TPi May 2019

Temperate region

Tropical region

Material strength

Fitting suppliers

Inland

Cost

Coastal

Offshore Coastal

Offshore

316L

Good

Poor

Poor

Poor

Poor

Average Numerous

Base

904L

Good Sufficient

Poor

Poor

Poor

Average Several

>316L

6Mo

Good

Good

Good

Good

Good

Good

Several

>904L

SD 2507

Good

Good

Good

Good

Good

High

Few

~904L

Alloy 625

Good

Good

Good

Good

Good

Good

Few >>6Mo

Comparative properties of stainless steel alloys

Choosing the optimal stainless steel grade

provides sufficient protection from corrosion. However, in a coastal or marine setting the system faces significantly greater exposure to corrosive species in the form of chloride in sea spray, which forms salt deposits on the tube and coupling surfaces, and initiates corrosion in stainless steels and nickel- base alloys. Stainless steel suffers two major types of localised damage in marine conditions: pitting corrosion and crevice corrosion. Pitting corrosion attacks in small concentrated pits, eventually penetrating the tube wall and causing leakage. In H&I equipment, this form of corrosion is most commonly seen on the tubes and fittings. Crevice corrosion is seen in tight spaces with limited oxygen access, most frequently beneath clamps or inside fittings. In severe cases, loose fittings can become dangerous projectiles when the system is highly pressurised. In components of type 316L stainless steel, ambient high temperature or hot fluids within the tube can cause stress corrosion cracking, as can external loads or cold working of the material. Corrosive damage has serious implications for industrial plants: expensive downtime, higher costs of maintenance and – eventually – destructive damage with potentially dangerous consequences. As we have seen, industrial H&I installations in marine conditions face particularly harsh pressures, and therefore the choice of stainless steel grade is crucial in determining plant safety, performance and longevity.

Stainless steel 316L tubing, combined with a plastic coating, provides some measure of corrosion resistance and is less cost-intensive than higher alloyed steel. However, the plastic coating must be stripped from fitting installation points, creating areas of weakness. Mechanical damage during installation and over the lifetime of the H&I system also reduces the integrity of the coating, as do leakage, high temperatures and UV radiation from sunlight. Brittleness and cracking result, leaving the underlying stainless steel exposed to corrosive attack. Using higher alloyed grades of stainless steel is a better alternative, as their chemical properties provide stronger defence against corrosion. These include austenitic grades such as 904L, 6Mo super-austenitic, nickel-based Alloy 625 and super duplex (SD) 2507. With regards to pitting corrosion, 904L is the least resistant, 6Mo and 2507 display similar levels of resistance, and Alloy 625 is the most resistant. Considering crevice corrosion, 6Mo and super duplex have similar resistance, far outperforming 904L. Alloy 625 is sometimes reported to display low resistance to crevice corrosion in seawater. Another consideration is ensuring the fittings used for the H&I installation match the tube material, to avoid lowering the overall performance. This uniformity is especially important in compressive fittings, where the ferrule and body of the

57

www.read-tpi.com

May 2019 TUBE PRODUCTS INTERNATIONAL