The local primary membrane stress (PL) is the stress generated due to a primary type load such as weight or pressure, but that is localized around a discontinuity such as a nozzle in a vessel.
According to ASME sec VIII div-2 part 5, A region of stress in a component is considered local if the distance over which the equivalent stress exceeds 1.1S does not extend in the meridional direction more than (Rt)0.5 where R and t are radius and thickness respectively.
Sometimes we face a big challenge to deal with these local stresses. Although they are very local, they are more than allowable. In these cases, the use of the elastic-plastic method instead of elastic analysis may help us to get rid of such stress categorizations.
Let’s take a look at the below example to elaborate more. As you can see there is a simple unreinforced lateral connection with a header of 1524mm OD and 16.05mm as the thickness. The OD of the branch is 1219.2 mm with a thickness of 9.7mm.
Fig 1. Mesh Model of the unreinforced connection
A 0.36 MPa Internal Pressure is the only load that would be applied to this connection and it is assumed that the working temperature is 3100 C with allowable stress of 119 MPa. The material is assumed A106-B.
Shell elements and elastic methods have been used to perform FE analysis for this connection. The below figure shows the applied load and Boundary conditions.
Fig 2. Load and Boundary Conditions
Two Fixed BCs have been applied to both sides of the header. The internal pressure load has been shown with red arrows. In the below figure, the contour of PL has been shown.
Fig 3. Stress contour of PL for the unreinforced connection
The maximum PL stress is about 205 MPa and has been located on the branch side next to the header weld, as it is expected.
The below figure reveals how local the highest stresses are. However, they have exceeded the allowable which is 1.5S or 179 MPa.
Fig 4. Extent of the highest PL stresses for the unreinforced connection
As you can see the extent of the highest stress zone that exceeds 1.1S (131 MPa) is much lower than (Rt)0.5 of the branch.
It seems that a reinforcement pad will solve this problem. To investigate how it works, a new geometry that is the same as the previous one and also includes a reinforcement pad has been analyzed. The width and thickness of the pad are 500 mm and 16.05 mm respectively.
Fig 5. Mesh Model of the reinforced connection
Now you can see the results. The maximum PL stress has changed to 153 MPa.
Fig 6. Stress contour of PL for the reinforced connection
Therefore, when a reinforcement pad is used the PL stress will be in the allowed range.
Also, the below figures show the extent of the highest PL stresses that is obviously much lower than (Rt)0.5 on both branch and header sides.
Fig 7. Extent of highest PL stresses for the reinforced connection
Another way to investigate this connection is the method that has been mentioned in para. 304.3.3 of the B31.3 code is about the reinforcement of welded branch connections. Following the mentioned rules reveals that using a pad reinforcement is not needed. Indeed, the basis for pressure design in this paragraph is to ensure excess area at discontinuity to account for material removed from the nozzle internally. This method does not attempt to discern stress at the discontinuity as well as some plasticity that may occur at design pressure.
But now the question is how we can justify this discrepancy between elastic FEA and the method mentioned in para. 304.3.3 of B31.3.
In the last step, an elastic-plastic method is used to study this connection without a reinforcement pad again. Diving into plastic analysis is not the intent of this short article. However, some of the assumptions are as follows.
The ASME sec VIII-2 part 3 multilinear stress-strain curve has been used. The material type is Ferritic steel while the amount of yield stress and tensile strength at working temperature is 177 MPa and 413.7 MPa respectively.
Since the convergence is achieved, there will be no collapse failure. Also, the amount of the highest plastic strain is about 0.08 and as it is expected has been located on the branch side. The below figure shows that the highest plastic strains are very local.
Fig 8. Plastic Strains distribution
The local strain limit criteria (ASME VIII-2) have been also passed and the maximum ratio of plastic strain over limiting triaxial strain ɛpeq/ɛL is about 0.36 which is lower than 1.
Fig 9. ɛpeq/ɛL distribution
Finally, it is concluded that the connection is qualified using elastic-plastic analysis and no reinforcing pad is needed.
Indeed, with the help of a more advanced method, we were able to optimize the design and make sure that the plastic strains are local enough to qualify an unreinforced connection even for such a pressure load.
Waste-to-Energy: Overview of the Gasification Process
This article introduces the main existing technologies to carry out a conversion line of a waste stream into an energy-usable gas mixture (syngas). The syngas also serves as an intermediate point in the synthesis processes of various high-added-value products.
Introduction
Faced with the problems of climate change and global warming, research and development have focused on the use of biomass as an alternative to fossil fuels, among other energy sources. The wide availability of waste has been widely recognized for its potential to provide greater amounts of useful energy with less environmental impact than fossil fuels.
Waste can be converted into a commercial product through biological or thermochemical processes. The biological conversion of biomass continues to face challenges related to low efficiency and cost-effectiveness. In the case of thermochemical processes, combustion, pyrolysis, and gasification are the three main conversion methods.
Biomass is traditionally burned to provide heat and electricity in industrial processes. The net efficiency for electricity generation from its combustion is usually very low, not exceeding 50%. Biomass combustion is usually limited to 10% of the total feedstock due to, among other things, carbon lock-in in existing feed systems. Pyrolysis, on the other hand, converts biomass in the absence of oxygen. Limited uses and the difficulty of downstream processing have restricted the wide application of biomass pyrolysis technology.
Finally, gasification converts biomass by partial oxidation into a gaseous mixture, with small amounts of carbon and condensable compounds. It is considered one of the most efficient ways to convert the energy stored in biomass and is becoming one of the best alternatives for the reuse of solid waste.
A process that is currently being developed in several parts of the world is the gasification of municipal solid waste (MSW), in which syngas is produced. After gasification, the syngas is treated to remove the main contaminants. Once the gas meets the necessary requirements, it can be used as an intermediate product for different processes, such as the production of SAF (sustainable aviation fuel) by Fischer Tropsch synthesis, or to produce DME and methanol, among other products of great interest on the world scene.
Fig. 1: Block diagram of the process of Syngas production from waste gasification
What is Gasification?
Gasification is a partial thermal oxidation resulting in a high proportion of gaseous products (CO2, H2O, CO, H2, and gaseous hydrocarbons), small amounts of carbon (solid product), ash, and condensable compounds (tars and oils).
Air, steam, oxygen, or a mixture of these is supplied to the reaction as the oxidizing agent. The gas produced can be standardized in quality and is easier and more versatile to use than the original biomass (e.g., it can be used to power gas engines and turbines or as a chemical feedstock to produce liquid fuels).
Gasification adds value to low-value feedstocks by converting them into marketable fuels and products.
Stages of Gasification
In conventional biomass treatment plants, energy is obtained through incineration or gasification.
The biomass gasification process allows energy to be obtained in the form of heat or electricity, using the syngas (synthesis gas) to drive the shaft of a turbine, or burning it as fuel to drive an engine. This process typically uses coal as a feedstock.
The chemistry of biomass gasification is quite complex. In general terms, the gasification process consists of the following stages (drying, pyrolysis, gasification, partial combustion):
Fig. 2: Gasification Stages
Gasification Reactor Design
Gasification reactor designs have been investigated for more than a century, resulting in the availability of several small and large-scale designs. They can be classified in several ways, although we will focus on classification by design.
Depending on the configuration, gasifiers are classified into three main types: fixed-bed, fluidized-bed, and entrained-flow.
These gasifiers can be divided into the categories shown in the figure. Fixed-bed gasifiers are ideal for small-scale biomass feedstocks.
Fluidized-bed gasifiers can be used to process biomass and refuse-derived fuel (RDF) from pre-treated waste feedstocks, which must meet size, composition, and moisture content specifications.
Entrained flow gasifiers are commonly used for coal because they can be fed in direct gasification mode, which makes feeding the solid fuel at high pressures economical. Short residence time, high temperatures, high pressures, and large capacities are the characteristics of these gasifiers.
Fig. 3: Classification of gasifiers and commercially available technologies by feedstock type.
Challenges and Opportunities to Improve Gasification
The main problem arises in the heterogeneous nature of the reactor feed stream, the limited experience under commercial conditions, and the quality of the syngas obtained. Some of the problems related to waste heterogeneity are overcome by pre-treatment of the waste at the inlet of the gasifier. However, some energy is required and must be accounted for to make a proper balance.
The raw syngas may contain the following contaminants: tars, sulfur, nitrogen, and chlorine-containing gases (NH3, HCl, HCN, H2S, COS), fly ash, and particles containing K, Na, and traces of other elements that may influence catalyst performance.
Syngas or Synthesis Gas
Synthesis gas or “syngas” is a mixture composed of carbon monoxide, carbon dioxide, hydrogen, and methane. It is produced by the gasification of a carbon-containing fuel to form a gaseous product that has a certain calorific value. Examples of synthesis gas production include the gasification of carbon-rich compounds, gasification of various wastes, and steam reforming of coke.
It is a gas that is used as an intermediate product to synthesize other substances, which is why it is called synthesis gas. It is also an intermediate in the creation of synthetic oil for use as a lubricant or fuel.
Main Contaminants in Synthesis Gas
Synthesis gas contaminants are composed of tars, nitrogen-based compounds (NH3, HCN, etc.), sulfur-based compounds (H2S, COS, etc.), hydrogen halides (HCl, HF, etc.), and trace metals (Na, K, etc.).
The presence of these contaminants in the synthesis gas poses several technical and operational problems ranging from corrosion (H2S) and fouling of equipment (tar), deactivation of catalysts (tar, H2S, NH3, HCl, and trace metals), or environmental pollution (NH3).
Most downstream applications of syngas have very stringent requirements in terms of composition, so they have different levels of cleanliness requirements depending on their application. Therefore, contaminant levels must be reduced by cleaning the gas to meet the requirements of downstream applications.
Cleaning of Syngas
Cleaning the synthesis gas is an essential step prior to its further use. Cold gas cleaning is considered the most widely used cleaning method due to its proven reliability and high efficiency in removing contaminants.
The main characteristic of this method is that it is carried out at a low temperature, usually at room temperature or below.
In terms of cost, this approach is more suitable for large-scale applications due to the need to treat solvent effluents generated during gas cleaning. Since gasification is carried out at about 800°C, the main disadvantage of this method is the efficiency penalty paid due to stream cooling, in addition to the secondary cost of treating or removing contaminant streams.
Cold gas cleaning uses either dry or wet processes. Wet cold gas cleaning processes employ scrubbing towers, impingement scrubbers, Venturi, and electrostatic precipitators or cyclones. These units remove contaminants by absorption, adsorption, filtration, or a combination of these.
Wet gas scrubbing processes are the most used because they allow the removal of more contaminants.
For example, ammonium (NH3), hydrochloric acid (HCl), and hydrogen sulfide (H2S) are readily soluble in water. Therefore, scrubbers, scrubbers, and cyclones that use water as a solvent will remove all these contaminants, although with different removal efficiencies depending on their solubility in water.
Uses and Market of Syngas
MSW is a raw material that is not in short supply, widely available and is not susceptible to geopolitical risks, risks in commodity price fluctuations, or risks of natural disasters. As a waste product, it has no real competitive uses.
On the other hand, it is important to note the role of synthesis gas in the market. Throughout the chemical industry, synthesis gas is an intermediate product.
The ammonia industry dominates the world market for synthesis gas (mainly from fossil fuels such as coal, natural gas, and oil). Hydrogen production for use in refineries and methanol processing are other important applications.
Conclusions
Syngas is defined as a gas, rich in H2 and CO, the main fuel components. Its main properties are flammability limit and laminar flame speed.
Syngas is produced from biomass/coal gasification or natural gas reforming, and the yield is measured by the mass of product produced per mass of feedstock.
The Fischer-Tropsch technique is one of the commercially available methods for making clean synthetic fuel from syngas.
On the other hand, products such as methanol, dimethyl ether, ammonia, etc. can also be produced from syngas.
Many industry newcomers feel a washer and a gasket are the same as both of them consist of a similar design and both are used in conjunction with other objects. Even though their looks are quite similar, both washers and gaskets are used for completely different purposes. In this article, we will learn the major differences between a gasket and a washer.
What is a Washer?
A washer is a disc-shaped thin plate having a hole in the center. Washers are generally used as a fastener along with nuts or bolts for the purpose of distributing the load. The hollow center of the washer is inserted into the bolt and then the nut-bolt arrangement is tightened. The washer then distributes the bolt’s load across its disc-shaped surface.
Washers are manufactured in various types like:
Plain washers
Spring washers
Cupped washers
Toothed washers
Locking washers
Torque washers
Tab washers
Wedge lock washers
What is a Gasket?
A gasket is a sealing element between two bodies. They are used to prevent leaks around the two mating surfaces. The irregularities of machined surfaces are filled by the gasket to prevent leakage. Click here to learn more about various types of gaskets and their working.
Depending on the application, they are manufactured into various types like
Washers vs Gaskets: the Differences between Washers and Gaskets
From the definition mentioned above, it is quite evident that both washers and gaskets are completely different items with different purposes. The major differences between a gasket and a washer are:
1.0 Washers distribute the loads applied on threaded fasteners while gaskets prevent leakage of fluids through flanged joints. Gaskets are used for sealing applications with flanges or valves whereas washers are used with nuts and bolts.
2.0 To distribute loads, the washers need to be rigid. On the other hand, a rigid gasket will not work as required.
3.0 Gaskets are manufactured in a wide variety of shapes and sizes whereas washers are normally made of uniform circular shapes. The central hole of the washers is small to fit into the bolts. The gaskets also have a central hole which is larger as compared to washers.
4.0 In general, washers are made from metals. On the other hand, gaskets are available in metals, metal alloys, non-metals like rubber, and other synthetic materials.
5.0 Sometimes a sealant is used with the gasket to improve its performance and longevity. But sealant is not used with washers.
Colors of Hydrogen: Green, Blue, Grey, Black, Brown, Pink, Turquoise, Yellow, Red, and White Hydrogen
We all know that Hydrogen is an invisible gas without any color. But still, we find people talking about different colors of Hydrogen. They usually talk about Green Hydrogen, Blue Hydrogen, Black Hydrogen, Brown Hydrogen, Pink Hydrogen, Turquoise Hydrogen, Yellow Hydrogen, Red Hydrogen, and White Hydrogen. So, what are these colors? In this article, we will learn about different types of Hydrogen colors and their meanings.
Colors of Hydrogen
As hydrogen atoms don’t exist on their own, they need energy to produce hydrogen. Hydrogen is highly abundant in form of water or natural gases but to create pure hydrogen, one needs to break those molecular bonds using some form of energy.
Depending on the process of how Hydrogen is produced, it is provided with 9 color codes. They are:
Green Hydrogen
Blue Hydrogen
Grey Hydrogen
Black or Brown Hydrogen
Pink Hydrogen
Turquoise Hydrogen
Yellow Hydrogen
Red Hydrogen, and
White Hydrogen
This color coding is unofficially provided by North American Council for Freight Efficiency (NACFE). However, still, there is no universal naming convention for the above hydrogen colors which may change in the future over time or even between countries.
Green Hydrogen
The name green hydrogen is given to the hydrogen produced using clean electricity from surplus renewable energy sources to electrolyze water. As this production method does not produce greenhouse gas emissions, the production of green hydrogen is environmentally friendly and sustainable. A device called an electrolyzer is used to produce green hydrogen.
Electricity generated by renewable sources like wind, solar, etc split the water into its components i.e Hydrogen and Oxygen using an electrochemical reaction. Thus the process emits zero carbon and helps in the net-zero carbon philosophy. As the production of green hydrogen is costly, it makes up only a small percentage of the overall produced hydrogen.
Blue Hydrogen
Blue hydrogen is generated using the steam reforming method of natural gas (Fossil Fuel). Natural gas and heated steam are brought together which produces hydrogen and carbon dioxide. This carbon-di-oxide is stored underground for industrial use using Carbon Capture and Storage (CSS) technology. As the steam reduction process does not avoid greenhouse gas creation, Blue hydrogen is also known as low-carbon hydrogen.
Grey Hydrogen
Grey hydrogen is the most common form of Hydrogen that is produced from natural gas, coal, or methane. A method called steam methane reformation is used for creating grey hydrogen. A smaller amount of greenhouse gases are also generated while producing grey hydrogen but those are not captured and released into the atmosphere.
Black or Brown hydrogen
Black and Brown Hydrogen is produced from black coal or brown coal (lignite). This type of hydrogen production is the most environmentally damaging. Most hydrogen produced in the United States is “brown hydrogen”. The steam-methane reforming process is used to create black and brown hydrogen. In this process, steam is used to split natural gas or coal into clean, zero-emission hydrogen fuel and CO2 gas.
Pink Hydrogen
When hydrogen production is done by the water electrolysis process which is powered by nuclear energy, the generated hydrogen is termed pink hydrogen. Sometimes, this hydrogen is also termed purple hydrogen.
Very high temperatures of nuclear reactors are used in this process which is highly efficient.
Turquoise Hydrogen
Turquoise hydrogen is produced using the methane pyrolysis process. Natural gas is broken down using the thermal process to create hydrogen and solid carbon. The thermal process to split the natural gas is powered by renewable energy and the generated carbon is permanently stored or used.
Yellow Hydrogen
When solar energy is used for the electrolysis process to generate hydrogen, It is termed yellow hydrogen. This method of hydrogen production is relatively new. Sometimes, various other mixed energy sources are also used to produce yellow hydrogen.
Red Hydrogen
A high-temperature catalytic process is used to split water with nuclear thermal power and the generated hydrogen is termed Red Hydrogen.
White Hydrogen
White hydrogen is the naturally-occurring geological hydrogen. This type of hydrogen is usually found in underground deposits. Sometimes, white hydrogen is created through fracking. At present, there are no strategies to exploit this hydrogen.
All the above discussions can be summarized in a tabular format as provided below:
Hydrogen Colors
Production Process
Green Hydrogen
Electrolysis of water using clean electricity from renewable energy sources.
Blue Hydrogen
Steam reforming of natural gas using CSS technology
Grey Hydrogen
Steam reforming of natural gas without CSS technology
Black and Brown Hydrogen
Steam reforming of black or brown coals
Pink Hydrogen
Water electrolysis using nuclear energy
Turquoise Hydrogen
Methane pyrolysis
Yellow Hydrogen
Electrolysis using solar power
Red Hydrogen
High-temperature catalytic process using nuclear power
White Hydrogen
Naturally occurring Hydrogen
Table 1: Colors of Hydrogen and Production Process
Blue Hydrogen vs Grey Hydrogen
The production of Blue and Grey hydrogen is almost similar. The main difference between the two processes is that in Blue hydrogen production, the greenhouse carbon-di-oxide gas is captured using CSS technology whereas in the Grey hydrogen production process the CO2 is not captured.
Blue hydrogen is an alternative low-carbon hydrogen production process but Grey hydrogen is not considered a low-carbon fuel.
Is Hydrogen a Clean Energy Solution?
As discussed above, most hydrogen production processes are not a solution to clean energy. However, green hydrogen is fully clean energy but the production process is costly. Green hydrogen is produced following the zero carbon emissions process and fully clean energy.
What is PSV Testing? Its Requirements, Procedure, and Standards
Pressure Safety Valves (PSV) or Pressure Relief Valves (PRV) are important instruments used to protect enclosed equipment or vessels from over-pressure scenarios. When the system pressure exceeds a certain limit (set pressure), the PSV/PRV pops up and vents out the overpressurized fluid to control the overpressure situation. In the oil & gas, power generation, steel manufacturing, chemical/plastics industries, water/wastewater, and aerospace/aviation industries PSV/PRV systems are widely used to protect people, property, and the process.
A PSV is a final link in the safety chain and these must work properly to safeguard the system. To ensure that the PSV will work smoothly and open when the need arises, they must be tested. In this article, we will explore the PSV testing requirements, procedures, and standards.
Requirements for PSV/PRV Testing
The PSV/PRV that is installed at the operating plant must be tested frequently based on the inspection plan or manufacturer’s guidelines to be assured that the intended operation will be successful when the need arises. Every PSV is usually tested every 1-3 years to find out that the mechanical parts are not stock closed, not worn due to corrosion, and valve seats are intact.
Types of PSV Testing
Depending on the location where the PSV testing will be performed there are two types of PSV testing procedures.
In-situ or Online PSV Testing and
Bench PSV Testing
Online PSV Testing
Online PSV testing is performed at the operating plant at their operating temperature and pressure condition. This is the most cost-effective PSV testing process as the plant remains operational. But the main difficulty with In-site PSV testing is reaching the set pressure.
Bench Testing of PSV/PRV
To avoid the set pressure challenge, the entire PSV is removed from the operating system and then taken to the lab for bench testing. This method is costly and time-consuming. Also, the PSV line remains in shut down condition till the PSV is tested and returned back to the line. However, many operating plants keep an extra PSV for keeping the plant operational during this time.
PSV Testing Procedure
The PSV testing is also known as Popping Test or Cracking Test. The test must be performed by an expert technician. The main elements required for the test are:
the PSV/PRV under test
a reference pressure gauge
an external pressure source.
The PSV testing process is performed as per the following steps.
Step 1: Check the PSV set pressure engraved on the riveted tag. Theoretically, the PSV should pop open when the system pressure reaches this value.
Step 2: Install the calibrated and certified pressure gauge to measure the pressure. Always ensure that the gauge has sufficient range to measure that pressure.
Step 3: Keep increasing the PSV inlet side pressure from the external pressure source until the PSV pops up with a sudden releasing sound. Note the pressure gauge reading at that instant.
Step 4: Now slowly decrease the pressure and note the pressure when the PSV closes (Reseat in position).
Step 5: Repeat the PSV testing process at least 3 times and record the pressures each time for confirmation.
Step 6: Send the readings to the certifying authority for certification and approval.
While performing the PSV test procedure, the following parameters need to be closely monitored:
The valve should open up at the set pressure of the PSV.
For the overpressure scenario, the discharge fluid rate shall be as per the PSV datasheet.
The valve should seat in position and close tightly when the pressure is lowered.
Codes and Standards for PSV Testing
The following codes and standards are generally followed for PSV testing guidelines:
ASME Sec VIII
API 520
API RP 527
API RP 521
API 581
API RP 576
ASME PTC 25
BS EN ISO 4126-1, 4126-2, and 4126-3
NZA 3788
PSV Testing Results: Pass/Fail Criteria
The results of a PSV/PRV testing process can be classified as passed, failed safe, or failed to danger.
A PSV test result will be termed as PASS when all the following criteria are satisfied:
The PSV pops up within +/-3% of its set pressure.
The valve reseats with +2.5% to -7% of the set pressure.
There are no visible leaks during the test
A PSV test result will be termed as failed safe when
The initial POP test lifts within a range of 103 to 110% or less than 97% of the set pressure.
The seat tightness is more than the limits specified in API 527.
A PSV test result will be termed as failed to danger when any of the following occurs:
If the relieving pressure is more than 110% of the set pressure.
If damage is found in its components like spring, bellows, leaking pilot diaphragm, blockage or leakage of the impulse and sensing line.
What are Pickling and Passivation? Their Meaning, Procedure, Advantages, and Differences
Pickling and Passivation are chemical processes used in the metal industry to protect metals from corrosion. Both pickling and passivation are widely used for stainless steel products. Some acidic chemicals are used over the stainless steel to create a passive layer or to remove contaminants. In this article, we will explore more about the pickling and passivation process. Let’s start with the definition of both processes.
What is Pickling?
Pickling is basically a metal cleaning process. In the pickling process, thin layers of metal (in form of stains, inorganic contaminants, foreign matter, grease, oil, rust or scale, etc) are removed from the surface of stainless steel. For the pickling of stainless steel, usually, a mixture of nitric and hydrofluoric acid is used.
Pickling is a popular process for removing weld heat-tinted layers from stainless steel surfaces. However, the Pickling process causes etching of the surface and affects the surface finish making it dull.
What is Passivation?
Passivation is a chemical treatment process where the stainless steel is treated with an oxidizing acid. Passivation dissolves carbon steel, and sulfide inclusions and removes iron and other surface contaminants from the stainless steel surface. At the same time, the acid promotes a chromium-rich thin but dense passive film (oxide protective layer) formation. This passive film imparts corrosion resistance quality.
Passivation of stainless steel is performed using nitric acid. Similar to pickled steel, passivated steel does not affect the metal’s appearance.
Advantages of Pickling and Passivation Process
The processes of pickling and passivating steel offer various advantages to the metal products like:
Both pickling and passivation remove surface impurities and contamination generated during manufacturing and fabrication.
Increase the durability and longevity of the stainless steel products with reduced corrosion possibility.
Weld hint tint or weld discoloration is removed and the metal looks smooth without imperfections.
Chemical film barrier against rust.
Reduced need for maintenance.
Pickling and Passivation Procedure
Pickling Process:
A range of methods can be applied to the pickling process. The most popular methods are:
Tank Immersion Pickling– Can be done on-site or off-site. Provides a facility for treating all the fabrication surfaces at the same time. This achieves uniformity of surface finish and optimum corrosion resistance.
Circulation Pickling– This type of pickling method is recommended for piping systems carrying corrosive fluids. In this pickling process, the chemical solution is circulated through a system of pipework.
Spray Pickling– Spray pickling is done for on-site treatment. Proper acid disposal and safety procedures must be ensured during the spray pickling process.
Gel Pickling– This is a manual pickling operation in which gels are applied on metal surfaces by brushing. It pickling method is quite useful for the spot treatment of welds and other intricate areas that require manual detail.
Passivation Process:
The passivation of stainless steel is performed using weak acids like nitric acid or citric acid. The main aim of passivation treatment is the formation of a passive layer that does not easily interact with the environment. Before the acid passivation process, the surfaces must be cleaned to make them free from oxide scales, oils, grease, and other lubricant, heat tints must be removed. After that nitric acid chemical/paste is applied to the material surface.
Passivation is a post-fabrication process used for newly fabricated stainless steel parts. The effect of passivation was first discovered by chemist Christian Friedrich Schönbein in the mid-1800s. However, the process of passivation become widely useful in the 1900s.
The weak nitric or citric acid chemically dissolves the free iron present on the surface. The chromium remains intact which creates a chromium oxide layer upon exposure to oxygen over the next 24 to 48 hours. This passive layer provides a chemically non-reactive surface.
Please note that passivation is not an electrolytic process, this is not a process to remove scale, and It does not change the surface color or appearance. The steps followed for the passivation process are:
Alkaline cleaning of the metal surface.
Deionized (DI) Water rinse
Nitric or Citric acid immersion bath
DI Water rinse
Drying of the part
Testing the effectiveness of the process.
Codes and Standards for Pickling and Passivation
Widely used codes and standards that govern the pickling and passivation process are:
ASTM A380
ASTM A967
ISO 16048
AMS 2700
ASTM B600
AMS-STD-753
BS (British Standard) EN 2516
The treatment duration for pickling and passivation treatment normally varies from 5 minutes to 45 minutes. The oxide layer left by passivation is roughly .0000001 inch thick.
Pickling vs Passivation: Differences
Both pickling and passivation are chemical surface treatment processes and are widely used for stainless steel. However, there are some differences between the processes in terms of the intensity of the treatments. The major differences between the pickling and passivation processes are tabulated below:
Pickling
Passivation
Acids used in the pickling process are more aggressive
Passivation uses weak acids (either citric acid or nitric acid).
Pickling makes greater changes to the surface
Passivation only creates a thin surface layer and does not change material properties.
The pickling process removes metal impurities on a sub-level basis.
Passivation normally does not remove metal impurities or contamination. It mainly makes the surface passive to corrosion.
