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내 용	1. INTRODUCTION Most equipment of reactor column or similar equipment in petro-chemical plant is equipped with internal trays which is using for scrubber function of oil refinery system. And the material of internals is of Carbon Steel or Stainless Steel to be decided by its life cycle of endurance. When this kind of internal material is of Carbon Steel.It should be necessary in cosidering how to connect internals and what should be of bolt material regardless of its kind of internal material because of metallurgy corrosion. Generally, material corrosion in petro-chemical occurs in major of two corrosion manner. The first one is of ionization of Ferrous material(Fe+2 + 2OH- -->FeOH2) Secondly, galvanic corrosion will be followed as various erosion and corrosion condition. Here, the characteristic of galvanic corrosion is generally described as Anode reaction which occurs between two different material, temperature and acidity in its potential difference. Galvanic corrosion take places in lower electric potential as high temperature and low acidity etc. When we place Carbon Steel into a smaller of surface area adjacent to Stainless Steel, Carbon Steel will be of Anode reaction against Cathode reaction of Stainless Steel, In this case we call this corrosion as galvanic corrosion. Therefore, The surface area of Carbon Steel should be larger than its Stainless Steel otherwise, change Carbon Steel material to same as Stainless Steel. In this theory and practice, how about in case of same ferrous material which chemical composition is little be different and moreover, its surface area is much different. Clearly smaller surface of material will be corroded as anode reaction side and the other side will be of anti-corrosion. For example, we can see that the electric potential of welding rod is more high against of base material due to considering this galvanic corrosion. Therefore, when we select any kind of material in using petro-chemical plant especially,it is necessary for consideration of this galvanic corrosion along with mechanical properties even though their equipment system is well protected with cathodic protection system. 2. BOLT AND TRAY MATERIAL CORROSION When we use bolt material with Carbon Steel or Stainless Steel material in reactor column It can not be avoid any kind of corrosion problems after long-run plant operation regardless of its used material because the surface area of bolt material is much smaller than internals. And those cause galvanic corrosion. Moreover, Stainless Steel means that its corroded rateis less than the other material like as Carbon steel or Alloy steel, It must be subject to corrosion such as Pitting, Erosion, Cavitation corrosion, Intergranular corrosion And galvanic corrosion as mentioned in previous introduction. Therefore, using of bolt material as fastener between two segment of internal piece of tray may not be disassembled or dismantled when it come to seriously corroded. Therefore, when Carbon Steel of Tray material corroded for life cycle years operation Its corrosion allowance was given for 4.5mm to Tray of Carbon Steel material refering to their engineering criterior we need to review that whether it be possible to disassemble corroded Tray material with power tool which is a practice dismantling method without flame cutting or other method. 3. THE STICKING BETWEEN BOLT AND TRAY As described in previous, Para2, Carbon steel would be corrosded as much as given corrosion allowance rather than Stainless Steel. In apractice, the corrosion rate of Carbon Steel is of 0.1mm per year, thus would come to 3mm corroded after 30 years of plant operation. And this corrosion occurs sticking problems between clearance between two metal. In a metallurgy, if there keeping clearance of 10^-6 mm, Two part of metals a complete bond condition without metal fusion. This is what we call as "metal sticking" 4. RESULTS OF TORQUE CALCULATION As results of bolt torque calculation comparing between available bolt torque and required bolt tightening or disassembly, The required bolt torque value of tightening or disassembly is nearly two times available torque value. Therefore, It is impossible to disassemble Hex bolt with power tool otherwise bolt body would be broken. And if bolt have an enough strong mechanical properties, Nut may not be turning because bolt torque in surface between Bolt and Tray is same as required tightening or disassembly torque, or will be spinning around itself.5. CONCLUSIONAs mentioned in previous article, the required material should consider metallurgy corrosion because required mechanical properties of Bolt is not much important for Bolt to connect tray than its corrosion problems considering the duration of plant operation. For example, as per ASTM A193, for each grade of Bolt material have a following order of tensile strength B5 > B6(410S) > B7 > B7M > B8(304S) > B8M(316S) However, metallurgy corrosion resistance is different order against tensile strength as follows. B5 < B6(410S) <B7 < B7M < B8(304S) < B8M(316S)Therefore, if available bolt should be decided considering anti-corrosion properties of material, The bolt of tray connection should be of ASTM A193Gr.B8 or A193Gr.B8M even if no more required material properties is on Tray Bolt. Prepared By Ryu-ChangMyong/Professional Engineer in Mechanical http://www.pes21.com 기술사 류창명/피이에스기술사사무소 대표 건설기계기술사/산업기계설비기술사
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