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API 571: 4.4.2 Sulfidation JAG
Terms in this set (27)
What is sulfidiation?
Damage mechanism that occurs during high temperatures when suplhur compounds are present.
What materials does sulfidation affect?
It affects all iron based materials, carbon steel, low alloy steel, 300/400 series SS. Nickel based allots are affected but depends on chromium content. Copper base alloys form sulphide at lower temps than carbon steel. The resititivty of a material against sulphidation is dependent with the amount of chromium present in the material.
Major factors that affect sulphidation?
Alloy composition, temperature and concentration of sulfur compounds.
What temperature in iron based alloys does sulfidation occur?
It occurs over 500F/260C
Susceptibility of an alloy is determined by what?
It is determined by its ability to form a protective sulfide scale
What is the relationship between temperature, chromium content and corrosion rate?
The higher the temperature (beginning at 500F) the faster the corrosion rate. Increase the chromium content and the corrosion rate drops (improved resistivity with higher % chromium). Decrease the chromium content and higher rates of corrosion will occur.
In general in iron and nickel base alloys, how is greater resistance against sulfidaiton achieved?
It is achieved by increasing the chromium content. 300 series SS are highly resistant in most refining process environments as well as nickel base alloys with a similar chromium content provide similar levels of resistance to sulfidation.
Where does sulfur come from?
Crude oils, coal and other hydrocarbons contain sulfur at various concentrations. Total sulfur content is made up of many different sulfur containing compounds.
What is the primary compound that causes sulfidation?
h2s and other sulfur reactive species as a result of thermal decomposition of sulfur compounds at high temperatures.
A sulfide scale can provide protection
it depends on the alloy and the severity of the process stream
What units and equipment are affected by sulfidation?
Piping/equipment that deals with sulfur process's and high temperature enviornments
Commong areas of concern
crude, fcc, coker, vacuum, visbreaker and hydroprocessing units
Fired heaters with oil, gas, coke and most other sources of fuel may be affected depending on?
Key conditions that need to be present before sulfidation can occur?
The presence of sulfur, high temperatures >500F
How does sulfidation appear ?
depending on service conditions, corrosion is most often uniform thinning but can also occur as localized corrosion or high velocity erosion-corrosion?
What is the main mechanism for erosion-corrosion?
It can occur in areas of high turbulence, erosion can remove the protective coatings/films exposing the bare material, any protective corrosion products are removed by erosion. If pitting occurs, then this can also create high turbulance areas. Corrosion can be accelerated due to the inability to protect itself with oxide layers and coating removal by erosion.
Sulfide scales can protect the surface, these can be thick or thin and dependent on?
the corrosiveness of the stream
presence of contaminents
How is protection achieved against sulfidation?
It can be achieved by upgrading to an alloy with a higher chromium content.
Stainless Steel and resistivity
Piping/equipment constructed by solid/clad 300/400 series SS can provide significant resistance to corrosion?
What type of coating can provide protection against sulfidation?
Aluminium diffusion treatment of low alloy steel components is sometimes used to reduce sulfidaiton rates and minimize scale formation, however it may not offer complete protection.
What is diffusion coating?
Diffusion coating is where a steel is protected by a non-corrosive material. Often the materials used are aluminium, chromium or silicon material. Being coated by these allows the metals to be used in a highly corrosive and high temperature enviornments
How is the coating applied?
The process is normally done at elevated temperatures in a controlled chamber
Aluminium and such coatings find place in atmospheres where sulfur containing fuels are used
Often Aluminium has been used on steel structures and with maintence work have been able to last around 10 years...
Inspection/monitoring for sulfidation?
Monitor process conditions for increasing temperatures and changing sulfur levels
How is temperatures monitored?
Tubeskin thermocouples and/or infrared thermography
Where evidence of thinning is suspected?
What is PMI?
Positive Material Identification - identification and chemical analysis of various metal alloys through nondestructive methods
THIS SET IS OFTEN IN FOLDERS WITH...
API 571: 4.3.10 Caustic Corrosion
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