Corrosion failures is very common problem in industial piping. Corrosion causes unexpected plant shut downs, reducing on productivity, high repairing costs and more importantly can threaten human life.
Good to say that corrosion is preventable problem by proper design, material and coating selection. Companies and governments can save big amounts of their expenses by making design and corrosion engineering in time.
In industry there are various corrosive environments in different sectors (Oil and gas, power, chemical, offshore, maritime).
Corrosion usually occurs because of internal and external conditions.
External Factors: Working environment. For instance, underground pipes will be under effect of different type of soil or underwater pipes are directly effected by seawater which can be very corrosive.
Internal Factors: Internal conditions are generally related with fluid type, fluid velocity and operating pressure. Oxygen or H₂S content, chlorid, HCl and other acidic substances can create corrosive environment for piping elements. Also temperature and pressure fluctuations increase the corrosion rate.
Some of important corrosion types:
Uniform Corrosion: As understood from its name, piping material uniformly rusts and get thinner. Corrosion rate defined with mm/year.
-Proper Material Selection
-Paint or other coating methods
2. Galvanic Corrosion: Can occur due to dissimilar metals usage. Active metal will be corroded according to galvanic serie.
-Proper Material Selection according to galvanic serie
-Elecrical insulation between dissimilar metals
3. Crevice Corrosion: Can occur in crevices like flange connections, bolting areas, lap joints.
-Proper Material Selection. High Mo content stainles steels or Hastelloys have good resistance to crevice corrosion.
-Usage of welding instead of other connection methods that exists gaps.
-Usage of non absorbent gaskets on sealing area.
4. Pitting Corrosion: Generally causes internal corrosion in pipeline, causes some of pits inside the piping area. Pitting corrosion is local corosion type and above Critical Pitting Temperature (CPT) given in ASTM G48-03, material’s pitting corrosion resistance decreases.
-Proper Material Selection. High Mo content stainless steels, Hastelloys, duplex stainless steel, Monel alloys.
-Control of pH.
-If fluid contents Chloride, low carbon alloy steels can be better than stainless steel.
5. Intergranular Corrosion: Can be seen on high temperatures. Stainless steels can be effected between 500-900⁰C temperature range. Heat treatment and also heat affected zone (HAZ) during welding process can be effected by this corrosion type. Carbides can be precipated between these ranges.
-Usage of low carbon stainless steel alloys.
-Ti, Nb content alloys like 321, 347.
-To remove carbides heat treatment at 1050⁰C 30 m.
6. Erosion and Cavitation Corrosion: Can be seen on directional changes like elbows, bends, heat excanger U tubes due to pressure changes and causes plastic deformation on pipeline.
-Avoid sharp corner designs
-Usage of high hardness steel.
-Making hard coatings.
7. Stress Corrosion Cracking: Causes brittle cracking and occurs due to high stress concentration. For stainless steels exist of Cl and oxygen increase the risk of SSC.
-Pipe Stress Analysis must be done properly and stress concentration must be kept at allowable values according to international codes like ASME B31.1/3.
-Proper Material Selection.
-Create compressive stress on pipeline, shot blasting can be useful.
DETRA Engineering can make optimum design to prevent possible corrosion defects by making proper material and coating selection and stress analysis after system evaluation.
Please contact with us for any of piping design enquiries.