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Caustic soda (sodium hydroxide) is one of the most widely used inorganic chemicals in water treatment process. Improper injection rate or concentration have led to many failures in the industry. These failures have been mostly identified as Caustic Stress Corrosion Cracking (CSCC).
Caustic Stress Corrosion Cracking (CSCC) is a form of Environment Assisted Cracking (EAC), characterized by surface-initiated cracks that occur in materials exposed to caustic environment. EAC is defined as a cracking process caused by the synergistic effects of stress, and environment on a specific material. All three factors–stress, aggressive environment, and susceptible material are necessary for EAC. The environment aggressiveness escalates with increase of caustic concentration and metal temperature, in case of CSCC.
A failure was reported in an alloy 800 (UNS N08800) high pressure steam superheater piping downstream of a thermal oxidizer in a Sulfur Recovery Unit of a Gas Plant. Unexpected cracking on the circumference was observed. No evidence of corrosion was found on the piping. A metallurgical failure analysis investigation established that the piping had failed due to intergranular environmental stress corrosion cracking (IGSCC), specifically Caustic Stress Corrosion Cracking (CSCC) cracking. The cracks most probably originated as a result of unexpected wet conditions and caustic carryover. It was recommended to review the water treatment program, prevent water condensation and inspect the rest of the piping for similar cracks.
External corrosion in uninsulated pipelines is normally able to be prevented by cathodic protection (CP). Generally, external corrosion on buried pipelines cannot occur if CP current is getting onto the pipe. CP is an electrochemical means of corrosion control in which the oxidation reaction in a galvanic cell is concentrated at the anode and suppresses corrosion of the cathode (pipe) in the same cell. For instance, to make a pipeline a cathode, an anode is attached to it.
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Corrosion under thermal insulations namely CUI (Corrosion under insulation) is among the key damage mechanisms which poses integrity risk to the hydrocarbon facilities. CUI is reportedly known as the reason behind 40-60% of failures in the facility piping whereas small bore piping (i.e., NPS < 4”) are even more sensitive to CUI failures, where up to 81% of reported failures in small-sized piping are known to be from CUI. Monetary spending to inspect and fix CUI-related failures cost 10% of overall maintenance budget in a typical medium-sized oil refinery. CUI risk is influenced by numerous operational and environmental factors which impedes its management in a typical AIM (Asset integrity management) program.
Pipelines have been the main transportation pattern of oil and gas because of their safety and economy, which are considered as the lifeline of offshore oil and gas transportation. With the booming development of offshore oil industry, the frequency of pipeline leakage is also increasing. Corrosion is one of the important factors due to some characteristics such as operating environment, service life and transportation medium, etc., which damages the integrity of the pipeline and damage the normal operation of pipelines. Furthermore, leakage accidents caused by pipeline corrosion have occurred all over the world, accounting for 70~90% of total accidents, which has caused huge economy losses and catastrophic environmental damage.