electrodes are suitable for welding only in flat position.<\/p>\n\n\n\n
Low Hydrogen-Sodium (EXXX5)<\/h3>\n\n\n\n \u2018Low hydrogen electrodes\u2019 is a common name given to those electrodes that produce a relatively lower amount of diffusible hydrogen in the weld metal deposited with these electrodes, in comparison to the electrodes described above.<\/p>\n\n\n\n
Hydrogen is an unwelcome element in the weld metal, hence these electrodes are in demand when low alloys of high strength are welded. These electrodes are also used in the coating of SMAW electrodes for stainless steel welding.<\/p>\n\n\n\n
These electrodes are also called \u2018basic-coated\u2019 electrodes. The weld metal produced by these electrodes has better mechanical properties than other varieties. The ductility is higher.<\/p>\n\n\n\n
Low hydrogen-sodium coatings contain a high content of CaCo3 (calcium carbonate) or CaF2 (calcium fluoride). Other elements such as cellulose, clay, etc. that have hydrogen in their molecular constitution are not used in these coatings. This helps meet the objective of low hydrogen<\/em>.<\/p>\n\n\n\nThe depth of penetration produced with these electrodes is moderate. The rate of deposition is medium as well. These electrodes require baking of fresh electrodes at a higher temperature, and are generally used with DCRP polarity. The storage of the electrodes should be done in dehumidified conditions. The packing of the electrodes should be such that no atmospheric moisture is let inside the electrode packet during storage.<\/p>\n\n\n\n
Low Hydrogen-Potassium (EXX16)<\/h3>\n\n\n\n This coating is similar to low hydrogen-sodium coating except that the sodium is substituted with potassium. This helps in ionization of the arc, and makes the electrode suitable for operation with alternating current, and can be used with DCSP as well.<\/p>\n\n\n\n
Low Hydrogen-Potassium (EXX18)<\/h3>\n\n\n\n This coating is the most widely used coating. Along with the contents of the EXX16 variety, it also contains iron powder. These electrodes can be used in all welding positions. The electrode E7018 is an example of this type of coating.<\/p>\n\n\n\n
Low Hydrogen-Iron Powder (EXX28)<\/h3>\n\n\n\n These electrodes end with a \u201828\u2019 in their designation. In this coating, the percentage of iron powder is higher than the EXX18 electrodes. This makes for higher rate of deposition, however, due to high amount of iron powder \u2013 these electrodes can be used only in flat position and horizontal position welding.<\/p>\n\n\n\n
In the above two types of coatings, when other metal powders such as molybdenum powder or nickel powder or chromium powders are added, it becomes possible to influence the chemical composition of the weld metal as well (the weld metal composition is otherwise largely dependent on the composition of the core wire).<\/p>\n\n\n\n
When metal powders are added, an additional letter must be added after the EXXXX to indicate the content of these additions. This suffix is separated from the main designation by a \u2018-\u2018.<\/p>\n\n\n\n
Iron Oxide-Sodium (EXX20)<\/h3>\n\n\n\n These electrodes have a \u201820\u2019 as the last two digits in their designation. The coating contains a high percentage of FeO2. This induces a large amount of slag on the solidified weld metal. So, the surface finish of the bead below this slag is excellent.<\/p>\n\n\n\n
Although this coating enables a high rate of deposition due to high amount of Fe, the arc is a little difficult to control. Moreover, the electrode can be used only in flat position. The spatter is low, while the penetration is moderate. The electrode can be used with all three currents, that is \u2013 DCSP, DCRP, and AC.<\/p>\n\n\n\n
Iron-Oxide-Iron Powder (EXX27)<\/h3>\n\n\n\n This coating is similar to the iron oxide-sodium coating, except that iron powder is added in a high amount. This increases greatly the rate of metal deposition. These electrodes can be used with all three types of current.<\/p>\n\n\n\n
Combinations<\/h3>\n\n\n\n There can be several other varieties that can be obtained by mixing the above covering types. Depending upon the application, the composition can be tailored to suit the needs of the application. The properties desired from the weld metal are conveyed to the supplier. Then, the supplier has to come up with the right coating that caters to the requirement. There can be multiple ways of reaching the same ultimate composition. Some of them are economical. Others, not so much.<\/p>\n\n\n\n
This is why, the exact composition of the covering is a trade secret of the supplier.<\/p>\n\n\n\n
Deposition Rates<\/h2>\n\n\n\n We have seen in above paragraphs that addition of iron powder in the flux coating increases the rate of metal deposition. The percentage of iron powder in the flux coating varies from supplier to supplier. The percentage is calculated by using the following formula. In US, the percentage is calculated by this formula.<\/p>\n\n\n\n <\/figure>\n\n\n\nIn US, this percentage is kept 10% to 50%. These percentages are related to the requirements of the American Welding Society (AWS) specifications.<\/p>\n\n\n\n
In Europe, the % of iron powder in electrode coverings is calculated differently. The following formula is used.<\/p>\n\n\n\n <\/figure>\n\n\n\nIf half of the weight of deposited weld metal came from the iron powder in flux covering, and half from core wire, the above formula would yield 200% iron powder. The US same electrode in US would yield a very different % of iron powder in coating.<\/p>\n\n\n\n
What Type Of Current For SMAW Electrodes?<\/h2>\n\n\n\n The content of flux coating influences what current type the electrode can be used with.<\/p>\n\n\n\n
The shielded metal arc electrodes are majorly used with direct current. Some electrodes function well with DCEP, while some function well with DCEN polarity. Some can be used with both polarities as well. In this regard, the electrode manufacturer\u2019s recommendation should be followed.<\/p>\n\n\n\n
Sometimes, the electrodes are required to function with alternating current. This is required when welding thicker sections where arc blow is a problem. Arc blow can produce slag inclusions, blow holes, and lack of fusion. Welding with alternating current helps in overcoming the problem of arc blow.<\/p>\n\n\n\n
The electrodes that can operate with alternating current come specially designed for this. The flux coating have a higher potassium content in them, that helps in ionization of the arc.<\/p>\n\n\n\n
So, this was about flux coating. Please share your thoughts in the comments section below.<\/p>\n","protected":false},"excerpt":{"rendered":"
When molten metal is exposed to air, it reacts with oxygen and nitrogen present in the air. As a result, metal oxides form which are detrimental to the integrity and strength of the weld. Hence, it is required to protect the molten metal from this oxidation. In covered electrodes, this protection is provided by the … Read more<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[108],"tags":[],"yoast_head":"\nTypes Of Coatings In SMAW Electrodes - mewelding.com<\/title>\n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n\t \n\t \n\t \n