Published by Nicholas Woodroof,
Hydraulic and instrumentation (H&I) systems are vital and often very large installations on both land-based plants and marine offshore applications. Normally, they consist of many kilometres of high-quality stainless steel, nickel base or copper alloy tubing with strict dimensional tolerances. The tubes are connected with fittings, each fitting being precision made and expensive. It is therefore not surprising that engineering companies and plant owners will try to find the most cost-effective material solution, while at the same time avoiding a compromise on quality and lifetime of the installation.
The pressures of the marine environment
When choosing material for H&I installations in inland regions, the most commonly used alloy is stainless steel of type 316L (UNS S31603) with matching fittings. Unless there is some specific plant-related condition requiring special consideration, this will provide the system with sufficient corrosion resistance for the environment to which it is exposed, thereby providing a cost-effective and long-term solution. However, in the case of coastal or marine installations, the conditions are significantly different, as the system will be exposed to sea spray, causing salt deposits to form on the tube and coupling surfaces.
Corrosion can occur naturally whenever metals are exposed to corrosive species. If corrosion is initiated, the rate of corrosion is governed by several factors, including metallic alloy temperature and concentration of corrosive species. In marine environments, it is the high concentration of chloride that initiates the corrosive processes on stainless steels and nickel base alloys. Consequently, the stainless steel grades most commonly used for inland applications are inappropriate under these coastal and marine conditions.
The marine environment subjects stainless steel to two main forms of localised damage: pitting corrosion and crevice corrosion. Pitting corrosion forms localised attacks which give rise to leakage of the tubes when the tube wall is penetrated. Crevice corrosion occurs in localised tight spaces where oxygen access is limited, causing corrosion within the crevice. With respect to H&I equipment, pitting corrosion is commonly seen on the tubes and fittings, while crevice corrosion occurs most frequently beneath clamps or inside fittings. In the case of severe crevice corrosion inside a fitting, the fitting may become loose and become a dangerous projectile when the system is pressurised. In stressed components of 316L type stainless steel, chloride-induced stress corrosion cracking can develop if the ambient temperature is high or if the process fluids inside the tubes are hot. The stresses in the material can be due to external loads or cold working of the material and cracking of the tubes will result in complete failure of that part of the system.
The corrosion processes can have significant consequences for industrial plants, resulting in costly downtime, high maintenance expenses and – if left undetected – potentially dangerous and destructive damage. Despite these risks, standard grades of stainless steel such as 316L are still often specified for these harsh environments, perhaps due to a lack of other options, a lack of awareness, or cost cutting. However, when specifying for an industrial H&I installation in a marine environment, the choice of stainless steel grade is a crucial consideration.
Selecting the right stainless steel
A choice of 316L stainless steel tubing with a plastic coating is one alternative when selecting H&I material for marine environments. This solution displays some measure of corrosion resistance, as well as involving lower initial cost than choosing a higher alloyed stainless steel. However, this solution often proves to be unsatisfactory in the long run for a number of reasons. First of all, the plastic coating must be stripped from the metal at all points where fittings are to be installed. The fittings then need to be protected by some sort of sleeve to limit corrosion, but even so this area will always be a weak spot in the system. Great care is also required during installation and throughout the service period of the H&I system to protect the coating from mechanical damage. Leakage inside the plastic coating will result in exposure of the underlying metal and a high risk of crevice corrosion. Many plastic coatings also degrade when exposed to ultraviolet (UV) radiation from sunlight, high temperatures or other atmospheric conditions, which can cause them to become brittle and eventually to crack.
Alternatively, operators of marine plants can turn to the higher alloyed grades of stainless steel, whose chemical properties render them more adept at withstanding corrosive environments. The most widely used options are austenitic grades such as 904L, 6Mo super-austenitics or nickel-based Alloy 625, but super duplex (SD) 2507 is also an option. As far as pitting corrosion is concerned, 904L is the least resistant, 6Mo and 2507 display similar levels of resistance, and Alloy 625 is the most resistant. When it comes to crevice corrosion in seawater, 6Mo and super duplex have similar resistance, performing much better than 904L. However, Alloy 625 has in many cases been reported to have an unexpectedly low resistance towards crevice corrosion in seawater, which may become an issue, for example beneath clamps. It is also important that the fittings used for the H&I installation should match the tube material as using lower alloyed fittings will lower the overall performance of the H&I system. Using the correct fittings is particularly vital for compressive fittings, where the ferrule and body of the fitting must have sufficient hardness and strength to be able to deform the tube material.
Field testing of different grades for H&I
To understand the performance of different grades exposed to marine environments, field investigations were performed with tubes made of 316L, Alloy 825, 904L and SD 2507. The tests were conducted at two separate marine coastal locations: the west coast of Sweden (temperate climate) and Florida (tropical climate). The tests were performed for a period of 12 or 13 months, and the samples were exposed to sea spray at both test sites. The test matrix included both tubes with fittings (not Alloy 825) and bent tubes in 316L to investigate whether stress corrosion cracking could be promoted in cold-formed material. The fittings used for all tests were made of 316L.
The results from the field testing show that 316L and Alloy 825 are not suitable material choices for marine H&I installations in any of the test environments, as both alloys suffered from extensive pitting corrosion after testing. Although the 316L showed no cracks in the bent tubes, there was crevice corrosion of the tube material under the fitting ferrule. The couplings in 316L were noted to develop corrosion on the threads for both test sites. In temperate marine conditions off the coast of Sweden, 904L showed some minor pitting corrosion on the tube but no crevice corrosion inside the fittings. Under the tropical test conditions in Florida, the only grade that did not show any pitting or crevice corrosion after the test exposure was super duplex (SD) 2507.
From the field tests it can be concluded that, for temperate coastal regions, 904L provides a level of minimal corrosion resistance that may be sufficient for some applications. However, for more severe conditions this alloy will not perform satisfactorily. Under such conditions, super-duplex is the only tested alloy that will not corrode. Alloy 825 actually performs slightly worse than 316L and is therefore not recommended for H&I systems in marine environments.
Higher alloyed materials for H&I systems
Alloy 904L would seem to be a good material solution for coastal installations in temperate climatic zones. However, for installations in tropical regions, an even higher alloyed grade must be considered. As discussed above, the typical options in these conditions would be SD 2507, super-austenitic 6Mo or Alloy 625. From a cost perspective, super duplex will be cheaper than the other two options and in addition offers the advantage of higher yield strength, which makes it possible for thinner tube walls to be chosen. Key drawbacks associated with super duplex tubes are that the high yield strength can render them more difficult to bend in the field, and that there is a limited number of fittings manufacturers.
Ultimately, the specific material grade that should be chosen will depend on the climate and particular environmental conditions in the region and location of the installation. There may also be restrictions as to which grades are allowed; for example, if operators are adhering to a set of specific industry standards which specify or exclude certain material options.
Super duplex most suited to marine H&I
In tropical and offshore H&I installations, SD (super duplex) 2507 is the most attractive solution, displaying no pitting or crevice corrosion during the field investigations carried out. As noted, it also has a high yield strength, which makes it possible to choose thinner tube walls. This results in a lighter system, which is of real advantage in a variety of applications, for example, freight ships or weight-sensitive offshore vessels.
The use of super-austenitic 6Mo grades is still common. Their corrosion properties are sufficiently high and the tubes can be easier to handle in the field due to their lower yield strength as compared to SD 2507. The availability of several fitting suppliers may also provide a reason for choosing this grade. Due to the higher cost implications for Alloy 625, this material choice is typically only considered in very special cases, for example when resistance to hydrogen sulfide cracking is crucial.
As the results show, SD 2507 displays excellent corrosion resistance properties in the marine and coastal environment, across a range of climatic conditions, as well as high material strength and mechanical properties. The higher initial investment that may be called for is rewarded with a robust damage resistance that prolongs service lifetime and limits the costs of maintaining the H&I system.
Author: Jonas Höwing, Sandvik Materials Technology, MENA
Read the article online at: https://www.oilfieldtechnology.com/offshore-and-subsea/12082019/marine-capable-materials/
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