| CORROSION, EROSION AND COATINGS* |
It is often necessary to put theremowells in extremely corrosive and/or erosive environments. In such cases the proper selection of materials becomes critical to the success of the installation.
*There is a technical difference between coating and hard surfacing but for convenience the term coating is used here to refer to either process.
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| CHEMICAL CORROSION RESISTANCE |
| While various coatings can ward off the effects of chemically corrosive environments, thermowells made entirely of corrosive resistant materials have definite advantages in that they are not susceptible to chipping, scratching or wearing away of relatively thin coatings. Materials such as Alloy 20, Aluminum, Hastelloy C, Monels, Nickel, Teflon, Titanium, and Zirconium have all proven effective against specific corrosives. There are of course environments which attack all but the most exotic and costly materials and applications which require special properties, in addition to corrosion resistance. In such cases protective coatings or sleeves are the method of choice to guard against corrosives. Corrosive resistant coatings are usually applied to flanged or Van Stone Type theremowells with the coating covering the entire "U" length and flange face. Among the more popular corrosive resistant coatings are: alumina, lead, tantalum, and teflon. Many other coatings are also available on request. Tantalum is available oversheath type configurations. |
| WEAR RESISTANCE |
| To date the best method available for slowing the effects of erosion (wearing away caused by particulate bombardments common in such operations as decoking) is carefully selecting a hard facing material and method of application. Because wear-resistant thermowell technology has been an area of specialization with Sandelius for many years, we make every effort to keep up-to-date on new coatings and application methods as they are developed. We would be happy to work with you to determine which of many available products would be best to try in your particular application. The following is a brief list of some of the more popular wear-resistant coatings in use today. |
Aluminum Oxide
Boron Nitride
Ceramics
Colmonoy No. 6
Colmonoy No. 75
Stellite No. 1 |
Stellite No. 6
Stellite No. 12
Wallex No. 1
Wallex No. 6
Wallex No. 50
Wallex No. 55 |
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HOW TO SPECIFY COATINGS After determining which coating is the best choice for your application it is important to specify it correctly. We cannot stress this point strongly enough. For example, it is obvious that a .020 inch thick coating of sprayed Stellite No. 6 is not equivalent to a .060 inch thick coating of T.I.G. welded Stellite No. 1. Yet both can truthfully be referred to as "Stellite" coatings.
Your order should include all the information necessary to insure you will receive the exact coating you require. The following 6 points should be included in all specifications for coated thermowell (Again if you need technical assistance to make the proper selection please call us anytime.) |
| 1. Specific Generic or Trade Name of the coating desired: Many otherwise excellent specifications fail on this simple point. An excellent example of the problem is the use of the trade name Stellite to indicate hard-facing is required. However, there are at least 14 different Stellite Alloys, each of which has it's own advantages. If you specify Stellite with no Alloy number you have a good chance of not getting the right one for your application. By specifying Stellite No. 1, Stellite No. 6 or any other exact Stellite Alloy any possible confusion is completely avoided. |
| 2. Method of Application: Some of the more commonly used methods of coatings are: Spray Gun or Flame Spray, Gas Tungsten Arc, Submerged-Arc, Open-Arc, Oxy-Acetylene Deposition, Shielded Metal-Arc Deposition, Plasma Arc, Diffusion Coating. |
| 3. The portion of the thermowell to be coated. Wear-resistant coatings are relatively expensive. To avoid wasting money they are usually applied only to the section of the thermowell which is subjected to wear. This is especially important on long thermowell where only the tips are actually being subjected to wear. |
| 4. The thickness of the coating: Depending on the type, coatings can be anywhere from a few thousands of an inch to a 1/4" or more in thickness. Often the same coating can be applied in any number of different thicknesses. Both the effectiveness and the cost of the coating are determined in part by its thickness (thicker coatings are not always better coatings). To avoid confusion specify the thickness desired as a thickness "per side". It is easy to think of a thermowell which starts as 1 O.D. and is coated to a finished O.D. of 1-1/8" as having a 1/8" thick coating. But in fact the thermowell has a 1/16" inch thick coating on each side adding a total of 1/8" to the O.D. |
5. The finish requirements of the coatings: Metallic coatings such as Stellites and Colmonoys can be left "as applied", machined smooth or even polished. In most cases a smooth or polished finish will provide superior wear resistance in service than an "as applied" coating. Desired finishes should be specified using standard RMS designations.
RMS 32 is equivalent to a very smooth machine finish.
RMS 16 is a lightly polished finish.
RMS 8 is a highly polished "mirror" finish. We recommend a minimum RMS 32 finish be specified on machineable coatings. |
| 6. The finished O.D. of the coated thermowell. As coatings are applied over a base material it is easy to become confused if the actual finished size is not specified as a finished size. For example an order calling for a 3/4" O.D. theremowell with a 1/16" per side coating can be interpreted in either 2 ways. First, it can be taken to mean the final O.D. of the completed thermowell is 3/4", or second, it can be taken to mean the thermowell is to be 3/4" O.D. before the coating is applied resulting in a finished O.D. of 7/8". |
