Stainless steel mesh cathodes studies for hydrogen generation

Stainless steel alloys woven mesh electrode and expanded mesh were evaluated for their suitability as cathodes in Microbial electrolysis cells. The meshes were prepared for use in the MECs by cutting metal mesh sheet in 3.8cm diameter discs with an exposed projected surface area of 7cm2. 12 SS 304 mesh of different sizes, a flat plate of SS 304 and lab made carbon cloth with a platinum catalyst.

Three electrode LSV system comprises of a working cathode, a counter electrode – platinum plate with a projected surface area of 2 cm2 and Ag/AgCl reference electrode. The performance of flat stainless steel and variety of stainless steel 304 mesh were evaluated on the basis of voltage required to initiate hydrogen production.

Active surface areas of different stainless steel mesh were determined by cyclic voltammetry by using a ferrocyanide solution. A solution of 5mM K4Fe(CN)6 containing 0.2M Na2SO4 deoxygenated with ultra high purity nitrogen for half hour was kept in the reactor shown in the LSV method with a Pt/C cathode as the counter electrode. Cathode was wet-proofed carbon cloth with a surface area of 7cm2 and platinum catalyst. The reactor was configured and filled with solution in an anaerobic glove box to prevent oxidation ferrous ion.

The MEC used for mesh comparison was a single cell cubic reactor. This reactor is developed from a solid block containing a cylindrical chamber. Anode was an ammonia processed graphite brush length.

MECs were operated in fed-batch mode. To prevent gas accumulation between mesh cathode and end plate, a part of the top of mesh was cut off and bent into the solution to assure no surface area loss. Whole gas developed at the cathode as gathered into an anaerobic gas collection tube glued to the top of the cubic reactor. The gas collection tube was sealed by a rubber stopper and aluminium crimp cap. Gas development was measured by using a respirometer.

A power source was used to apply voltage from 0.6 – 1.2 V to the MECs.  To enhance the functionality of MEC, tree types of sandwich type electrodes were used to reduce electrode spacing. Anode electrodes were heat processed carbon mesh. The cathodes were stainless steel mesh. The difference among three electrode arrangements was the separators kept between two electrodes. First type of separators was glass fiber mats with thickness of 1mm. Another type was round hole perforated plastic separator. Another type construction used double pieces of a small plastic separator stated aove to support the top and bottom of electrodes, creating an empty space between the electrodes.

The composition of MEC headspace and gas bags were tested by using gas chromatography. Nitrogen, hydrogen and methane were tested with gas chromatograph and CO2 with a separate GC. Nitrogen gas was a dilution gas and hence it as eliminated in the calculations to find the contents of H2, CH4 and CO2 developed by the system. Woven mesh electrode was found to be more effective in increasing current than expanded mesh. It was also a more effective catalyst for hydrogen evolution in MECs and used in further studies.

Mechanical strength characteristics of nickel base alloys

The potential to withstand the combined onslaught heat and corrosion makes nickel base alloys a supreme choice for aggressive elevated temperature conditions. Nickel alloys have extreme application for chemical plant systems subjected to corrosive process streams at high temperatures up to 1000oF. In various cases, high strength, chemical resistant nickel alloys are recommended, if not only, practical material for hostile conditions beyond the capability of austenitic and superaustenitic stainless steels. However costlier than iron alloys, supreme performance properties of heat and corrosion resistant nickel base alloys usually makes them a very economical choice with long term service.

Heat and corrosion resistant alloys have wide applications in chemical processing. Every alloy has specific UNS number. Inconel alloys 600, 601 and 625 are commonly used. The physical properties of nickel alloys are similar to series 300 Chromium-Nickel stainless steels. For each alloy heat conductivity and expansion properties significantly vary and should be considered in equipment design. The mechanical characteristics ofnickel alloy Monel 400 bars are extreme in strength and ductile characteristics.

At 1500oF, nickel alloys retain 45 – 75% of their room temperature yield strength whilst stainless steels only retain 20 – 35% of strength. Stainless steel lose their valuable strength at temperatures about 2000oF and higher. Nickel alloys can still perform significantly for moderately stressed parts. For instance 1000 hour rupture strength at 2000oF is about 1 ksi for Inconel alloys 600, 601 and 1.4 ksi for Inconel alloy 617.

ASME boiler and pressure vessel code contains permitted stress for the alloys inspected except for Haynes 214.

Another essential characteristic in alloy choice for high temperature applications is metallurgical stability that is also called as heat stability. It refers to resistance to develop brittle microstructural phases or precipitates upon aging that after extensive exposure at high temperatures. It is called age embrittlement manifests  basically a decreased ductility and toughness and can also impair corrosion resistance.

Some alloys such as Inconel alloy 600 and Inconel 601 are virtually immune to age embrittlement, many undergo different levels of impairment. Among those adversely influenced is Inconel alloy 625 that may experience a remarkable drop in ductility and impact strength when subjected in the range of about 1200oF – 1400oF. At higher temperatures, these characteristics are partly restored due to dissolution of brittle precipitates. System failures featured to decreased ductility and hardness are infrequent that can be ascribed to the very high beginning properties usually of unaged nickel alloys.

The most prevalent form of attack in high temperature chemical processing environment is gaseous corrosion, usually oxidation, sulfidation and halogenations. Various forms o damage occurred in extreme high temperature conditions are carburization, nitridation and hydrogen corrosion. Those are not classed as corrosion in the traditional sense of word, as there is no metal loss or surface recession. Instead, damage manifests as metallurgical or mechanical impairment – usual in the form of embrittlement.

The directional influence of alloying elements on retarding or exacerbating high temperature chemical corrosion of nickel base alloys is evaluated. The influence of chromium, molybdenum, copper, tungsten, silicon and aluminum can be either suitable or harmful, depending on the specific exposure conditions, significantly temperature and reducing versus oxidizing condition. 

Fine and coarse strainer elements for demanding screening applications

Strainers are made for high flow volumes and low pressure loss. A strainer is fitted with a flow detector, preventing the barrier of the screen insert in front of the outlet and permitting contaminants to stay on the filter bottom. Here they can be easily eradicated from the below through an offset discharge port. The strainer is prepared for the connection of 2 presure gauges, permitting to control pressure loss occurring on the screen insert. Strainer is made with different screen insert types.

Perforated

A wide range of perforation sizes are available. For easy selection, a standard perforation size is recommended that is suitable for general application of each type of strainer. A general perforation size is specified to offer the excellent balance of open area ratio, hole arrangement and gauge thickness that results in minimal pressure drop. You should refer to every strainer’ specification sheet for standard perforation size.

Where possible, a 60 degree staggered round hole order is used for its excellent strength and large open area ratio. In small perforation sizes, a straight line is used, round hole pattern that allows for a large open area ratio while not compromising gauge thickness. Generally as the hole diameter becomes smaller and the open area ratio increases, gauge thickness gets thinner.

Wire mesh

Sintered Wire mesh strainer elements are recommended for finer straining applications. Plain weave mesh with large open area ratio and nominal flow resistance is commonly used. Other types of weaves used are plain Dutch and Twilled Dutch weave. Generally with perforated straining elements, standard mesh sizes are developed fit for standard service for every type of strainer. Unsupported wire mesh straining elements are only fit for strainers under 2 inches in size, made of 20 or 30 mesh and operating with low pressure applications. For large strainers, fine mesh sizes and higher pressure applications reinforced mesh lined screens should be used.

Wire Mesh Lined

In various cases, mesh strainer elements are strengthened with heavier gauge, perforated metal to offer additional support. Perforated metal backing 5/32 inch offers outstanding support without considerably reducing the open area ratio.

Considerable factors

Purpose:  If a strainer is used for protection rather than direct filtration, standard screens will suffice in various applications.

Service:  With services that need very durable screens like high pressure and temperature applications or services with high viscosities, perforated screens without mesh liners are preferred. If a mesh liner is needed to receive a specific level of filtration, a trapped mesh combination is used.

Filtration level: While choosing a perf or mesh combination, focus should be on to prevent overstraining. As per a general rule, the specific filtration level should not be lower than half of the size of particle to be eliminated. If very fine filtration is needed, the pressure drop through the strainer will increase quickly, feasibly causing damage to the screen.

Very fine mesh sizes such as of 5 micron and coarsed perforated plate of ½ inch dia offer innovative solutions. 

Efficient and economical Hydrogen production by using Mesh cathodes

Fossil fuels such as oil, coal and natural gas supply 85% of World’s energy consumption. Considering the oil market report from International Energy Agency, Global oil product demand increased by a robust 2.5% - 88.2% mb/d in 2008. The large demand has come closer than ever to exceeding world’s known production ability. if oil production remains constant until is vanished, it is anticipated that it will last for another 42 years. Similarly there is sufficient of natural gas to serve the demand for 61 years and coal for 133 years. Everyone realizes that fossil fuel will become scarce and costly in the nearly years.

In addition of petroleum crisis, increased use of fossil fuel resulted into higher release of carbon dioxide, a major component of green house gas. Increasing magnitudes of greenhouse gases in atmosphere are resulting into increase in global temperatures, with potentially harmful results for the environment and human health. Inevitably a conversion to sustainable energy sources is forthcoming. Alternative energy sources are renewable such as wind, solar, geothermal, hydroelectric and biomass. As compare to conventional energy sources, they have lower carbon emissions. Until now, only hydroelectricity and nuclear power have been major alternatives to fossil fuels.

Hydrogen is the most abundantly found element on the earth and is present in infinite magnitudes. Hydrogen gas has one of the highest energy density values. Presently many people claim a hydrogen economy that is based on using hydrogen as an energy carrier.  Hydrogen fuel cell combines hydrogen and oxygen chemically to develop electricity, water and waste heat and hence it doesn’t create pollution. Fuel cells are costlier to produce as compare to standard internal combustion engines however with new technologies and production systems, they are becoming more cost-effective.  

Hydrogen gas can be developed in various ways from hydrogen containing compounds like water, biomass and fossil fuel. Presently commercial bulk hydrogen is developed from natural gas through steam reforming. Renewable methods of sustainable hydrogen production include water electrolysis and biological processes for example biophotolysis, photo or dark fermentation. Only 4% hydrogen is produced from water splitting by electricity received from different sources at a standard energy efficiency about 56 to 73%. The efficiency of biophotolysis conducted by algae and photosynthetic bacteria is low and needs large surface area for the process. Hydrogen can be developed from different types of biomass such as carbohydrates for example glucose and polysaccharides.

Microbial electrolysis cells offer a suitable method to develop hydrogen from renewable biomass and wastewater. To create an economical and highly efficient mesh cathode, technology advancement is required. Considering the factor that stainless steel is more economical than platinum as catalyst, stainless steel mesh with larger surface areas than flat sheets are used as cathodes in the Microbial electrolysis cells. Analysis shows that mesh could have about three times active surface area than flat sheet. The relative position of mesh in correspondence of current density in MEC is in agreement with linear voltammetry studies at small bubble coverage. Mesh cathodes widely promise for the hydrogen development at low cost.

Combustion wire spray process for corrosion protection

Combustion wire spray process uses a set of drive rolls powered by an air turbine or an electric motor to draw a metal alloy wire through combustion spray gun. At the gun nozzle, fuel gas of acetylene, propane or MAPP is combined with oxygen in precise volumetric proportions by using a siphon plug and ignited to develop a flame that is then shaped at the gun’s air cap by compressed air. The metal wire is fed concentrically in the flame, melted and atomized by the compressed air and molten drops are propelled towards a prepared surface where they solidify and bond to the substrate to develop a coating.

Combustion wire spray is a wide choice for machine element repair and corrosion coatings. Hard or soft wires can be used.

Characteristics of combustion wire spray process

·         A range of alloys and pure metals for restoration, corrosion coatings and other applications

·         Installations from economical manual systems to fully automated production systems

·         Portable for onsite coatings

·         High spray rates with low gas use

·         Coatings can be machined to final dimensions and finish

·         Covering areas where coating is possible

·         Coating of internal shapes

·         Simple to use and maintain

Process

An electric arc spray wire process involves the use of two metallic wires, often the same composition. Both wires are electrically charged with opposing polarity and are supplied into the arc gun at matched and controlled speeds. When the wires are placed together at the contact location, the opposite charges on the wires produce sufficient heat to continuously melt the wire tips. Compressed air atomizes the molten material and accelerates it on the sample surface to produce the coating.

In electric arc wire spray, the coating weight that can be accumulated per unit of time is a function of the electrical power of the system and density and melting point of wire. Considering the columnar strength of the wire, the push, pull mechanisms can be used to feed the wire at a constant rate.

Attributes of electric arc wire spray

A range of alloys and pure metals for restoration, corrosion coatings and other purposes in solid wire or cored wire

Compact and self-contained systems

Outstanding portability for on-site coatings

Does not need any process water or gases except compressed air

High spray rates

Develops coatings that are easy to machine

Coating of internal shapes

Simple to use

Thermal spray wires have been available for decades. The wires work in multitude of applications successfully. Wire technology has revolutionized double wire arc spray. Cored wires are developed by creating a thin strip of metal in a wire, at one point in the process material is added to the center of the strip as it is being formed, the material can be metallic elements or non-metallic like carbides. Thermal spray wire materials have a proven history of excellent spray ability and are treated to improve feed ability and productivity.

How fretting occurs on metal and its effects

To design and manufacturing practices, a clearance between tube and tube supporting device in steam generators and heat exchangers is needed. Vibration in these tubes makes them sensitive to impacting and rubbing with the supporting system or adjacent tubes resulting in local wear damage. The tube vibration may be activated by cross-flow perpendicular to the tube centerline and longitudinal flow along the tube or tubes. Although the real flow in the practical conditions is mixed, hence the tube oscillates in different directions causing some type of combined sliding and impact motion between the tube and supporting equipment and feasibility between adjacent tubes.

Considering the vibratory nature of the impact and rubbing motions, the damage is usually accounted a result of fretting, however fretting in its ordinary manner is featured by nominal reciprocating motion between the wear materials held together by a normal force.

Fretting corrosion was corrosion to include a chemical factor, oxidation and mechanical factor, welding and shearing of metal asperities. However it has been discovered that a corrosive media is not essentially for fretting to occur and that some materials that do not oxidize do fret. Three mechanisms by which fretting corrosion can arise-

a.       Eradication of metallic particles by grinding or by the development of welds at the points of contact after tearing. Subsequent oxidation of the particles is assumed to have no contribution in causing wear.

b.      The elimination of metal particles that subsequently oxidized develop an abrasive powder. An abrasive action is then considered to be more severe factor causing wear.

c.       Direct metal oxidation and continuous removal of oxide layer by the scraping of one surface over the other.

The mechanism of fretting refers that fretting is three stage process. At first, a surface developed oxide layer prevents metallic interaction, it is distributed by an oscillatory motion, then adhesion, plastic deformation and metal flow occurs. The transferred particles can get oxidized and dislodged to become discrete wear particles or the moved particles can create into surface forming a moderate zone, partially oxidized surface region preventing further transfer, the fretting action then develops loose wear particles. Eventually a steady state reaches that is attributed by a general disintegration and dispersal of zones influenced by the early stages. Shortly, three stages are adhesion and metal transfer, development of oxidized debris and eventually attainment of steady wear rate.

Damage caused by fretting varies from discoloration of the mating surface to the damage of large magnitude of materials. The frequency, total count of cycles, amplitude of motion, normal pressure and physical properties of interacting materials and environmental conditions all add to the results. The slip amplitude is normally considered as the major parameters have an impact on fretting.

Monel 400 wire and tube are found to be more resistant to fretting wear as compare to plain steel. The wear rate by oxidized wear debris developed on the damaged surface is more severe and it reduces with time. 

Corrosion Performance of alloys in Various Acid Mixtures

In various processes, mixtures of various acids or acids and salts occur. Corrosion resistance in these conditions is sometimes predictable qualitatively. In some condition, anomalous effects can be developed. However, it is impossible to mention the corrosion rates of alloys in several acid mixtures within the constraints.

The corrosion resistant wrought nickel alloy families include commercially pure nickel, nickel-copper alloys, nickel – chromium – molybdenum alloys and nickel – chromium – iron – molybdenum alloys. Similar alloys categories are feasible for cast alloys. Nickel and nickel alloys for example stainless steels offer an extreme level of corrosion resistance. Although nickel can accommodate larger magnitudes of alloying elements, mainly chromium, molybdenum, copper and tungsten in solid solution comprising of iron. So nickel base alloys can be used in more vigorous environments and offer supreme resistance to general corrosion, pitting, crevice and intergranular corrosion and stress corrosion cracking.

Sulfuric and Nitric acid solution – Alloys comprising of chromium and attain active passive behavior, inclusion of nitric acid or nitrates to sulfuric acid will decrease the corrosion rate. In nonchromium alloys such as Hastelloy B2 and Monel alloy 400, inclusion of nitric acid will increase the corrosion rates. The nitrate reduction reaction enhances the redox potential in sulfuric acid solution, the redox potential in sulfuric acid solution, the redox potential of sulfuric acid solution is controlled by hydrogen ion reduction reaction. In nonpassivating alloys for instance, Hastelloy B2 in which the corrosion current increases monotonically with potential, increase in potential increases the corrosion rate. In passivating alloys, increase in potential can move the alloy from active state to the passive state, hence decreasing the corrosion rate. In high nitrate concentrations, the passive current density increases that increases the corrosion rate. For alloy C76, an increase in corrosion rate is only observed with nitric acid addition. It is feasible that for lower concentrations of HNO3 a reduction in corrosion rate could be noticed.

Sulfuric acid and hydrochloric acid-Inclusion of alkali chloride salts or hydrochloric acid to sulfuric acid increases the corrosion rates of all alloys. In deaerated conditions, Hastelloy B and Hastelloy B2 are the most versatile with alloys C276 and HastelloyC22 bar. Normally the higher molybdenum concentration offers the better performance of alloy in mixture of sulfuric acid and hydrochloric acid.

Nitric acid and Hydrochloric acid mixtures- The influence of nitric acid to hydrochloric acid are similar to influence of nitric acid to sulfuric acid. Although in HNO3 + HCl mixtures, pitting causes corrosion, instead uniform corrosion that occurs in mixture of sulfuric acid and nitric acid. Additionally, nominal variations in HCl content can create wide changes in corrosion rates.

Nitric acid and Hydrofluoric acid mixtures – The inclusion of nitric acid to hydrofluoric acid decreases the corrosion rate initially however above 10% HNO3, the corrosion rate increases. Increasing HF content results into increased corrosion rate. Although unlike to HCl inclusion, higher chromium alloys normally showed nominal rates, irrespective of molybdenum concentration. Intergranular corrosion was also noticed in various alloys. In these cases, higher temperature resulted into increase in corrosion rates.