Pitting and crevice corrosion of materials in seawater
The oil and gas plants often choose to configure stainless steel tubing in the instrumentation and sensor components in addition of chemicals, hydraulic lines, impulse lines and various utility applications for a broad range of temperature limits, pressure and flow values. It is found that stainless steel 316 tubing offers moderate corrosion resistance in the offshore applications in the various parts of the world.
Corrosion is a serious issue that affects the tubing performance. Two types of localized corrosion attack are noticed- pitting and crevice corrosion. Various factors cause the onsite localized attack such as Inefficient tubing material and installation procedures can cause damage of tubing surface within a few months.
Contamination also damages the tubing surface that may be caused by iron traces that remain on the tubing after welding and grinding, drilling, blasting etc.
Pitting and Crevice Attack
Pitting corrosion of an alloy tubing causes shallow pits that become visible very soon if left untreated. This type of attack begins as soon as the chromium based oxide layer on the alloy 316 stainless steel is degraded in the chlorine based service media. Greater chloride content and high temperatures accelerate the process of layer damage. After the degradation of the layer, an electrochemical cell forms that starts the formation of pits on the tubing wall and the depth of such pits increases significantly with the passage of time, ultimately are leaving the tube unusable.
Crevice is a complicated kind of attack that is sometimes impossible to inhibit in the tubing materials. It occurs among tubing and its supports such as clamps and tubing runs and beneath contamination and accumulations made on the surface of tubes. The tight crevices are widely hazardous in nature. General corrosive attack on the tubes in a tight crevice results in increasing oxygen content in the fluid that is present in the crevice. Smaller oxygen content increases the possibility of damage of security surface oxide resulting in the formation of a thin pit.
Opposite to pitting corrosion, the production of pit is enclosed by a crevice to increase the concentration of Fe ions in the fluid present in the gap. The chloride ions also diffuse into the gap and combine with Fe ions. The produced ferric chloride solution speeds up the corrosion process of tubes in the crevice.
The chosen tubing material should resist the variety of corrosion types such as general corrosion, localized corrosion, galvanic corrosion, microbial based, chloride induced stress corrosion cracking as well as sour gas cracking. The tubing alloy should have sufficient mechanical strength particularly to handle the fluid pressures. It must be resistant to erosion corrosion. The atmospheric effect must also be considered and marine life also attacks the tubing material.
An alloy for offshore tubing applications should have high PREN (Pitting resistance equivalent number) and superior resistance to localized corrosion.
Alloys for offshore applications
The selection of a suitable alloy is essential when it is observed that austenitic stainless steel grade 316 tubing attains severe corrosion. Duplex stainless steel 2507 does not receive any kind of attack. It was used in the distillation plants in the Gulf oceans where it got only nominal chloride crevice corrosion however no perforations were seen.
Various alloys were used as the candidate materials in the installations that demand resistance to marine attack. The commonly employed alloys have been austenitic stainless steel 300 series such as SS 316.
The duplex stainless steel alloys for tubing offer an outstanding set of characteristics that make them incomparable for specific applications in the worldwide projects. A selected alloy must have higher critical pitting temperature than its service temperatures. On the base of application, it is crucial to choose a material that has critical crevice corrosion temperature higher than its service limit.
To secure the tubing surface it must be installed after the extensive building practices to prevent the deposition of construction materials such as debris. Or, tubes must be shielded from the contamination or completely cleaned.
The super duplex alloys 2205 and 2507 perform excellently in the demanding seawater applications. It is discovered that large concentration of chromium, molybdenum and nitrogen increases CPT and CCT limits of austenitic and duplex stainless grades. Comparatively lower content of costly nickel and molybdenum, they provide service identical to the high austenitic stainless steels.
The duplex grades offer suitable resistance to localized corrosion and have good mechanical characteristics therefore they are eligible for high pressure operations. Duplex steel 2507 offers high yield strength that is almost three times to strength as offered by SS 316l.
Localized corrosion is not the single only attacking factor for application materials in the seawater. Additionally a mildly corrosion resistant alloy can be chosen and then it can be protected. Suitable security is given by thermoplastic polyuerathane jacket that is economically extruded on the regular tubing. Recent configurations in the Gulf ocean pair this concept with superduplex tubes and this offers outstanding service. The extrusion of a thermoplastic coating on a tubing is an economical option. SS 316 tubing is chosen and then coated with polyuerathane. However the jacket offers consistent security from corrosive fluids, it should meet the additional needs. It prevents impact, abrasion, and UV damage.
Various kinds of tubing supports and clamps are used. Some are considerably attacked by crevice corrosion by producing tight crevice with extensive crevice areas that reduce the oxygen content so that oxide layer does not reproduce. The support components made from aluminum alloys show good service.
A latest design is developed for pipe supports in which stainless steel tubes are kept between two partially round rods that are made from thermoplastic material. The round tubes are perpendicular to the round support rod that reduces the feasibility of crevice attack.
When you need tubings for marine applications, contact Heanjia Super-Metals, your one stop shop for all materials.