Stainless steels-The core application material of industries

Iron and iron alloy such as steel are poor materials as they rust in air, corrode in acids and scale in furnace conditions. There is a group of iron-base alloys, the iron-chromium-nickel alloys are called as stainless steels, that do not rust in sea water, are resistant to concentrated acids and that do not scale at temperatures up to 1100oC.

Good mechanical properties and manufacturing features give the stainless steels their raison and make them vital tool for the designer. The usage of stainless steel is nominal as compared to carbon steels that attain a steady growth, unlike to the constructional steels.

Stainless steels as a family is certainly more non-uniform than the constructional steels and their properties are in diverse cases relatively unfamiliar to the designer. In some ways, the stainless steels are an undetermined world however to take the benefit of these materials will need a better understanding of their basic properties.

Use of stainless steel

Steel is the major industrial constructional material. Dominant product form for stainless steels is cold rolled sheet. The applications are dominated by major sectors: consumer products, equipment for oil and gas plants, chemical process plants, food and beverage industry.

The commonly used stainless steel grades are SS 304 and SS 304l that form more than 50% of the global production of stainless steel. In order to receive a perspective of the development of stainless steels, it is right to consider the background.

Alloying elements have a specific influence on the properties of the steel. It is the combined influence of the allying elements, to some level, the contaminants that determine the feature of a specific steel grade. The influence of alloying elements on the important material properties is evaluated.

The most important feature of stainless steel is their corrosion resistance. The reason for the good corrosion resistance of stainless steels is that they develop a thin, invisible surface film in oxidizing conditions.

Aqueous corrosion refers to corrosion in liquids or moist conditions at temperatures up to 300oC, often in water-based conditions. The corrosion process is electrochemical and needs the presence of an electrolyte in the form of a liquid.

High temperature corrosion allows corrosion in hot gases at temperatures up to 1300oC.

Stainless steels are significantly different from carbon steels in some respects. There are also significant differences between the different categories of stainless steels.  Heat conductivity for stainless steels is usually lower than for carbons steels and reduces with increasing alloying level for every stainless steel category.

Austenitic steels are featured by supreme corrosion resistance, good toughness and excellent weldability, they are the most common stainless steels. The properties of Hastelloy wire grades include resistance to general corrosion, pitting and crevice corrosion and stress corrosion cracking. Low carbon grades attain supreme resistance to intergranular corrosion and as a result higher alloyed steels are only available with low carbon concentrations. Austenitic stainless steels are used in all types of applications and industries. Common areas include piping systems, heat exchangers, tanks and process vessels for food, chemical, pharmaceutical, pulp and paper and other process plants.

Durable sintered mesh elements for filter cartridges and methods to clean them

Filter cartridges made from sintered wire mesh are suitable for applications that need a robust filter cartridge resistant to extreme chemicals. They are made in varied sizes to suit industry standard filter structures even also in diverse non-standard sizes and specifications. The screens made from 316L grade stainless steel and configured without using resins or adhesives.

Sintered stainless steel mesh filter cartridges use sintered mesh structure for filter configuration that combines filter efficiency with strength. Standard mesh construction would consist of an external protective layer, an internal main filter layer, drainage layer and eventually two strengthening layers to offer strength. The fiver mesh layers are sintered to develop a single sheet of filter media.

The mesh cartridges can be in cylindrical or pleated form and in different lengths, diameters and endcap configurations.

How to clean sintered mesh filter cartridges

Reverse Flow: When much of contamination is larger than the pore size of the filter media, reverse flow of liquid or gas through the element will often be sufficient for cleaning. Usually, a flow of minimum two times the forward flow gives complete cleaning.

Ultrasonic cleaning: Surface contamination can be eliminated by ultrasonic cleaning in bath containing detergent, immersed particulate may not be eliminated.

Chemical cleaning: Different chemicals are used to dissolve the contaminant or process fluid in case of hardening on contact with air.

Heanjia can provide you with a wide set of sintered mesh elements used in filtration. It is crucial in the development of synthetic fibers and we can eventually enhance and innovate your conventional filtration. The metal to metal gaskets used in the polymer industry by the diversity refers to the production and quality of our tools. The screens can be made in any shape or dimensions available in different alloys.

We offer a solution allowing a reduced consumption of filters and an enhanced quality of filtered polymer. We can offer a wide range of material to be used as filling sands in the stainless steel, Inconel, aluminium and other high performance materials.

We are specialized in the development and production of metal filtration solutions. We offer filter elements for industrial gas filtration, depending on the special benefits of metal fiber media. Our filter elements are fit for use in high temperature, corrosive chemical and petrochemical process, for process filtration and exhaust gases. These elements are the perfect solution for applications demanding high mechanical stability, long service life, low pressure drop, easy cleaning and chemical and heat resistant are needed.

Filter materials are used for temperature resistance up to 1000oC. Filtration of elements in applications where small dust loads are separated from a gas stream. The contaminants are held in multiple layer structure of the filter medium. Advantage of our depth filter systems that combine high temperature and corrosion resistance with high dirt retaining capacity and outstanding off-line cleaning feasibilities, the excellent solution for your applications. 

How the use of arc spray coatings improve corrosion protection of a component

Coatings are developed to offer protection from corrosion and erosion to secure the material from chemical and physical interaction with its environment. Corrosion and wear issues are of great relevance in diverse industrial applications as they result in the degradation and eventual failure of components and systems in the processing and manufacturing industries and in the service life of various components. Different technologies are used to deposit the suitable surface protection to provide protection under specific conditions. They are often distinguished by coating thickness.

Most thermal coating processes are used at atmospheric pressure in air, except plasma spraying, usually served in soft vacuum. Plasma spraying can be performed in an inert condition or vacuum and cold spray is usually performed at atmospheric pressure however in a controlled condition chamber to collect reuse the spray gas due to the large gas flow rates required.

Extreme uses of arc spray wire are now in culture against wear and corrosion as well as heat and also for functional purpose. The choice of coating process is firmly based on the required coating properties for the application and coating cost. Coating characteristics are determined by the coating material and the form in which it is used, as well as the setoff parameters used to conduct the coating process. Thermal spray coatings are usually featured by a lamellar structure and the real contact between the splats and substrate or the earlier deposited layers determine to a large level of the coating properties like heat conductivity, Young modulus etc.

The real contact area ranges between 20-60% of the coating surface parallel to the substrate. It increases with impact speed of particles. Therefore the coating density increases with increase from flame, wire arc, plasma and gun spraying and thereafter re-fused.

The variety of corrosions particularly for coatings can be categorized as general corrosion, related to 30% failure, where the average rate of corrosion on the surface is uniform and localized corrosion, about 70% of failures. The galvanic corrosion occurs when two different metals are in contact with each other in a conductive solution, the more anodic metal is attacked where each other in a conductive solution, the more anodic metal is attacked, while the more cathodic one is uninfluenced. The electrolyte plays a significant role, and the relative surface contact area, small anodic to cathodic area ratio refers to severity of anodic metal corrosion.

The corrosion can also be intergranular as well as transgranular when cracking occurs. The coating material and is microstructure plays a crucial role in this type of corrosion. The coatings provide protection from corrosion, sacrificial coatings, thicker coatings offer longer protection.

Corrosive wear occurs when the influence of corrosion and wear are combined, resulting into faster failure of material surface. A surface that is oxidized can be mechanically weakened and can wear at a higher rate. Stress corrosion failure results from the combined influence of stress and corrosion. With thermal spray coatings, such types of metallic failures can be significantly reduced. 

Performance of Nickel based super alloys in industrial chloride conditions

The commonly accepted application of commercially pure nickel is to handle the highly concentrated solutions. Nickel shows lower corrosion rates in hot caustic solutions than alloyed nickel as alloying elements like chromium and molybdenum dissolve commonly from alloy in hot caustic solutions. Nickel can also withstand cold reducing acids due to slow discharge of hydrogen on its surface. Hot reducing acids and oxidizing acids quickly attack pure nickel. 

The key application of Monel bars is in handling pure hydrofluoric acid. Although if oxidants like oxygen exist in hydrofluoric acid, Monel alloys may experience intergranular attack. Monel alloys are slightly more resistant to general corrosion than Nickel 200 in hot reducing and oxidizing acids like sulfuric acid and nitric acid. Ni-Mo alloys usually called as Hastelloy B type alloys, are made to withstand reducing HCl at all concentration and temperature limits. As costlier materials like Hastelloy alloys are also used in handling other corrosive reducing conditions like dilute sulfuric, acetic, formic and hydrofluoric acids. Alloy B2 has the minimum corrosion rate in boiling 10% sulfuric acid. Although Hastelloy alloys show poor performance in oxidizing acids, for instance, in hydrochloric acid contaminated with ferric ions.

There are several commercially available Ni-Cr-Mo alloys. They are derived from original C alloy, the advanced grade is Hastelloy C2000. Although the more common grade in industrial applications is Hastelloy C276. NiCrMo alloys are the most versatile nickel alloys as they comprise of molybdenum for protection against corrosion under reducing conditions and chromium that secures the component from corrosion in oxidizing conditions.

Hastelloy C276 has nominal corrosion rates in reducing and oxidizing conditions. One of the major applications of NiCrMo alloys is in the presence of hot chloride containing solutions. In these environments, most of steel grades receive crevice and pitting corrosion as well as stress corrosion cracking. Although NiCrMO alloys are extremely resistant it is not immune to chloride induced corrosion in major industrial applications.

Nickel based corrosion resistant alloys are NiCrFe alloys. They also contain smaller magnitudes or molybdenum and copper as in Incoloy 825. Nickel-Chromium-Iron alloys are usually less resistant to corrosion as compare to Nickel-Chromium-Molybdenum alloys, although they could be less costly and hence find a great range of industrial applications where the application of stainless steels is limited. The corrosion rate of Inconel 600 in sulfuric acid is higher than corrosion rate of Incoloy 825 as grade 825 contains nominal magnitudes of molybdenum and copper that are advantageous alloying elements for resistance to sulfuric acid. Incoloy 825 has nominal corrosion rate in nitric acid as it comprises of larger magnitudes of chromium. The common applications of Ni-Cr-Fe-Mo alloys like Hastelloy G30 is in the industrial development of phosphoric acid and in highly oxidizing conditions like nitric acid.

Cold processed Nickel 200 is resistant to cracking in NaCl and in chloride concentration of CaCl2 and MgCl2 at 121oC, 149oC, 177oC, 204oC and 232oC. 

Corrosion resistance behavior of nickel super alloys in caustic solutions

Nickel based super alloys are widely used in the chemical plants in the targeted applications. For instance, applications of Hastelloywire grade B2 is in handling hot reducing acids as it offers nominal corrosion rates in this media. Commercial nickel grade Ni 200 is used to handle hot caustic solutions. Other grades of Hastelloy family such as C276 containing Ni, Cr and Mo are versatile and can be used in almost each condition, although their function in hot reducing acids would be lower than Nickel-Molybdenum alloys and in hot caustic it would have a higher corrosion rate as compare to Nickel 200. Unlike austenitic stainless steels, nickel alloys prevent stress corrosion cracking in hot chloride conditions. Although nickel alloys may experience stress corrosion cracking in conditions of hot caustic and dilute hydrofluoric acid conditions.

Caustic Conditions

Caustic conditions involve highly concentrated solutions of sodium hydroxide or caustic soda, potassium hydroxide or caustic potash and calcium hydroxide or caustic lime that may be seen in the industries of oil refineries, pulp and paper. It is likely that the cracking sensitivity of Nickel alloys is related to a dealloying phenomenon.

Cross section of 0.6mm thick sheet of C276 that was in use for ten months in a heat exchanger between water and 50% NaOH and traces of perchlorate at temperatures about 100oC. Cracking occurred in the dealloyed layer subjected to the caustic solution. The supreme performing material in caustic conditions is commercially pure Nickel. Large magnitudes of molybdenum in nickel alloys are detrimental and chromium seems to be an advantageous element in high concentrations. During dealloying chromium and molybdenum dissolve leaving behind a porous pure nickel layer even also the alloy is plated on the surface with pure nickel. In slow strain rate conditions, Hastelloy C276 was prone to transgranular cracking in 50% NaOH at 147oC. Mill annealed and aged for 24 hour at 677oC, Hastelloy C22 resist cracking when kept in 50% NaOH solution at 147oC for 720 hours.

Inconel 600 experiences stress corrosion cracking in hot caustic solutions. Lab SCC test was conducted by using cylindrical slow strain rate samples and spring loaded bend beam samples of grades 600 and 800 in deaerated 10% sodium hydroxide solution at 550oF. Stress corrosion racking was noticed in both alloys, although alloy 600 offered better resistant to cracking as compare to alloy 800, feasibly due to higher nickel concentration. It is found that resistance to stress corrosion cracking increases with magnitude of Nickel in tan alloy, however there was an extensive variation in results depending on the hydroxide concentration and temperature. 

Machining of Nickel base super alloys

Machining of Nickel alloys should be done carefully by using sharp tools with positive rake angles. Adequate feed rate and depth of cut are essential and tools should be controlled to prevent rubbing. Even in the supreme conditions, stress can occur that may cause distortion of the work. For the highest dimensional stability, it is recommended to rough the part to size, stress relieve it and then finish it to size. Stress relieving has nominal influence on shapes, however may influence mechanical properties.

Category of alloys

Group A: Alloys comprise of 95% or more nickel. They have average mechanical strength and high hardness. They are hardened by cold processing. The alloys are gummy in the annealed and hot processed condition and cold processed material is preferred for the supreme machinability and smooth finish.

Group B: Comprises of standard nickel-copper alloys. They have higher strength and nominally lower hardness than those in group A. They are only hardened by cold processing. Cold drawn and stress relieved material provides the supreme machinability and smooth finish.

Group C: Comprises of solid solution nickel-chromium-iron alloys that are similar to the austenitic stainless steels. They are only hardened by cold processing and are machined readily in the cold-drawn or cold-drawn and stress relieved condition. These alloys are Inconel 600 wire, 601, Incoloy 800, 825 and Monel K500.

Group D: Comprises mainly of the age hardenable alloys. Group D1 comprises of alloys in the unaged condition. Group D2 comprises of alloys of group D1 in the aged condition, and many other alloys in aged and unaged conditions.

Cutting Fluid

Any cutting fluid can be used in machining nickel alloys. They respond well to general sulfurized mineral oil, sulfur offer enhanced lubricity and anti-weld properties. If the temperature of oil and work material is sufficient during machining to result in brown sulfur staining of the material. The stain can be removed with a cleaning solution of the sodium cyanide. It should be performed before heat processing including welding due to exposure to high temperature the staining may cause intergranular surface corrosion. To prevent intergranular corrosion, the components should be immersed in cleaning solution for sufficiently long time to prevent the stain. High speed machining operations create high temperatures that preclude the use of a sulfurized oil because of sulfur embrittlement of carbide tools. 

Materials for Solid fuel oxide cell for performance at high temperatures

Solid fuel oxide cell serves at high temperature with fuel like hydrogen gas or reformed natural gas on the anode side and air on the cathode zone. Moisture could occur on both electrodes hence in contact with metallic interconnects. Sulfur contaminants present in the fuel gas contact with the metallic interconnects. Sulfur contaminants present in the fuel gas stream are also anticipated to occur, however upstream desulfurization has been applied to reduce the sulfur contamination level to sub ppm or ppb levels. Hence, besides oxidation, interconnect could also experience sulfidation, hot corrosion and carburization. 

Thermal stresses produced in the SOFC stack because of large temperature gradients across the current collector could also speed up the corrosion process due to premature cracking and spallation of the oxide layer. The availability of complex gaseous species in the fuel condition also cause establishment of grain boundary corrosion, internal oxidation and localized metal loss causing overall reduction of component service life. Sulfidation refers to vigorous corrosion resulting into combined effects of oxidation and reactions with sulfur that may present in the fuel gas streams.

The metallic interconnect is also needed to have sufficient strength to help maintain the structural integrity of the stack during Solid oxide fuel cell service at high temperatures and under thermal cycling. The high temperature alloys for interconnect should have thermal fatigue resistance against feasible structure fracture during thermal cycling, creep resistance to maintain the size stability at high service temperature and rupture resistance to withstand peak thermal stresses produced during SOFC operation. The above stated strengths can be more or less correlated to the yield strength. For stainless steels, the compositions with higher yield strength often possess high creep and fatigue strengths.

Many alloys except annealed low carbon steels do not have standard yield strains, the stress is referred as yield strength. When feasible, the yield strength from bar analyses at room temperature and high temperature was gathered.

Nickel based superalloys

Depending on the ratio of chromium and aluminum, nickel based super alloys are classified containing Cr and Al into these categories:

A NiO scale with Cr2O3 and Al2O3 internal oxides for low chromium and aluminum concentrations.

An Cr2O3 scale with Al2O3 internal oxides for high Cr above 15% however low Al below 3%. An exclusive alpha- Al2O3 scale for considerably high chromium above 15% and high aluminum above 3%. The presence of an inner layer drastically decreases the local oxygen activity at the metal –interface so that an enrichment of alumina particles occurs. Alumina layer develops below the Cr3O3 inner layer that significantly improves the oxidation resistance and also acts as an electrical insulating layer. So, an aluminum concentration of 3% was established to be critical maximum. But none of used nickel base alloys contained chromium content higher than 18% and aluminum higher than 3%. Recommended nickel superalloys Inconel bars for use are Inconel 625.

Stainless steels

Stainless steels are popular for their oxidation resistance. They contain the compositions to withstand all temperatures and hence do not strengthening by heat processing although a few grades need. Austenitic steels are featured by larger linear thermal expansion coefficients.