Sodium Chlorate Transfer Line Corrosion Failure & New Material

Sodium Chlorate Transfer Line Corrosion Failure & New Material Study Josh Lauer – WestRock Mill, Tacoma, WA

ABSTRACT Preventing leaks in chemical transport lines is essential in minimizing product loss and preventing personnel safety issues. Sodium chlorate used in the production of chlorine dioxide (ClO2) has caused numerous failures in existing piping materials resulting in increased maintenance expense and emergency temporary repairs. This paper reviews the reasons behind historical material choices, the difficulties encountered with our current system, and new materials that are currently being tested. It also includes diagnostics on previous material failures along with availability and cost evaluation of current vs. potential replacement options. This investigation evaluated the viability of using Duplex 2205 in sodium chlorate transfer systems versus more traditional materials. The availability and costs were compared to what is currently used and were found to be comparable. After conducting the examination, it was decided to implement several test sections of pipeline to prove out the material change. If the new sections show no signs of failure in the coming months, the remainder of the system will be changed over as well.

INTRODUCTION Historically the Tacoma Mill has had difficulty maintaining the sodium chlorate chemical transfer system and preventing leaks. Proper chemical handling is an important safety measure and cost reduction tool. This is particularly important in applications where chemical cost and reactivity or hazardous properties are higher. The chemicals used in the manufacture of ClO2 are hazardous and can be challenging to handle and transport. Sodium chlorate, sulfuric acid, and methanol each have their difficulties. This document will focus primarily on sodium chlorate. Product loss is comprised of inefficient operation and material or maintenance failures. Operational efficiency is variable and can be addressed with better engineering and administrative controls. There are procedures in place to evaluate these types of failures and develop solutions. Material failures need to be addressed and corrected as better practices and information becomes available. The primary causes of chemical loss from material failure are broken pipes and failed gaskets. It is tolerable to have some small chemical loss depending on the material and location. If sodium chlorate leaks but is contained and can be cleaned up it is of lower concern than chlorine dioxide or concentrated sulfuric acid. Chlorine dioxide leaks must be immediately corrected, and follow-up investigations addressed.

SINGLE VESSEL PROCESS (SVP) - CHEMICAL REACTION & EFFICENCY Sodium chlorate is one of the components used in the production of the chlorine dioxide manufactured on-site that is consumed in the bleaching process. The sodium chlorate is shipped to the facility in a dry crystalline form via railcar and made down into an aqueous solution. Dissolving sodium chlorate is endothermic and requires steam addition to completely dissolve the crystals. To dissolve the sodium chlorate the slurry is circulated in a loop from the tank, out to the pump, through the rail car it’s shipped in, and back into the tank. This section of pipe line is subjected to a harsher environment than the line section after the crystal is fully dissolved due to the abrasive chemical slurry.

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12 8 3 → 12 3 4 9 [1]

The theoretical maximum efficiency ratio for the conversion of sodium chlorate to ClO2 is approximately 1.57:1. An exceptional conversion target goal is 1.65:1 or better. Currently the Tacoma Mill average conversion is near 2.05:1. This is wasteful and has driven an effort to reduce cost by increasing efficiency. The main issue noted were leaks that occurred in the piping system allowing chemical to drain to the sewer system. The source of leaks can be narrowed down to one of two possible issues; the first being a gasket failure and the second being material failure. Gasket failure is due to improper installation technique or the incorrect material being used. Gasket failure is possible even when properly installed due to age and heat cycling, but it is rare. Material failure is due to heat cycling, inadequate weld penetration, improper weld procedure, or assembly error allowing stagnant wetted spaces. Assembly error and inadequate weld penetration become more prevalent when repairs must be made in the field under adverse conditions.

EXISTING MATERIAL FAILURES To date material failures have been experienced in fiberglass, 316 stainless steel, and Polyvinylidene Fluoride (PVDF) lined pipe. Fiberglass (FRP) pipe is not recommended in chlorate applications when the chlorate is only partially dissolved. The chlorate slurry will rapidly erode the interior gel coat and fiberglass mesh. Even high grades of fiberglass tend to erode in slurry applications. Experience has shown that FRP is also not desirable in fully dissolved chlorate applications if the temperature exceeds about 160 degrees F. Even at temperatures below 160 F heat cycling will limit the FRP service life. Fortunately, FRP can usually be repaired with an external patch and depending on the application can remain in service for some time (Figure 1).

Figure 1 – Patched FRP Overflow Line

PVDF [2] lined carbon steel pipe has also been used with mixed success. The lined pipe generally has a good service life once installed with the correct torque. The primary failure method is the chemical attack on the lining over time and the heat cycling due to the dissimilar materials. The PVDF lining has a different thermal growth rate than carbon steel which leads to fatigue in the PVDF. After significant fatigue the PVDF lining starts to blister (Figure 2), which leads to cracking that allows chemical to reach the carbon steel pipe. Total failure can be expected quickly depending on the material that reaches the carbon steel pipe. The most significant problem with lined pipe is the long lead time for standard items and particularly long lead time for custom ordered components. This necessitates using temporary fiberglass line sections which increases costs, causes repeat work, and possible additional leaks.

Figure 2 – 2” Blister in Lined Pipe

316 Stainless steel has been another popular choice and is the recommended material by company standard in addition to FRP and titanium. Major issues with the use of stainless steel are: stress corrosion cracking (SCC), pitting, chloride attack at intergranular boundaries and any stagnant area resulting in crevice corrosion. All these issues are due to assembly issues and operating conditions. These failures can be caused by improper welds during assembly, pipe strain, gaps left during assembly for process chemicals to stagnate in, or even using substandard material. At present the most commonly seen type of failure is due to chloride attack in crevices left during the assembly or welding process. Crevice corrosion yields highly localized metal loss in the isolated electrochemical cell and the environment is much more aggressive than in the bulk of the chemical [3]. Proper fabrication requires full penetration welds and eliminating pockets or crevices that would allow material to stagnate. It is also important to use quality materials to avoid those with poor casting which can contain voids in the material. Poor castings with void space inclusions can accelerate intergranular corrosion and make the material more susceptible to SCC. An assortment of leaking pipe sections were sent out to our internal lab for analysis this past spring. The test results showed crevice corrosion (Figure 3), pitting, and indicated poor quality fabrication. A section of PVDF lined pipe was also analyzed and found to have failed due to cracks that developed in the lining, likely due to repeated heat cycling. Figure 3 – 316 Stainless Steel Crevice Corrosion

A final word on recommended materials; company specifications recommend the use of 316L stainless steel, titanium, PVDF lined carbon steel, or FRP for fully dissolved sodium chlorate solutions. For partially dissolved sodium chlorate slurries only 316L stainless steel or titanium should be used. Outside of these recommendations internal specialists [4] have supported the use of Hastelloy C-276, Inconel 625, and Duplex 2205. Given the recommendation of possible material choices, Duplex 2205 was chosen to evaluate due to its increased durability and relative ease of assembly. Duplex 2205 shows high resistance to: fatigue, stress corrosion cracking, chloride pitting and crevice corrosion, and low thermal expansion. The primary reason for consideration in this case is the resistance to chloride pitting and crevice corrosion. A strong secondary reason is the similar weldability and workability to 316 stainless steel.

MATERIAL AVAILABILITY Material availability will vary dependent on the quantity ordered and the warehouse location. Generally, 316 stainless steel can be sourced in 1-2 weeks if it’s not stocked on-site. At present there is a large selection of material stored at the site warehouse that can be used and then will be restocked. Lined pipe can be custom ordered in 3-4 weeks if needed, but there is also a small selection of common sizes that are stored in the on-site warehouse. Titanium and Duplex 2205 are still mostly special order at this point. Only custom machine parts and valves are kept on-site due to long lead times. The lead time for the pipe in specialty metals is usually between 4-6 weeks. When considering new (or replacement) installations the most efficient process is to drop ship directly to the fabricator’s shop. Some of the available contract fabrication shops are also able to source materials directly at competitive prices. As always, the lead time will vary depending on amount ordered and how common the sizes are. The longer lead time for specialty materials is a consideration during the work order planning phase but is becoming less of a concern. As older, more worn, materials are replaced with metals that are more durable reliability increases and the repairs shift from reactive to planned maintenance. A cost comparison of materials was completed this spring (Figure 4) to support the reevaluation of the mill piping material selection. To obscure vendor pricing stainless steel was used as a basis since it is currently the most abundantly used material in sodium chlorate systems at the Tacoma Mill. All other materials were converted to percentage of 316 stainless steel. The most commonly used materials at this time are PVDF lined pipe and 316 stainless steel. The relative cost comparison shows that while the stainless costs are quite low the lined pipe is very high. The interesting point is that 2205 is mostly stable across each category and cheaper that lined in every case. Figure 4 – Standard Material Costs

Material Cost Comparison

2" 4" 6" 2" 4" 6" 2" 4" 6" Pipe Elbow Tee

316 SS 2205 Titanium Lined

The price stability of 316 stainless and lined pipe is normally very good. The use of old quotes to estimate for new projects is usually accurate to within 10%, even if the quotes are dated. Titanium is the opposite of this; the quotes are only valid for one week and if a purchase order is not issued in time the material will have to be re-quoted. The difference from one quote to the next can be significant and can even cause project delays. There is also one other innovative option that is worth mention, and that is elastic strain preload fittings (ESP). The ESP fittings are considered to be a weld equivalent solution and are capable of passing a helium leak test. They are available in 316 SS, compatible with ordinary pipe, and are relatively quick and easy to use in adverse conditions. The drawbacks in recent experience are the need for specialized install tooling, long lead times for items not stocked locally, and limited size selection. The ESP fittings are also approximately 20 times more expensive than the comparable Duplex 2205.

FABRICATION A standard pipe section containing a wide selection of fittings and sizes was chosen as the basis of comparison. This section (Figure 5) was used to estimate material and fabrication costs. Figure 5 – Design ISO Drawing

Fabrication costs between materials can be minimized if the work can be performed in the shop. There is approximately a 30% increase in fabrication cost between 316 stainless and titanium, but only about a 15% difference between 316 stainless and Duplex 2205. These differences are minor when compared to the cost increase of field welding vs. shop fabrication. Due to the difficulty and extra labor involved in field fabrication we see an increase of 2.2 to 2.3 times the cost regardless of material selected. A precaution now taken when welding Duplex 2205 and titanium is to use a self-feeding automated TIG welder when able and to purge the interior and exterior with the appropriate inert gas. The automated TIG welder can use a process that’s known as a keyhole weld to join the pipe sections. This eliminates the need to bevel grind the material and do root and finish passes. The automated TIG welder also eliminates the variability that would result from a free-hand weld. The process initially takes additional time to set up the jig, but once set is much faster. Depending on the fabrication shop schedule and the size of the order, replacement sections can usually be completed in about a week after materials arrive at the shop. There is always the option of rush ordering in an emergency, but that can quickly devolve into using whatever material is available first.

TEST SECTIONS After examining the cost and availability of Duplex 2205 the decision was made to proceed with several test sections of pipe. These sections were fabricated using the automatic TIG equipment. The fabrication shop was able to deliver outstanding quality welds that would pass x-ray testing. The first two test sections of pipe were installed February 2019 and as of July are showing no signs of leaking. There was also a small section of pipe installed using the ESP fittings in an easy to service location to test the new technology. After experiencing long lead times, high expense of the fittings, and pipe strain on installation; it was decided to hold off on further testing despite the fittings performing well.

CONCLUSIONS To reduce expense due to lost chemical and excess maintenance the existing and historical pipe materials were evaluated against possible new alternatives. After examining material failures and the recommended guidelines; both corporate policy and from the materials specialist, Duplex 2205 was chosen as a candidate for evaluation. Quotes for materials and fabrication came back favorable and the decision was made to proceed with several field tests. The results of this investigation have shown that Duplex 2205 being the more cost-effective solution; while not being necessarily as durable as Hastelloy and Inconel, is certainly superior to 316L stainless steel. The Duplex 2205 also shows price stability over time and similar costs over the size range. It is also significantly cheaper than PVDF lined pipe in every application, even accounting for the added fabrication cost. If the test sections perform well over the coming months, it will be recommended to replace the remaining sections of material with new Duplex 2205.

REFERENCES [1] Rollbuhler, Tom, 2018, SVP-Lite Chlorine Dioxide Operator Training, AkzoNobel, p. 32. [2] 2019, “Kynar Linings,” https://www.electrochemical.net/corrosion-protection/kynar-linings, July 2, 2019, Electro Chemical Engineering & Manufacturing Co., Emmaus, PA. [3] Parrott, R. BSc PhD MIMMM CEng, Pitts, H. MEng PhD, 2011, “Chloride Stress Corrosion Cracking in Austenitic Stainless Steel,” p. 3-4. [4] Willis, Jim, 2019, private communication, Richmond, VA, July 1, 2019. [5] Clingman, Amy, 2019, “Design ISO Drawing.”

Gateway to the Future

SINGLE VESSEL PROCESS (SVP) - CHEMICAL REACTION & EFFICENCY

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12 8 3 → 12 3 4 9 [1] Gateway to the Future

EXISTING MATERIAL FAILURES Gateway to the Future

FRP - Fiberglass Gateway to the Future

KYNAR – Polyvinylidene Fluoride, PVDF Lined Pipe Gateway to the Future

KYNAR – Polyvinylidene Fluoride, PVDF Lined Pipe, Cont. Gateway to the Future

316 STAINLESS STEEL – Welding Issues Gateway to the Future

316 STAINLESS STEEL – Pitting Gateway to the Future

316 STAINLESS STEEL – Crevice Corrosion & Stress Corrosion Cracking Gateway to the Future

MATERIAL AVAILABILITY & RELATIVE COST

Material Cost Comparison

2" 4" 6" 2" 4" 6" 2" 4" 6" Pipe Elbow Tee

316 SS 2205 Titanium Lined Gateway to the Future

FABRICATION Gateway to the Future

TEST SECTIONS – Duplex 2205 Gateway to the Future

TEST SECTIONS – LokRING Gateway to the Future

CONCLUSIONS Gateway to the Future

QUESTIONS?