Corrosion resistant alloys for different types of media
The high performance alloys attain outstanding mechanical strength and offer good creeping resistance at the elevated temperatures, high stability and resistance to corrosion and oxidation. They are usually austenitic face centered cubic crystal structure with main composition of nickel, chromium, cobalt and iron. Super alloys have been mainly made for use in the aerospace and power production industries.
The corrosion resistant nickel based alloys are commonly used in the severe conditions where resistance to heat and corrosion is essential for the service product. The industries that commonly use the super alloys are chemical and petrochemical plants, power production units, oil and gas refineries, marine, nuclear reactors, heat exchangers, gas turbines, oil and gas industries.
The high performance alloys have become the premiere option for corrosion resistant and versatile materials. Heanjia Super-Metals supplies the superalloys in sheet, plate, bar, pipe and tube, rod, wire and mesh forms.
Common nickel based alloys are Hastelloy C-276, Hastelloy X and Inconel 718.
Types of Corrosion against which super alloys are used:
- Uniform Corrosion
It is also named as general corrosion. The surface effect developed by various direct chemical attacks is a uniform etching of metallic material. In the polished surface, such kind of corrosion is initially observed as a general dulling of the surface and if allowed to progress, the surface becomes hard and chilled in appearance. The discoloration of metal developed by exposing it to the high temperature is not considered as uniform etching.
- Galvanic corrosion
This is an electrochemical action of two unlike metals in availability of an electrolyte. This kind of corrosion occurs when two different metals come in contact of each other.
The galvanic corrosion occurs when two metals are placed closed to each other. Cathodic protection can also be used to limit the effects of galvanic corrosion.
Concentration Cell Corrosion-
Concentration cell attack takes place when the several surfaces of a metal are placed in the different concentrations of the same solution.
The high concentration of metal ions will exist under faying surfaces and small concentration of metal ions will exist near to the crevice developed by the faying surfaces.
- Pitting Corrosion
Stainless steels prevent attack in the severe media and offer good performance for a long period. In case of corrosion occurs, it produces at random in pits. Pitting initiates in the presence of chloride ions, paired with these depolarizers as oxygen or oxidizing salts. To limit pitting it is important to maintain the clean surfaces, use of a secured coating and inhibitors for immersion service.
Pitting corrosion can result into unwanted catastrophic system failure. The pitting corrosion can be small on the surface and large underneath the surface.
- Microbial Attack
Microbial corrosion is a type of corrosion that is resulted by the availability and function of microbes. This corrosion occurs in the several forms and can be limited by biocides or by traditional corrosion control methods.
There are several mechanisms related with this kind of corrosion. Usually MIC takes the form of pits that produce under colonies of living organic matter and mineral and biodeposits. This bio layer develops a secured climate where the conditions can become extremely corrosive and corrosion increases.
Microbial attack can be a severe issue in static water. The use of biocides and mechanical cleaning techniques can decrease MIC however where stagnant water is to collect is a place where MIC can take place.
Corrosion can occur if any chemical needs to be reduced. In several conditions the chemical that is reduced. In several media, the chemical that is reduced is dissolved oxygen or hydrogen ions in acids. In anaerobic conditions, bacteria can flourish. The bacteria offer the reducible chemicals that permit corrosion to happen.
- Crevice Corrosion
Crevice is a type of attack that is developed at the area of contact of metals with metals or metals with non-metals. It occurs in the washers, barnacles, sand grains, below the security layers and at pockets produced by threaded joints.
Irrespective of the pitting resistance of stainless steels, they are always sensitive to such kind of corrosion as nucleus is not essential. Cleanliness, the suitable use of sealants and secured coatings are suitable to control this issue. The stainless steels such as SS 316/316l suffer from increased crevice corrosion resistance. The super alloys are used that are resistant to crevice corrosion at high temperatures. The materials that usually suffer from crevice corrosion are screw and fasteners.
- Filiform Attack
This kind of attack or corrosion occurs below the painted surface when the moisture penetrates the coating. This problem usually happens with Lacquers paints, so their use should be limited to prevent any adverse effect. When the coating is needed, it should attain small water vapor transmission features and outstanding adhesion. This kind of corrosion is limited by careful surface preparation before coating.
- Intergranular Corrosion
Intergranular corrosion occurs along the grain boundaries of a metallic material. A vast cross-section of several commercial alloys describes its structure. This structure incorporates the magnitudes of individual grains and each of these small grains have well specified boundary that chemically varies from the metal inside the grain center. Heat processing of stainless steels highlights this issue.
- Exfoliation Attack
It is a kind of intergraular corrosion that develops itself by raising the surface grains of a metallic material by the impulse of an enlarging corrosion that occurs at the grain boundaries beneath metal surface. It is a proof of interganular corrosion and usually observed an extruded section where intensity of grains is lower than in rolled forms. Such kind of corrosion may occur in carbon steel.
- Stress Corrosion Cracking
Stress corrosion cracking is resulted by the immediate effects of tensile stress and a specific corrosive media. The stress can be caused by applied load, residual stress, stress created during the production process etc.
- Corrosion Fatigue
Corrosion fatigue is a type of stress corrosion cracking that is resulted by the combined influences of cyclic stress and corrosion. No metal is can prevent the reduction in its corrosion resistance if the metal is placed in a corrosive media. The metal loss due to corrosion fatigue is more severe than the combined damage by cyclic stress and corrosion.
It can be reduced by decreasing the cyclic stress and corrosion.
- Fretting Corrosion
This type of corrosion quickly occurs at the interface among contacting, highly loaded metal surface when undergone vibratory movement is called as fretting corrosion.
Fretting corrosion is usually seen in the bearing surfaces in the machine equipments for example connecting rods, bearing supports that result into fatigue breakdown. It also occurs in the structural components for example trusses where the loaded bolts are utilized and a relative motion develops among the bolted members.
This type of corrosion is widely opposed when the contacting surfaces can be adequately lubricated as in the machinery bearing surfaces in order to prevent direct contact with air.
- Erosion Corrosion
Erosion corrosion is the product of a combination of severe chemical media and high fluid surface speeds. It can be caused by the high speed fluid flow for example in the oil-field check valve or can be resulted from the fast motion of an object in a static fluid for example it occurs when the propeller in a ship moves on the ocean.
The surfaces have received erosion corrosion are normally very clean opposite to those that receive different kinds of corrosion. Erosion corrosion can be limited by using the harder alloys or by using highly corrosion resistant alloys. Alterations in fluid velocity and variations in the flow patterns can also decrease the influences of erosion corrosion.
Erosion corrosion is usually the outcome of the damage of the security layer on the surface of metallic material. The oil field development tubes when deal with the variable pressure in the well that causes multiphase fluid flow. The force of damaging gas bubbles results into damage at the welds where the tubes are joined and turbulence is higher.
It is usually considered that erosion corrosion is related with turbulent flow. It is valid as several piping systems need turbulent flow – the fluid does not travel quickly if lamellar flow is maintained. The erosion corrosion can be featured to multiphase fluid flow.
Dealloying is a rare kind of corrosion that occurs in copper based alloys. Dealloying occurs when the alloy loses the active part of the metal and keeps the highly corrosion resistant part in a porous sponge on the metal surface. It also occurs by re-accumulation of the noble part of the alloy. The use of corrosion resistant alloys prevents this type of corrosion.
Stagnant seawater is extremely corrosive that the general corrosion resistant alloys are also attacked, here the use of cupronickel 70/30 alloy is preferred.
- Hydrogen damage
Hydrogen results into different kinds of corrosion issues. The common type of problem is hydrogen embrittlement that occurs in titanium, steels and other metals. Hydrogen blistering occurs when the hydrogen penetrates the steel due to reduction reaction on the metal cathode. The nacent hydrogen atoms diffuse into the metal to join with another atom. The product diatomic hydrogen molecule is big enough to migrate and penetrate. Ultimately a gas blister develops up and breaks down the metal.
Hydrogen blistering is monitored by reducing corrosion in acidic media. It is not an issue in neutral or caustic media or in high steels.
Carbon Dioxide and General Corrosion
Carbon dioxide (CO2) is the main source of general corrosion. If water is present, CO2 develops carbonic acid that is extremely corrosive in nature. In the gas areas if the temperature drops below the dew point, water condensation occurs. It can specifically be an issue during commencing and shutting down the operation unit. Further, temperature, pH, flow rate and oxygen also make the conditions worst.
Chloride, oxygen and pitting attack
Chlorides and oxygen can damage the security oxide layer of chromium, causing pitting and crevice corrosion. This form of corrosion occurs quickly, they are tough to find and usually catastrophic. Besides, the pitting corrosion is based on temperature, pH and velocity.
Chlorides in availability of tensile stresses also result into Stress Corrosion Cracking at the high temperatures. The alloys containing high nickel content prevent SCC.
H2S and environmental assisted cracking
H2S corrosion in combination with tensile stress causes catastrophic damage due to hydrogen embrittlement that is caused by hydrogen. These damages are usually posed however occur quickly without any warning. Factors improving environmental assisted cracking are partial pressure of H2S and CO2, pH, chloride content, stress and temperature limits.
Corrosion Resistant Alloys at Heanjia Super-Metals
The excellent corrosion resistant alloys find their use in the endless applications varying from military to aerospace. An inclusion of chromium significantly prevents the acidic damage of metal that just not strengthens the alloy in fact develops a passive oxide layer that basically seals in the metal. For the special orders in corrosion resistant alloys, there is simply no other destination than Heanjia.
We take full control on melting and forming of every metal and alloy that we sell. You are privileged to order large magnitudes of corrosion resistant alloys or simply 100 kg of bar.
For your any kind of needs, we are the perfect place to find your product. Stainless steel is introduced in the different product varieties and categorized by name.
Selection of corrosion resistant alloys
The selection of corrosion resistant alloys to use in the gas and oil plants, can prove to be a complicated problems. People select the alloys in the various ways to handle the required flowline and well environments. If big research facilities are approachable, then it would be easy o test the material in the specific condition. On the base of results, a set of alloy is selected. Rather testing most of the available alloys at once, it is recommended to test a few of them that are more suitable for your application.
This selection method may easily take a long time to provide the required results and can associate to a reasonable price. Another way to choose corrosion resistant alloys is the evaluation of corrosion data that is applicable in the anticipated service environments that can easily cut down the inappropriate materials. The commonly used corrosion resistant alloys are:
- Austenitic Stainless steel 316l- SS 316l is widely used in vessel cladding, surface piping and linepipe. This steel grade is primarily used in these applications. Precautions should be followed ensure the complete deareation of the application. The alloy will pit in availability of oxygen in cold seawater.
Alloy 316l shows extremely sensitive functionality in the presence of chloride ions. In the conditions that are free of chlorides, SS316L offers consistent performance in sour gas units however pitting begins when the chloride ions are present.
2. Incoloy 825 and Inconel 625
Alloy 825 and 625 offers good resistance to attack caused in H2S conditions with the rising temperatures. They prevent attack in all magnitudes of CO2 and limited content of H2S and temperature limits. These alloys are not extremely sensitive to chloride induced corrosion excluding the high chloride concentrations. This information should be studied in detail before making a final decision.
In the presence of gaseous media containing elemental sulfur, these alloys are mildly affected. Elemental sulfur is also called as free sulfur. It results into extensive pitting and catastrophic damage of these alloys in specific conditions.
3. Hastelloy C-276
Alloy C276 offers similar corrosion resistance as of Incoloy 825 and incoloy 625 however it is the only alloy that is also highly resistant to sulfur based corrosion.
4. Duplex stainless steels
Duplex stainless steel such as 2205 and 2507 offer high corrosion resistance. These are reinforced by cold processing. They are significantly resistant to H2S and chloride induced corrosion. They can be utilized up to 300oC, over 1500 psi partial pressure of CO2 and 1000 psi partial pressure of H2S.
Heanjia Super-Metals delivers corrosion resistant alloys in European and Southeast Asian Countries. Contact us at anytime for any kind of assistance.