Stainless Steel Study

Ultralow-Manganese vs. Low-Manganese & Standard 316L Alloys

By Gerhard Schiroky & Gary Henrich

During the past 10 years, a number of publications have suggested that 316L stainless steel alloys with extremely low concentrations of manganese (referred to as ultralow-manganese alloys, which contain less than 0.05 percent Mn) are superior in corrosion resistance compared to more conventional 316L alloys, i.e., low-manganese alloys (which contain less than 0.5 percent Mn) and standard 316L alloys (which contain less than 2.0 percent Mn). According to these publications (see references), the generation and redeposition of manganese weld fumes are responsible for causing pitting corrosion in as-welded 316L alloys during exposure to gaseous atmospheres containing halogens (i.e., fluorine, chlorine, bromine) and minute amounts of water vapor. While this argument has been made, there has been a lack of convincing data and meaningful thermodynamic-based discussion on the corrosion characteristics of ultralow-manganese versus low-manganese and standard 316L alloys.

A recent study conducted by Swagelok Company, a manufacturer of stainless steel fluid handling components and systems, examined the corrosion behavior of ultralow-manganese, low-manganese, and standard 316L alloys in different gaseous environments, aiming to answer the following questions:
• Which elements evaporate during orbital autogenous welding of the 316L alloy test samples that contain varying amounts of manganese?
• How does a low-humidity, halogen-containing environment affect as-welded test samples during short-term (24 h) and long-term (28 day) exposure?
• How does the corrosion resistance of post-weld passivated samples compare when they are exposed to a low-humidity (100 PPM moisture), halogen-containing gas atmosphere?

Materials

Table 1. Chemical composition of alloy heats
used for sample preparation

The majority of tests were performed on two low-manganese and two
ultralow-manganese VIM/VAR 316L stainless steel alloys. A limited number of tests were also performed on a standard 316L AOD alloy. Table 1 lists the chemical compositions of the individual alloys and the tests in which they were used.

Weld-Fume Analysis
Tube sections machined from bar stock or cut from electropolished tubing were orbitally welded into longer sticks. The weld fumes were collected and analyzed for chemical composition.

Preparation of Tube Sections
Tube sections measuring 1.00 in. (25.4 mm) in length, 0.250 in. (6.4 mm) outside diameter (OD), and 0.035 in. (0.89 mm) wall thickness were machined from bar stock or cut from tubing, depending on product form. Alloy D tubing had a wall thickness of one mm. The ends of the samples cut from tubing were squared using a facing tool. All tube sections were thoroughly cleaned and dried.

Sections machined from bar stock were electropolished, passivated in nitric acid, rinsed, and cleaned in an aqueous ultrasonic bath. The Alloy D tubing and AOD Alloy E tubing were electropolished in the as-received condition.

Welding Procedure
Several tube sections of each alloy were used to determine the welding conditions for producing welds according to Swagelok specifications. The root weld width equaled approximately twice the wall thickness, causing it to fall in the range 0.052 to 0.088 in. (1.34 to 2.26 mm). Eleven tube sections of each alloy were welded into 11 in. (27.9 cm) long sticks containing ten orbital welds each. Three sticks were prepared for each of the four VIM/VAR alloys.

Fume Collection
During welding, argon purge gas flowed from the downstream tube section through a short piece of Tygon tubing into a nonfritted glass impinger. A solution of 15 mL of 2 percent nitric acid was used in the impinger as the trap solution to collect weld fume particles. During welding of the test specimens, the inside diameter (ID) purge gas from the entire stick was passed through the acid solution. The impinger and the Tygon hose connecting the weld stick to the impinger were rinsed thoroughly with deionized (DI) water after each stick had been welded. The rinse water was added to the trap solution. A separate trap solution was generated for each individual stick.

Figure 1. Weld-fume analysis

In order to extract weld fume deposits from the inner surface of the weld sticks, each was heated in a convection oven set at 120 F (48 C), capped at one end with a PTFE cap, filled with 5 percent nitric acid and allowed to soak for 15 minutes. The extract from each was combined with the trap solution for that stick. In addition, the interior walls of each stick were rinsed with DI water, and the wash solutions were added to the trap solution.

The combined trap and extract solution of each stick was diluted with DI water to a final volume of 20 mL and analyzed by inductively-coupled plasma atomic emission spectroscopy (ICP-AES) for iron, chromium, nickel, manganese, and molybdenum against a reagent blank solution.

Results
The results of the weld-fume analysis are shown in Figure 1. The total measured amounts of iron and manganese were divided by 10 (the number of welds per stick) to determine the amount of metal per each weld. None of the alloys emitted detectable levels of chromium, nickel, or molybdenum during welding. The lower detection limit of the analysis method was 0.4 µg per element for each of the trap solutions, which amounts to 0.04 µg per weld. Figure 1 shows the reproducibility of the weld-fume results is very good for sticks of the same material.

To determine whether any iron and manganese extracted from the inner tube
surface during the nitric-acid soak performed to dissolve the weld-fume deposits, extraction tests were performed on as-received (not welded) Alloy D and AOD Alloy E tubing. The result of these tests showed that small amounts of iron were leached from the unwelded tubing. The amount of iron shown in Figure 1 is estimated to be maximally 10 percent higher than the amount of iron traceable to weld fume deposits only. The extraction tests on the unwelded tubing showed that manganese was not extracted during the nitric acid soak. Hence, the manganese amounts shown in Figure 1 are traceable to weld-fume deposits only.

Discussion
The low-manganese and ultralow-manganese 316L VIM/VAR alloys emitted similar amounts of iron during autogenous orbital welding. Compared to the amount of iron, a relatively small amount of manganese was emitted from the low-manganese Alloys A and B. The amount of manganese emitted from the ultralow-manganese Alloys C and D was below the detection limit of the ICP-AES analysis technique. The amount of iron emitted during welding increased when higher power levels were necessary to obtain welds meeting the specification for weld bead width.

Exposure to Corrosive Gas Mixtures with Low Moisture Concentration

Sample Preparation
Tube sections were prepared as described above, except that the individual tube sections for this test had a length of 1.5 in. (38.1 mm). The sections were welded to Swagelok specifications into three-inch (76.2 mm) long test samples using a gas mixture of 95 percent Ar and 5 percent hydrogen (H₂) as the purge and shield gas. The flowrate of the high-purity gas mixture (two PPM oxygen) through the tubes was 12.5 ft³/h (354 L/h). Orbital weld speed was 10 r/min. Ceriated tungsten electrodes 0.08 in. (2.0 mm) in diameter were used to weld with an arc gap of 0.03 in. (0.76 mm).

Equipment
A corrosive gas flow bench was used for exposing specimens to a relatively low level moisture environment (100 PPM) containing 5 percent of the corrosive gas, either chlorine or hydrogen chloride and nitrogen. Pure gases used in the tests were 99.999 percent N₂, 99.997 percent Cl₂, and 99.995 percent HCl.

Figure 2. Area downstream of weld zone after samples were rinsed with distilled water.

Electronic mass flow controllers were used to control the gas flows. All mass flow controllers were protected with inline filters.

In the corrosive gas flow bench, the dry carrier gas (N₂) was split into two streams and used either as dry diluent gas or as the carrier gas to deliver moisture. The system was also plumbed to provide total system drydown by introducing dry nitrogen via a purge assembly located between the corrosive gas source cylinder and its gas regulator.

The desired moisture condition was produced by flowing the dry nitrogen through a permeation chamber containing a 20 cm long permeation device calibrated to deliver two ng H₂O/min per centimeter of device length at 100 C. The permeation device released a given amount of moisture according to temperature and independent of flowrate. The corrosive gas stream was maintained at 5 percent of the total gas volume and all gas pressures were set at 20 PSIG (1.3 bar).

The test chamber consisted of a horizontal 12 by 1.5 in. (305 by 38.1 mm) OD Pyrex glass tube that was connected to Swagelok Ultra-Torr fittings at either end. Specimens were positioned in the chamber by removing a fitting at one end and sliding in or out a Pyrex glass cradle that supported the specimens in a horizontal position. The moisture was monitored online using a digital hygrometer located immediately prior to corrosive gas introduction. A second hygrometer was used to monitor the moisture of gas downstream from the chamber during system drydown.

Corrosion Tests
Corrosion testing was performed on as-welded and post-weld passivated tubular samples of Alloy A, Alloy B, Alloy C, Alloy D, and AOD Alloy E tubing. After welding, a two-inch (50.8 mm) length of the three-inch (76.2 mm) long samples was cut open to facilitate characterization of the internal surfaces following exposure to the corrosive gas mixtures.

The samples were exposed to the flowing, hydrogen chloride-containing and chlorine-containing gas mixtures for 24 hours and 28 days, respectively.

Results
Short-Term Exposure (24 Hours): Following the 24-hour exposure to the flowing, hydrogen chloride-containing and chlorine-containing gas mixtures, the low-manganese samples showed some discoloration just downstream of the weld zone; however, the discolored band was barely noticeable. It was even more difficult to detect any discoloration in the ultralow-manganese samples. The AOD Alloy E sample developed a more pronounced yellow-brown band.

The surfaces of the exposed samples were then characterized by scanning electron microscopy (SEM). All samples showed the presence of submicrometer-sized particles, primarily downstream of the weld zone. Analysis of the particles by EDS indicated the presence of chlorine. Identification of metal constituents was inconclusive because the small particles were in intimate contact with the alloy surface.

After the particles had been rinsed off with distilled water, the tube samples were examined by SEM for signs of corrosion. No evidence of corrosion was detected in any of the tube samples.

No brown bands were observed in any of the post-weld passivated samples. Examination by SEM of these samples did not reveal any particles or signs of corrosion.

Long-Term Exposure (28 Days): Following the 28-day exposure of welded tubes of Alloy A, Alloy B, and Alloy D to the hydrogen chloride-containing gas mixture, all samples showed a band of discoloration both upstream and downstream of the weld zone. The width of the band was approximately 0.4 in. (10.2 mm) on either side of the weld zone.

Analysis by SEM showed that the bands consisted of individual particles approximately one to 10 µm in size.

The areas between particles appear to have been severely etched during exposure and show signs of pitting corrosion. In locations where the particles have been rinsed off with distilled water, the substrate surface appears to be similarly etched and pitted (Figure 2).

The samples were also characterized by SEM in areas farther downstream of the weld zone, approximately 1.0 in. (25.4 mm) beyond the weld. Very small (0.1 µm dia.), isolated particles were detected. The surfaces between the particles appeared featureless and resembled the surfaces of as-welded (unexposed) tubes. The extremely small size of the particles made it impossible to determine whether they were associated with any signs of corrosion.

Discussion
Short-Term Exposure (24 Hours): The presence of a yellow-brownish band on the exposed AOD Alloy E sample may have been a result of the evaporation and redepositing of manganese. The number and density of the observed particles were highest for the AOD Alloy E samples. There were no signs of corrosion in the alloy samples exposed for 24 hours. This finding suggests that under 24-hour test conditions, the presence of manganese in the weld-fume deposits did not lead to corrosion of the low-manganese alloy samples.

Long-Term Exposure (28 Days): The most significant finding of the 28-day corrosion test was that both ultralow-manganese (Alloy D) and low-manganese (Alloys A and B) experienced comparable attack on surfaces in close proximity (0.4 inch [10.2 mm]) to the weld zone, as determined by SEM (Figure 2). Particles with similar morphology formed in the three alloys and, upon rinsing with distilled water, caused remnants to stay behind. The surfaces of the alloys, both between the particles and at locations where the particles had been located (prior to rinsing), appeared etched and pitted.

Based on the observations made by SEM, all three alloys showed signs of similar, significant corrosion in locations where the discolored bands had been observed. No such corrosive attack was observed in locations beyond the bands. Hence, it is likely that corrosion was caused by the presence of iron weld fume deposits. SEM analysis of the samples did not provide any evidence that the presence of manganese in the weld fume deposits accelerated or exacerbated corrosive attack.

Key Findings
During autogenous orbital welding of 316L stainless steel tubes, the molten weld pool reaches temperatures high enough for measurable quantities of alloy constituents to evaporate. As long as the manganese concentration of the alloys is sufficiently low, more iron evaporates than manganese. The emitted metals redeposit on the colder tube surfaces adjacent to the weld zone.

The iron-rich surface film adjacent to the weld zone serves as an initiation site for corrosive attack on the tubes. Most likely, iron reacts with hydrogen chloride or chlorine to form iron chloride. In the presence of moisture, the chloride hydrates. This reaction liberates hydrochloric acid that subsequently etches the surface of the alloy.

Because the low-manganese alloys and the ultralow-manganese alloys released approximately the same amount of iron during welding, and because the corrosion tests led to very similar observations in low-manganese and ultralow-manganese alloys, it is believed that the iron-rich weld deposits adjacent to the weld zones are responsible for the observed corrosion behavior of welded samples made from either type of alloy.

Gerhard Schiroky is manager of materials technology for Swagelok Company, where his responsibilities include the improvement of materials and processing techniques, analysis of fluid system components in Swagelok’s Metallurgical Laboratory and Chemistry and Corrosion Laboratory, and the characterization of materials and their interactions in components that are in the design stage. Mr. Schiroky earned his Ph.D. in Materials Science and Engineering at the University of Utah. He can be reached at gerhard.schiroky@swagelok.com or 440 248-4600, ext. 4388.

Gary Henrich is supervisor of the Chemistry and Corrosion Laboratory at Swagelok Company, where his responsibilities include development of chemical and corrosion tests for supporting new alloy materials, environmental compliance testing, and cleanliness monitoring of Swagelok products and processes. Mr. Henrich earned his bachelor’s degree in chemistry and microbiology at Miami University, Ohio. He can be reached at gary.henrich@swagelok.com or 440 349-5657, ext. 4869.

www.swagelok.com

A version of this article appeared in Future Fab International (www.future-fab.com).

References
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