A new weapon in the battle against corrosion

The anticorrosion, mechanical, and physical properties of steels for hydraulic systems are a growing concern both inside and outside the marine and offshore industry. Hydraulic and instrumentation tubing tends to pit and corrode when placed in inaccessible chloridic environments, such as beneath clamps, support trays, and connections. Because of this, standard steel grades are proving insufficient in topside, downhole, or other challenging settings due to corrosion-related tube failures. As found in field tests by Shell, Swagelok, BP, and laboratory tests by Sandvik, even high-quality ASTM 316L tube has a service life of five years, and even less than a year in some cases.

Subsequent to these findings, engineers at Sandvik further researched the affects of final heat treatment on the performance of steel grades in chloridic environments. Types of final heat treatment greatly impact the quality of the tube surface, and any surface irregularities can be settings for corrosion initiation. The smoother the surface, the lower the risk of general corrosion.

To investigate relationships between types of final heat treatment and corrosion media, Sandvik engineers compared bright-annealed to open-annealed and pickled steels in a series of tests.

Bright annealing versus open annealing

Bright annealing is a solution-annealing operation performed in a vacuum or controlled atmosphere containing hydrogen. This ensures that oxidation is reduced to a minimum and the steel surface remains relatively bright. Aside from a bright surface, the oxide layer is very thin after this type of annealing.

Subsequent pickling is not necessary — and pickling can potentially damage the oxide layer. As a result, with a cleaner and brighter appearance and a controlled inside-surface condition, bright-annealed tubing has a much smoother surface, which give the component better resistance to pitting corrosion. In contrast, open annealing heat treatment is performed in an open furnace where atmospheric oxygen causes scaling on the surface of the metal.

Standard specifications for stainless-steel tubes require that they be delivered with a scale-free surface. Pickling is the usual procedure to remove the scaling after tubes have been passivated in an alkaline solution. The pickling process removes the hard scale from the tube surface, but the tube’s surface is also chemically attacked.

Pickling takes place in a bath of diluted nitric and hydrofluoric acids at a temperature not exceeding 40°C. Rather than one single operation, it is a series of dips in different acid baths and water. Water removes residues from the pickling bath, and the internal and external surfaces of the tubes are then flushed with high-pressure water.

Complete removal of the pickling acid in the tubes is critical to avoid the risk of corrosion. Washing procedures are especially difficult when the tube ID is small. The pickling process ends with rinsing in warm water to prevent water stains and leaves the tubes with a dull, silver-white surface.

Marine environment testing in Florida

Extensive tests by Sandvik examined the influence of these solution-annealing processes on hydraulic and instrumentation tubing. Twelve months of atmospheric tests were carried out in a marine environment at the Corrosion Technology Laboratory (CTL) at the NASA Kennedy Space Center in Florida. The environment was chosen for its high temperature, ultraviolet levels, salt concentration, and humidity. Engineers compared tubes from two different production routes: open annealed followed by pickling, and bright annealed followed by surface finishing. The aim of the study was to compare the corrosion resistance of the outer and inner surfaces of the tubes in order to assess the impact of final heat treatment on the quality of the tube surfaces.

Because Sandvik Materials Technology only produces bright-annealed tubes with a 12.7-mm OD and 1.24-mm wall thickness, open annealing and pickling were performed on bright, annealed tubes. The open annealing was performed at 1,100°C followed by water cooling.The tube samples were 250-mm long, and test racks were designed according to ASTM G-50 (1997) “Standard Practice for Conducting Atmospheric Corrosion Tests on Metals.” Samples were mounted on racks using ¼-in. polyoxymetylene rods through the center of the tubes, with each rod secured in place on an aluminum rack. Once the test rack was populated, it was transported to NASA’s Beachside Corrosion Test Site and placed at a 30° angle facing the ocean.

The site is roughly 1½ miles south of NASA’s Launch Complex 39A, directly on the Atlantic Ocean and about 150 ft from the mean high-tide line. This introduced the test specimens to aggressive and very corrosive, high-salt, high-humidity, and high-UV Florida seacoast environmental exposure.

Detailed analysis of the tube surfaces was carried out following the atmospheric test using an optical microscope. The surfaces were cleaned in warm water prior to the inspection, wiped with tissue paper and dried with compressed air. Surface roughness, Ra, was measured with an interferometer.

Optical microscopy analysis, conducted at Sandvik Research and Development facilities in Sweden, found that both tubes were discolored after exposure, especially the underside of the tubes. However, the annealed and pickled tube had considerably worse corrosion evidence than the bright-annealed tube. As shown in Figure 2, the annealed and pickled tube has many pits.

The outer surface roughness of each tube was also measured, and results are shown in the Table. The surface roughness of the annealed and pickled tube was clearly higher than for the bright-annealed tube.

Each tube was then split in two to examine the inner surface with optical microscopy. As clearly shown in Figure 3, the bright annealed sample exhibited higher corrosion resistance of the two tubes. Its inner surfaces are still bright and shiny after one year of marine exposure. On the other hand, the open-annealed sample exhibited significant corrosion. Small pits were found in the inner surface of the open-annealed tube, which may have been caused by the residual of the acid used during pickling.

Effects of metallurgy on anticorrosion

Overall, the results of the exposure test shows that the bright- annealed and surface-finished specimens exhibited considerably higher corrosion resistance in this environment than the open-annealed and pickled counterparts. Sandvik has applied this knowledge to develop steels that can outperform insufficient standard grades, like 316L, in hydraulic systems. It is worth examining the metallurgical properties of these grades in more detail to better understand the advantages of bright-annealed and surface-finished tube and also to determine why standard grades are underperforming in hydraulic and instrumentation tubing applications.

Two useful examples are Sandvik 3R60, an austenitic chromium-nickel steel, and Sandvik SAF 2507TM (UNS S32750), a high-alloy and high-strength superduplex tube. Both come in a solution-annealed condition, either white pickled or bright annealed, and each has been shown to outperform standard materials, both in laboratory examinations and in field tests. Each was developed to exhibit the high mechanical strength required for hydraulic cylinders in aggressive environments, such as warm chlorinated seawater and acidic, chloride containing media. The anticorrosion properties of steels are integral to their mechanical strength; chromium content is particularly important for pitting resistance, and alloying with molybdenum and nickel has also proven beneficial.

Figure 4 compares the key alloy content of Sandvik 3R60 alongside levels typically found in standard 316/316L steels. Alloy levels in standard grades are often kept to a minimum in conformance to ASTM standards. As shown, Sandvik 3R60 has higher levels of Cr, Ni, and Mo than equivalent standard grades. In combination, these steels give the material a higher Pitting Resistance Equivalent (PRE) value than of typical steels. Sandvik SAF 2507 has a nominal PRE value of 43 (minimum 42), significantly higher than the PRE values for other duplex 25Cr stainless steels that are not superduplex. This is partially due to the fact that the material contains 4.0% Mo, giving it performance comparable to 6.0% Mo austenitic steels.

These characteristics have been put to the test with ASTM G48, one of the most-severe pitting and crevice-corrosion tests applied to stainless steels. The modified ASTM G48 test exposed Sandvik SAF 2507 to 6% FeCl₃, with and without crevices, for periods of 24 hr. When pits were detected together with a substantial weight loss (>5 mg), the test was interrupted; otherwise the temperature was increased by 5°C, and the test continued with the same sample.

The scatter band in Figure 5 shows that Sandvik SAF 2507 and 6Mo+N alloys have similar resistance to pitting, and critical pitting and crevice temperature (CPT and CCT) values are within the range shown. Yet, Sandvik SAF 2507 has distinct advantages over these 6.0% Mo steels. For one, it is more readily available, giving it lower initial costs.

Tests of Sandvik 3R60 reveal that low-carbon content imparts higher resistance to intergranular corrosion than that exhibited by type ASTM 316 steels. Elsewhere, Sandvik 3R60 was exposed to corrosion testing for 24 hr in boiling Strauss solution (12% H₂SO₄, 6% Cu₂SO₄). Figure 6 shows results in a TTC-diagram and indicates that the material’s resistance to grain boundary attack is much better than ASTM 316L — an advantage in complicated welding operations.

As with Sandvik 2507, the superior resistance of Sandvik 3R60 to these types of corrosion is partly due to its 2.6% Mo content. The 3R60 material has substantially higher resistance to attack than steels of ASTM 304 steels, and also better resistance than ordinary ASTM 316/316L steels with 2.1% Mo.

Conclusions

Laboratory and field tests established that bright-annealed and surface-finished heat-treatment processes hold distinct advantages in manufacturing tube for severe and chloridic hydraulic applications. Furthermore, enhancements to levels of chromium, nickel, and molybdenum improve these steels’ mechanical strength, corrosion resistance, and life over those of equivalent standard grades, such like ASTM 316L.

For these reasons, Sandvik SAF 2507 is widely used in highly corrosive conditions for hydraulic and instrumentation applications in tropical marine environments. Sandvik 3R60 is relied upon for a wide range of industrial applications where steels of type ASTM 304 and 304L have insufficient corrosion resistance such as hydraulic systems, heat exchangers, condensers, and pipelines.

Eduardo Perea is Global Technical Marketing manager for Sandvik Materials Technology, Sandviken, Sweden. For more information, visit www.smt.sandvik.com.