Mechanism of Pitting corrosion and how it affects a service material?

Alloying elements added during the production of the steel increase corrosion resistance, hardness or strength. The metals widely used as alloying elements in stainless steel include chromium, nickel and molybdenum. Stainless steels are separated as martensitic stainless steels,  ferritic stainless steels, austenitic stainless steels, duplex stainless steels and precipitation hardening steels.

Stainless steel prevents corrosion, maintains its strength at high temperatures and is easily retained, it is widely used in fields automotive, propulsion shaft for high speed craft and food processing products, medical and health equipment. The replacement of a high speed craft is lighter than the displacement of a traditional ship. The displacement factor is essential to receive quick and suitable sea transportations. High speed craft permits for the use of non-traditional ship manufacturing materials provided a safety standard similar to a conventional ship is received.

Stainless steels are used in numerous applications for offering corrosion resistance. However steels have excellent general resistance, they are not sensitive to pitting corrosion. This localized dissolution of an oxide-covered metal in specific severe conditions is one of the most common and catastrophic causes of failure of metallic structure.

Pitting corrosion is a localized accelerated dissolution of metal that occurs due to a breakdown of the security passive film on the metal surface. The mechanism of pitting corrosion is similar to crevice corrosion, dissolution of the passivating layer and regular acidification of the electrolyte due to inadequate aeration.

In the pits, a highly severe corrosive environment occurs that may tolerate nominal similarity to the bulk corrosive condition. For instance, in the pitting of stainless steels in water containing chloride, a micro- environment necessarily showing hydrochloric acid may occur in the pits. The pH in the pits reduced considerably, in combination with an increase in chloride ion concentration due to electrochemical pitting mechanisms in these systems. Pitting is usually discovered in conditions where general corrosion resistance is conferred by passive surface layers.

Localized pitting corrosion is discovered where these passive layers have damaged. Pitting corrosion caused by microbial activity like sulfate reducing bacteria required special consideration. Generally pitting corrosion in stainless steels occurs in neutral to acid solutions with chloride or ions containing chlorine.

The identification and valuable evaluation of pitting corrosion often shows an extreme challenge. Evaluating pitting corrosion can be further complicated by a distinction between the beginning and distribution phases of pitting processes. Extremely sensitive electrochemical noise method may create early alert of extreme damage by featured pit. Pitting can occur without any anticipitation and with nominal overall metal loss.

The pits may be hidden under surface deposits and other corrosion products. A small narrow pit with nominal overall metal loss can cause to the failure of an whole engineering system. Pitting corrosion is almost a common denominator of all types of a localized corrosion, may be with different shapes. Corrosion of metals and alloys by pitting comprises one of the extreme failure mechanisms. Failure caused by pits by perforation and engender stress corrosion cracking that affects the life cycle of application material.

We produce Pitting corrosion resistant alloys Inconel 718wire and Hastelloy grades for use in the severe application environments. 

Alloys for hydrocracking process in refineries

Hydrocracking is a combined desulfurization and cracking operation that can convert a complete range of hydrocarbon feedstocks into valuable products. The transformations occur in the presence of high pressure hydrogen, so hydrocracking is hydrogenation or inclusion of hydrogen to molecules. Although differences occur from plant to plant, specifically in the various reactors and arrangement of heat exchangers. Many systems have more than one reactors with large plans having a desulfurized reactor first, after a hydrocracking reactor.

The reactions occur at temperatures of 650oF to 8500oF at pressures about 1200 to 3000 psi and in the availability of a hydrogen rich recycle gas stream. After heat exchange with the hot effluent, the charge oil and recycle gas streams are combined and heated in a feed heater to the required inlet temperature. To control the temperature of the highly exothermic reactions, cool recycled quench gas is injected between the beds in the reactor. The reactor effluent is quenched, flashed and washed in the high pressure separator, flashed at low pressure and fractionated.

The corrosion issues and factors and compensatory remedies are much similar to those in hydrotreating. The stainless steels used in hydrocracking desulurizing reaction section are stainless steels 304, 321 and 410. The austenitics are recommended because of their high temperature strength and due to the problems of 885oF embrittlement with ferritic stainless steels. Although ferritic stainless steels the grades are used as tube material for feed- effluent exchangers operating below 700oF. 

The stabilized grades of Inconel bar are used to prevent sensitization particularly in the welds and the feasibility of polythionic stress corrosion cracking.

With feed and recycle gas flowing through heat exchangers, it is safe to consider that the steam temperature will be above 550oF or 288oC and the application of stainless steel is justified. The reactor effluent on the opposite side is specifically above 650oF or 343oC, so irrespective of the material types used on the feedside, the effluent needs stainless steel materials.

The tube sheets for feed effluent exchangers are made from stainless steel 304 or 321. The channel section is made of the same grades. The shell is often clad with stainless steel to prevent sulfidation from the hot effluent stream.

In the feed pipes as soon as the temperature reaches 550oF, it depends on the plant size whether it is made from wrought austenitic stainless steel or centrifugally cast HF modified. In the modification, there is often lower carbon and controlled ferrite, it offers more resistance to polythionic cracking. The wrought alloys are popularly chosen for pipes are grades 347 and 321. In the feed heater, grades 304 or 321 furnace tubes are used, even they are aluminized to reduce scaling.

Some service companies aluminize stainless steels for added security against polythionic corrosion feasibilities and to decrease high temperature sulfide scale development. This method is used by these firms in hydrotreaters and for the desulfurization stage in hydrocracking. An alloy is aluminized that is corrosion resistant, the feed effluent exchanger is used as a stabilizer feed heater. 

Premier materials for use in aircraft components construction and repair

Chrome-Nickel steels are corrosion resistant metals. The anti-corrosive degree of this material is determined by the surface condition of the metal and composition, temperature and concentration of the corrosive material. The key element of stainless steel is chromium. The corrosion resistant steels are most usually used in aircraft manufacturing.

The stainless steel can be rolled, drawn, bent or formed to any shape. As these materials expand about 50% more than mild steel and conduct heat by 40% as fast, they are more complicated to weld. Stainless steel can be used for any component of an aircraft. The common applications are in manufacturing exhaust collectors, stacks, structural and machined components, springs, castings, tie rods and control wires.

The chrome-vanadium steels are made from 18% vanadium and about 1% chromium. When heat processed, they have strength, hardness and resistance to wear and fatigue. It can be folded and flattened without signs of breaking or failure. They are used in the production of springs, balls and roller bearings.

Inconel alloys are used in aircraft exhaust systems. Inconel grades have high nickel concentration and the electrochemical test detects nickel. Uses of Inconel wire in aerospace industry are significant.

Aluminum is another one of the most commonly used metals in modern aircraft construction. It is crucial to the aviation industry as of its high strength to weight ratio and easy fabrication. The excellent property of aluminum is being lightweight. Aluminum metals at low temperature about 1250oF.

Pure aluminum has a tensile strength of 13,000 psi, however its strength may be about doubled by rolling or other cold processing methods. The combination with other metals or through heat processing, the tensile strength can be increased to 65,000 psi or to within the strength range of the structural steel.

Titanium is a widespread metal, it is used in diverse commercial units and is in constant demand for the numerous items like pumps, screens, tools and fixtures where corrosion attack is prevalent. In aircraft manufacturing and repair, titanium has major use for fuselage skins, engine shrouds, firewalls, longerons, frames, air ducts and fasteners.

Titanium is used in the production of compressor disks, spacer rings, compressor blades, vanes, turbines and liners and hardware for turbine engines. Titanium looks similar to stainless steel.

Titanium has high melting point. The corrosion resistance of titanium is featured by the development of a security surface layer of stable oxide or chemi-absorbed oxygen. The layer is usually developed in the presence of oxygen an oxidizing agents.

Monel is widely used in gears and chains to operate retractable landing gears and for the structural components subjected to corrosion. In aircraft industry, Monel 400 is used in parts requiring high strength and resistance to corrosion like exhaust manifolds and carburetor needle valves and sleeves.

In choosing substitute metals for the repair and maintenance of aircraft, it is crucial to check the suitable structural repair manual. Aircraft manufacturers design structural members to meet specific load requirements for a specific aircraft.

All steel is hot worked from the ingot into some form from which it is hot or cold processed to the finished shape. 

Strengthening of Hastelloy alloys for performance in challenging environments

High nickel corrosion resistant alloys are called as Hastelloy grades. Hastelloy alloys are not just high strength materials in fact many of them keep such as high level of their room temperature strength at very high temperatures that structural applications at high temperatures are not atypical. Their strengths are variable depending on chemistry and form however normally these alloys have tensile strengths about 100,000 psi and yield strengths of about 50,000 psi. They are used basically for applications need outstanding corrosion resistance. Modification of Hastelloy wire is performed by the fabrication by forming and welding.

Hastelloy alloy B is significant for its exceptionally high resistance to all concentrations of hydrochloric acid at temperatures up to the boiling point. It is also resistant to other non-oxidizing acids and salts and has significant high temperature properties in that it keeps over 2/3^rd of its room temperature strength at 1600oF in oxidizing conditions. It can be used at high temperatures in reducing atmospheres. It should not be used for firmly oxidizing acids or salts.

Hastelloy alloy C has an excellent resistance to oxidizing solutions, particularly those containing chlorides, and hypochlorite solutions and moist chlorine. It prevents nitric, hydrochloric and sulfuric acids at average temperatures, has outstanding resistance to diverse corrosive organic acids and salts and is resistant to oxidizing and reducing conditions up to 2000oF. It should not be used above 120oF in nitric and hydrochloric acid and salt combinations.

Hastelloy X as outstanding strength and oxidation resistance up to 2200oF and is significant for many industrial furnace applications for its resistance to oxidizing, neutral and carburizing atmospheres. It has major applications in aircraft components like jet engine tail-pipes, after-burners, turbines, blades and vanes.

The line of demarcation between heat resistant alloys and super alloys is rather tenuous as indefinite as the dividing line between ferrous and nonferrous. Single phase alloys like Nichrome and Inconel alloy 600 are weak above 1250oF however their high temperature strength and resistance to creep are enhanced by the introduction of stable, hard phase or phases i.e. precipitated carbides or intermetallic compounds. Most of the high nickel super alloys are of the Aluminum-Titanium age-hardenable type. Chromium offers oxidation resistance along with auxiliary strengthening. Columbium, molybdenum, tungsten and tantalum are usually present to provide solid solution reinforcing of the matrix. The major part of the strengthening at high temperatures, is because of the precipitation of the Ni3(Al, Ti) compound, normally designated gamma prime phase. Precipitation hardening significantly  improves the stress-rupture properties of some nickel base alloys. 

Evaluation of Fracture morphology of sintered fiber felt

The fracture morphology of stainless steel sintered fiber felt sample with 80% porosity sintered at 1000oC for 60 minutes is observed. It is found that most of the randomly distributed fibers at facture region are oriented to the tensile direction with obvious network structure loosing and necking. With high porosity and low sintering temperature, the development and growth of sintering joints in the 3D skeleton of sintered fiber felt become tough that results in low fiber to fiber bonding strength. The fiber felt are multilayer sintered 3D network material by stacking randomly laid stainless steel fibers. The sintering joints are easily damaged shear stress under tension that results into low tensile strength of sintered fiber felts. As a result, most of the randomly distributed fibers at frcture region move towards the tension direction without any obstacles owning to the sintering joints breaking.

Additionally the fracture process is slow due to the resistance coming from fibers rotation, randomly orientation fibers alignment and friction among fibers. So the sintering joints breaking by shearing off mechanism is an important failure mode for the sintered fiber felt in tension loading owning to high porosity, low sintering temperature, or short sintering time. The tensile strength of sintered fiber felt is low.

The fracture morphologies of sintered fiber felt with 75% porosity sintered at 1200oC for one hour. Necking phenomenon, network structure loosing and fibers alignment cannot be noticed.  Considering relatively low porosity, adequate sintering temperature and sintering time and the sintering joints among fibers can develop and grow successfully that result in strong interfiber joints. Fiber filament fracture occurs mainly to the joint rupture when tensile stress reaches its fracture strength.

As sintering joints are not damaged, the randomly distributed fibers cannot rotate towards the tension direction without any problems and the 3D network structure does not become loose. Tensilestrength of sintered fiber felt  is high and relatively low porosity, suitable sintering temperature and sintering time. 

Capacity of wire mesh screens to hold tension

In the world of technical screen printing, process challenges have increased widely with the passage of time. Increasing demand for the extreme levels of precision in industries like solar technology and electronics calls for the development of premium sintered wire mesh screens, we offer a wide selection of steel meshes, made particularly for screen printing applications. These materials have been developed for several years for use in leading screening applications.

Heanjia use the best raw materials for the production of fine meshes, to exacting wire diameters. Our excellent quality wire screens are woven in cleanroom conditions in a special air conditioned building, by using state-of-art looms prepared in house by our experts. For specifically challenging applications, the fine mesh screens are cleaned and tested individually. With exhaustive quality testing and process security ensure a premium value end product and full reproducibility.

Properties of Mesh screens

·         Advanced precision levels

·         Widely limited thickness tolerances

·         Stable weave

·         Above average flexural strength

·         Clean and uniform surface area

·         Outstanding abrasion resistance

·         No static loading

·         Uniform appearance without transition

·         Large open area

·         Easy to fabricate

Results

·         Suitable print quality

·         Advanced load capacity and service

·         Controlled link and paste deposit

·         Nominal loss of tension during tensioning and printing processes

·         Outstanding ink and paste flow

·         Increased service life

·         Highest registration accuracy because of higher tensioning properties

·         Enhanced contact properties

·         Rapid stability after tensioning

·         Fast commissioning of processes

Applications

·         Precision screen printing for electronics and solar cell technology

·         Advanced generation of screen printing meshes for high tensioning values. It enhances the end screens and registration accuracy

·         Advanced rigidity and high levels of stability.

·         Screen printing demanding increased color lay down, altered shape, fewer contact points

·         Economical screen printing meshes, exclusively for printing on glass and ceramics

The wire mesh screens attain the highest levels of flexural strength and the nominal percentage of elongation in terms of elastic yield point.

For printing on non-absorbent substrates, off contact distance should be used. The chosen off contact distance allows the screen directly behind squeegee to dissociate from the substrate. Only thereafter the screen separates cleanly and fully from the color, a prerequisite for a sharp print with uniform color lay down.

The required off contact distance is specified by three factors:

Mesh tension, Rheology and Squeegee speed

With higher mesh tension, ink viscosity reduces and squeegee speed reduces, hence small off contact distance is needed.

While printing with off contact distance, the mesh deforms nominally that creates a negative effect on registration accuracy.

The woven wire mesh screens are generally tensioned to higher values as compare to synthetic meshes and the regular development and refining of mesh production extends this trend. The screen should be able to handle and retain any levels of tension. For achieving high value and reproducible printed results, the screen attains high dimensional stability.

Suitably tensioned screens can attain outstandingly long service lives.