During the past 10 years, a number of publications1-15 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% Mn) are superior in corrosion resistance as compared to more conventional 316L alloys, i.e., low-manganese alloys (which contain less than 0.5% Mn) and standard 316L alloys (which contain less than 2.0% Mn).

Several publications5-9 claim that during welding, the manganese evaporates from the weld pool and redeposits in the heat-affected zone, primarily downstream of the weld pool. Pure molten manganese has a much higher vapor pressure than the major alloy constituents—iron, chrome, nickel, and molybdenum. As a result, as long as the manganese concentration in the alloy is sufficiently high, a measurable amount of manganese will evaporate during welding. When the manganese concentration is reduced, a point is reached at which more iron than manganese will evaporate.

It has been claimed that 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 in numerous publications, convincing data and a meaningful thermodynamic-based discussion have not been provided to date.

The objective of this study was to determine the corrosion behavior of ultralow-manganese and low-manganese 316L alloys in different gaseous environments and to generate answers to 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 hr) 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

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 inch (25.4 mm) in length, 0.250 inch (6.4 mm) outside diameter (OD), and 0.035 inch (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 1.0 mm. The ends of the samples cut from tubing were squared using a Swagelok® facing tool. All tube sections were cleaned using an industrial alkaline detergent (10% strength) by fixturing the test specimens in a sieve and immersing in an ultrasonic bath set at 160°F (71°C) for 15 minutes. The wash step was repeated twice with fresh wash solution. The specimens were flushed with deionized (DI) water, placed in a beaker containing DI water, and ultrasonically agitated for 10 minutes. The specimens were then placed in fresh DI water, and the wash step was repeated until the rinse water remained clear after rinsing. The specimens were dried in an oven set at 230°F (110°C) for 30 minutes.

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 equals approximately twice the wall thickness, causing it to fall in the range 0.052 to 0.088 inch (1.34 to 2.26 mm). Eleven tube sections of each alloy were welded into 11 inch (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 non-fritted glass impinger. A solution of 15 mL of 2% 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 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.

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% 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 any 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. This figure shows that the reproducibility of the weld fume results is very good for sticks of the same material.

In order 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% 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 ultra-low-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 (Figure 2).

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 inch (38.1mm). The sections were welded to Swagelok specifications into 3.0 inch (76.2 mm) long test samples using a gas mixture of 95% Ar and 5% hydrogen (H2) as the purge and shield gas. The flow rate of the high-purity gas mixture (2 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 inch (2.0 mm) in diameter were used to weld with an arc gap of 0.03 inch (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% of the corrosive gas, either chlorine or hydrogen chloride and nitrogen. Pure gases used in the tests were 99.999% N2, 99.997% Cl2, and 99.995% HCl. 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) is split into two streams and used either as dry diluent gas or as the carrier gas to deliver moisture. The system is 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 2 ng H2O/min per centimeter of device length at 100°C. The permeation device releases a given amount of moisture according to temperature and independent of flow rate. The corrosive gas stream was maintained at 5% 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 inch (305 by 38.1mm) 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, the 3 inch (76.2 mm) long samples were cut open, as shown in Figure 3, 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 inch(10.2 mm) on either side of the weld zone.

Analysis by SEM showed that the bands consisted of individual particles approximately 1 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 were rinsed off with distilled water, the substrate surface appeared to be similarly etched and pitted (Figure 4).

The samples were also characterized by SEM in areas farther downstream of the weld zone, approximately 1.0 inch (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 manganese content of AOD Alloy E was 1.57%, and that of Alloy A and Alloy B was only approximately 0.3%, leading to the expectation that a substantially larger amount of manganese would evaporate and redeposit during the welding of AOD Alloy E.

Also, the number and density of the observed particles were highest for the AOD Alloy E samples. The easily visible brown band appeared to be related to the larger number of chlorine-rich crystals observed downstream of the weld zone.

Neither deposits of iron nor of manganese led to any signs of corrosion in the alloy samples exposed for 24 hours. This finding suggested 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 4). 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 accelerates or exacerbates corrosive attack.

Conclusions

Based on the results from the weld fume analysis and the corrosion tests, the following conclusions are reached:

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.

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