Oil & Gas Drilling Rig on Water

Stress Corrosion Cracking and Stainless Steel in Marine Environments

When stainless steel is put in marine environments, pitting corrosion or crevice corrosion will take place. Similar to crevice corrosion is galvanic corrosion which is also the result of low resistivity. The salt content in these environments ranges from 10,000 – 19,000 mg/L of chlorides.

Pitting Corrosion (or Crevice Corrosion) happens when the passive oxide film on an alloy surface breaks down in a chloride-rich environment. The higher chloride concentrations, more acidic environments, and elevated temperatures, the more likely there will be a breakdown of this passive film. The higher an alloy’s Pitting Resistance Equivalent Number (PREN), the higher its resistance to localized corrosion.  This is calculated as PREN = %Cr + 3.3(%Mo + 0.5%W) + 16%N.

Certain alloys are susceptible to Stress Corrosion Cracking (SCC). SCC occurs when chloride ions interact with the material where tensile stresses are highest. This produces stress levels that are below the yield strength of an alloy and failure can occur.

“Stainless steel is a steel alloy with a minimum weight content of 10.5% chromium (Cr), the least amount of Cr required to produce a passive chromium oxide (Cr2O3) film that prevents surface corrosion by blocking the diffusion of oxygen into the steel.”

Corrosion consultant Roger Francis and foundry and metallurgical consultant Stan Hebdon comment that “a strong correlation has been established between the PREN for particular types of stainless steel and their resistance to crevice corrosion in seawater. In their paper, they cite several studies that support the following conclusions: a higher PREN indicates greater resistance to corrosion; only alloys with a PREN=40 will resist crevice corrosion in seawater; austenitic stainless steel alloys need a higher PREN than duplex stainless steel alloys to be resistant to crevice corrosion in seawater; and at various water temperatures (46˚ to 77˚ F), austenitic stainless steel grades with 6% Mo and super duplex stainless steel grades with 25% Cr (both grades have a PREN=40) resisted corrosion attack under all conditions. According to NORSOK, alloys for seawater must have a PREN >40.”

“’The super duplex and the austenitic stainless steel with 6% molybdenum have proven corrosion resistance in seawater,’ Francis says. ‘They’ve been used in the North Sea since the early 1990s.’ He explains that the ferrite phase in the duplex stainless steel alloys gives them strength, so they are at least 50% stronger than the austenitic stainless steel alloyed with 6% Mo. Whether the extra strength is needed depends on the design of the seawater pump. ‘A seawater pump is a pressure vessel,’ he says. ‘The seawater comes in at a low pressure and exits at a high pressure. The whole pump body has to be strong enough to resist that pressure.’ Furthermore, he notes, the strength is needed to resist erosion-corrosion. Since duplex stainless steel alloys are stronger than the austenitic stainless steel, pumps cast of duplex stainless steel can be designed with thinner castings to reduce the component’s weight and cost while retaining its strength.”¹ 

Stainless Steel Type 316

Type 316 is a chromium nickel austenitic stainless steel containing molybdenum. The molybdenum addition enhances the corrosion resistance over that of Type 304 in halide environments as well as in reducing acids such as sulfuric and phosphoric acid. Type 316 can be dual certified as 316L when the composition meets the lower carbon limit of 316L. Type 316L should be specified for welded applications as the low carbon version eliminates chromium carbide precipitation and enhances the corrosion resistance in the as-welded condition.

Type 316 resists atmospheric corrosion as well as in moderately oxidizing environments. It also resists corrosion in marine atmospheres and has excellent resistance to intergranular corrosion in the as-welded condition. Type 316 has excellent strength and toughness at cryogenic temperatures. Type 316 is non-magnetic in the annealed condition but may become slightly magnetic as a result of severe cold working.

Stainless Super Duplex 2507

2507 Stainless Steel was designed for demanding applications which require exceptional strength and corrosion resistance, such as chemical process, petrochemical, and seawater equipment. As a duplex (ferritic-austenitic) stainless steel, 2507 combines the most desirable characteristics of both ferritic and austenitic steels.

The duplex microstructure gives this grade high strength, excellent resistance to chloride stress corrosion cracking, high thermal conductivity, and a low coefficient of thermal expansion. In addition, 2507’s high chromium, molybdenum, and nitrogen levels provide excellent resistance to pitting, crevice, and general corrosion that is similar to the 6% Mo super austenitic stainless steels.

Stainless Nitronic 50 (XM-19)

Nitronic 50 (XM-19) is a high strength and corrosion resistant austenitic stainless steel. XM-19 remains non-magnetic even after being severely cold worked and it maintains strength at high temperatures as well as sub-zero temperatures. Nitronic 50 is a very versatile alloy and with Nitronic 50’s outstanding corrosion resistance it is an effective alloy for use in the petroleum, chemical, marine, nuclear, pulp and paper, textile, and food processing industries.

Nickel Alloy 405 | MONEL® R-405

In addition to these stainless steel marine alloys, Alloy 405 is a ductile nickel-copper alloy with resistance to a variety of corrosive conditions. R405 has good weldability and formability and is used widely in marine equipment and hardware, chemical processing and marine related production due to its high corrosion resistance.

At Double Eagle Alloys, we have a wide range of Type 316 and Stainless Super Duplex Type 2507 products in our inventory and the in-house capabilities to customize our stock to you needs. We also stock a wide range of bar products in Nitronic 50 (0.500” – 15.00” OD) and Ni-Cu 405 (0.500” – 3.50” OD).

Crevice corrosion, stress corrosion cracking, and pitting corrosion are all obstacles faced when choosing the right materials in the marine industry. Make sure you have the right material. Contact Double Eagle Alloys today to learn more about how our industry experts can help you throughout the procurement process.