Effect of Molybednum on Ss and Nac Resistance

Copyright 2008 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole must be in writing to NACE International, Copyright Division, 1440 South creek Drive, Houston, Texas 777084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.

The Effect of Molybdenum on Stainless Steels and Naphthenic Acid Corrosion Resistance

Glen Gallo Shell Oil Products US

8505 S. Texas Rd Anacortes, WA 98221

Jim Edmondson Shell Global Solutions US Inc.

3333 Highway 6 South Houston, TX 77082

ABSTRACT

Molybdenum containing austenitic stainless steels are specified for naphthenic acid corrosion resistance in crude units. There are many unanswered questions as to how and why molybdenum increases resistance to this corrosion mechanism. This paper will try to document the current state of knowledge on the protective mechanism and the history of increasing molybdenum content versus increased TAN (Total Acid Number) values. Material selection of stainless steels with 2% Mo, 2.5% Mo, 3% Mo, and 6% Mo will be discussed along with case histories of successes and failures. Using information collected from laboratory and operating experience we will attempt to define factors affecting resistance versus molybdenum content and draw conclusions as to what level of molybdenum content is needed as a function of TAN.

KEYWORDS: naphthenic acid, TAN, molybdenum, corrosion, austenitic stainless steel

INTRODUCTION

The use of molybdenum (Mo) containing stainless steels for naphthenic acid corrosion (NAC) is a common practice in the refining industry. When asked, what makes Mo the alloy element of choice in naphthenic acid containing environments, many corrosion engineers would simply state that Mo stabilizes the passive protective film against organic acids like naphthenic, acetic, and formic.

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Paper No.

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Passivity of austenitic stainless steels is common knowledge and can be defined as the formation of a corrosion product on the surface that resists further corrosion in an environment. Over the years, many detailed studies of the stainless steel passive oxide layer have been performed with very slow advances in the understanding of the passive layer composition and why it is positively affected by small alloy additions like Mo. The difficulty in understanding the oxide film composition and factors affecting passivity can be attributed to the complexity of variables. It can also be partially attributed to the very small thickness of the film, which is on the order of 1-3 nanometers. This paper will abide by the following definitions of the passive film: austenitic stainless steel becomes passive when a protective film, Cr2O3, forms on the surface. Cr2O3 is a non-porous oxide with a greater volume than the equivalent metal consumed. Passivity can be measured by corrosion rate.1

There are numerous papers on NAC, the measurement of naphthenic acid content, the variables that affect corrosivity, experiences in different refinery units, and compositional differences in crudes and their crude fractions. Many papers have statements that Mo containing stainless steel alloys are used to resist NAC. There is however, some disagreement on the Mo content required to resist NAC. Much of the difference stems from the number of variables affecting the breakdown of the passive layer and the corrosivity of naphthenic acids and sulfur species that occur together in the crude fractions. It is reasonable to say that a more systematic examination of corrosion rates under varying conditions is required to determine the level of Mo content required for a particular service. Theoretical data, while helpful, is not at the level of understanding needed to explain the mechanism of Mo strengthening the passive layer against NAC. A deeper understanding will lead to improved materials selection for new or replacement components, better inspection methodology for current installations, and drive future work to fill in the gaps.

Purpose of the paper:

Summarize available knowledge on Mo bearing stainless steels in NAC service with respect to: Passivity and corrosion mechanism for Mo bearing stainless steel Parameters affecting corrosion of Mo containing stainless steel Material selection guidelines for Mo content versus NAC Corrosion failures of austenitic stainless steel

DISCUSSION

Naphthenic Acid Corrosion Mechanism for Mo bearing stainless steel

The corrosion reaction for carbon steel and low chrome alloys in NAC is not the same as it is for austenitic stainless steels. While the corrosion reactions of carbon steel have been widely published the reaction and mechanism for corrosion of stainless steels and the role of its protective oxide layer have not been published nor are they well understood. Simply stated, Mo addition causes a significant increase in the protection afforded by the passive film and protection depends on the ability of the passive layer to reform faster than it can be removed. If the passive film is removed or broken, local corrosion will occur and may re-passivate the area. Accelerated corrosion will occur when the passive layer is removed faster than it can reform.

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A promising line of research is looking at the effect of Mo on the microstructural changes of stainless steels.2 As Mo is increased from 0 to 7% the structure moves from a single phase to a dual phase with Mo-rich ferrite and Mo-depleted austenite. Along with the increase in volume fraction of ferrite, the microhardness will also increase. EDX analysis after corrosion testing showed the Mo rich areas to be more resistant to velocity effects in NAC. This supports the belief that a dual phase structure and increased microhardness are beneficial to erosion-corrosion resistance.

Relevant theoretical research for Mo as an alloying element and its effects on pitting resistance and passivity breakdown, not specifically for NAC, have been performed and written about by members of the Electrochemical Society.3 The paper describes Point Defect and Solute-Vacancy Interaction models that provide valuable help in understanding the gap in knowledge surrounding passivity breakdown. A second paper4 provides a good description and schematic (Figure 1) of a passive film. It states, Passive films form as bilayer structures, consisting of a defective oxide (the barrier layer) adjacent to the metal and an outer layer that forms from the reaction of metal cations with species in the solution (including the solvent). Solution phase species may be incorporated in the outer layer, but not in the inner layer, whereas alloying elements from the substrate may be incorporated in both layers.

Figure 1 - Schematic of Processes that Lead to the Formation of Bilayer Films on Metal Surfaces

Since the mechanism of passive film protection by an oxide with Mo in the alloy is not mature enough to predict corrosion under specific operating conditions, the refining industry is using

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corrosion rates obtained from historical successes and failures, lab corrosion data, and empirical corrosion rate models. As stated earlier this is an acceptable means to measure the protection afforded by a passive layer. This method will work provided sufficient data has been collected to account for all of the significant factors that affect the corrosion reaction and passive film. The focus for NAC and protective oxide or sulfide layers has predominantly been on carbon steel and low-alloy steels, not on testing levels of Mo in austenitic stainless steels. Even the level of knowledge on the corrosion of carbon steel and low alloys in naphthenic acid environments is considered insufficient and more detailed lab testing is needed.

Breakdown of the passive layer starts at localized defects on the metal surface. Breakdown is affected by multiple environmental factors and because of variability of the steel. Environmental factors include the concentration and types of the naphthenic acids at a specific point in the process, temperature, velocity (wall shear stress), and whether a change in phase is occurring, i.e. condensation or vaporization. High velocity and condensing conditions have been shown to have the most pronounced effect. The mechanical agitation imposed by high velocity reaches a critical stage where the formation or self-repair of the passive film is prevented.5 The actual Mo content of the stainless steel first and foremost affects variability of the passive layer protection in NAC. Thermal condition of the steel is important, whether it is the original heat treat condition and/or the weld fabricated condition. Homogenous conditions are preferred because fewer defects occur in the passive film that can lead to a rupture in the film.

Alloy and Mo content

One of the earliest noted papers on NAC 6 states that type 316 stainless steel (SS) has proved to be very resistant and Mo content of 1 percent is quite sufficient for NAC. In the refinery of today the Mo content, and alloy, for the worst case conditions has evolved to alloys with as much as 6% Mo.

The application of Mo bearing stainless steel to prevent NAC is not attributed to a particular individual in the refining industry. There is a correlation to the chemical process industry that performed corrosion testing on low molecular weight organic acids like acetic and formic acids in aqueous solutions. One set of corrosion data generated a curve (Figure 2 1) that shows a step change to lower corrosion rates at 1.25% Mo. It also shows that a minimum of 2.2% Mo is needed for acceptable resistance to corrosion in condensate from boiling acetic acid. Mo has become the key alloy addition for organic acids, including naphthenic acid.

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FIGURE 2 - Curves Showing Effect of Molybdenum on Corrosion Resistance of Cr-Ni Steels in Condensate from Boiling Acetic Acid

Curve 1: Heated 5 min. at 1100 to 1150 C (2010 to 2100 F) and air-cooled. Tests conducted n condensate from boiling 80% acetic acid.

Curve 2: Heated 5 min. at 1100 to 1150 C (2010 to 2100 F) and air-cooled. Tests conducted in condensate from boiling 96% acetic acid.

Cost of Mo Content

Current cost (September 2007) of the alloys in Table 1 fluctuates on a quarterly/monthly basis so that rough baseline percentage costs are shown but accurate prices per pound cannot be stated. Estimating factors used in unit, equipment, or piping projects for some of the alloys are available, e.g. 304/304L, 316/316L. Depending on the accuracy of the estimate, some of the alloys are not specifically listed in the industry estimating programs, e.g. 316L + 2.5 Mo, 317/317L, 317LM, 904L, and 6 Mo. As estimating accuracy increases with each subsequent phase of a project, the program will require current pricing information to be collected from suppliers and input of this data into the software. In the context of this paper the cost of each alloy can be estimated as a function of its elements and compared to each other using 316/316L as a baseline. Relative costs can vary greatly with country of origin and quantity of order.

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Table 1 Chemical Composition of Stainless Steels used for NAC

Alloy % Mo % Cr % Ni % Fe Cost 304/304L nil 18-20 8-12 65-74 - 5 to -10% 316/316L 2-3 16-18 10-14 61-72 Baseline 316L + 2.5 Mo 2.5 min 16-18 10-14 61-72 + 10 to 30% 317/317L 3-4 18-20 11-15 65-71 + 30 to 60% 317LM 4-5 18-20 13.5-17.5 65-71 + 50 to 100% 904L 4-5 19-23 23-28 42-50 + 100 to 150% 6 Mo 6-7 19-21* 20-25* 42-56* + 100 to 200%

*Not exact data for the range of alloys in the 6 Mo family

Types of NAC on Mo bearing SS

NAC of Mo bearing alloys is affected by many of the same factors as carbon steel and low alloys. As mentioned earlier, Mo content has the greatest contributing factor and is also the one most controllable by a corrosion and materials engineer. Crude and environmental conditions like TAN, temperature, velocity, phase, condensing/vaporizing must be known to accurately predict corrosion and provide suitable material selection.

NAC of carbon steel and low alloys has been classified into three types 7.

Type I pure naphthenic acid corrosion sulfur compounds have little or no effect, if they are present Type II sulfidation corrosion, accelerated by the presence of naphthenic acids Type III naphthenic acid corrosion, inhibited by hydrogen sulfide

Austenitic stainless steels do not follow the same classification because sulfidation may have little to no effect on the passive oxide layer depending on the sulfur species present, their concentrations, the TAN value, temperature, etc. The concept used in this paper is that low reactive sulfur concentrations process streams do not affect the corrosion protection of stainless steels because the Cr2O3 passive layer is not converted to a sulfide based layer.

To promote our understanding of NAC on Mo bearing stainless steel the same classification system might look like:

SS Type I - pure naphthenic acid corrosion, little to no passive oxide film protection SS Type II naphthenic acid corrosion, corrosion at defects in passive oxide layer SS Type III naphthenic acid corrosion, inhibited by the passive oxide film

SS Type I corrosion would be accelerated corrosion of an active stainless steel with a Mo content insufficient to prevent breakdown of the passive film as a result of velocity or vaporization/condensation. General corrosion, grooving or gouging, over the entire area subject to the high velocity, turbulence, and/or condensation is observed.

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SS Type II corrosion rates would be moderate and localized, resulting from NAC where TAN or temperature is elevated. Velocity and condensing conditions are not contributing factors and corrosion rates while not acceptable, are manageable. Metal loss would be pitting type corrosion. An increase in velocity might cause a move to Type I corrosion.

SS Type III corrosion is the mode where operating conditions and material selection match and corrosion rates are acceptable. An increase in temperature or TAN might cause a move to Type II corrosion while an increase in velocity or stream condensation might cause a move to Type I corrosion.

Material Selection for Mo Content Versus NAC

Very early work on aqueous organic acids resulted in Figure 1 that shows a step change to lower corrosion rates at 1.25% Mo and good corrosion resistance to hot organic acids when Mo is greater than 2.2%. It is evident from field and lab testing that 316 SS was an effective solution to solving NAC problems and little additional work was done on what level of Mo provided protection. Industry is now starting to realize that the level of Mo is an important factor but still generalizes the use of 316, 316 with 2.5 minimum %Mo, and 317 with minimal data to support the differences in protection.

The first rule of thumb was to use 316 SS to prevent NAC when a carbon steel or low alloy failed. That rule has evolved in a couple of ways. A TAN value was added to determine NAC severity and help identify when an alloy, Mo grade stainless steels included, was required. Original TAN levels for alloy upgrades were set at 0.5 for the crude and 1.5 for a cut basis experience with California acid crudes 8. As some failures of 316 SS occurred the level of Mo in 316 SS was set at a minimum of 2.5%. General industry consensus is that Mo levels in 316 SS moved closer to the 2% minimum allowed by the specification as steel making practices improved. Failures of 316 SS with < 2.5% Mo have been documented in NAC services where > 2.5% Mo has not failed 9. However, conclusive testing on corrosion rates for 2% through 3% Mo contents is still needed.

A 316 SS with a minimum 2.5% Mo rule is commonly used for NAC when TAN is > 1.5 in a cut and velocity or condensing conditions do not exist. The rule of thumb is suitable for general discussion but should not be used for material selection where process data and specific crude and environmental data are available. There are many cases histories of where failures of 316 SS occurred.

Corrosion curves that address TAN, velocity, and temperature need to be developed for different grades of austenitic stainless steel and Mo contents. An excellent example of a curve is represented in Figure 3.2 An interesting observation made from this curve is that, for Naphthenic Acid Erosion Corrosion (NAEC), there is no clear threshold of Mo content where corrosion rates decrease markedly. The often-stated 2.5% minimum Mo threshold is not observed. Others have reported similar observations. 7

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FIGURE 3 - Dependence of NAEC rate on the Mo content in stainless steels (TAN = 12.0 mg KOH/g; testing time: 40 h)

Relevant API Documents

API RP 57110 paragraph 5.1.1.7 Naphthenic Acid Corrosion (NAC) has a good description of damage with critical factors that mention condensing vapors. Mo content is also mentioned. Unfortunately the Mo content mentioned, 2% to 2.5%, is not consistent with API 581, which describes 2.5% minimum Mo as having the best resistance. Another shortcoming is that most of the discussion concerns carbon and low-alloy steel but includes austenitic stainless steel without stating which factors apply to which material. Discrepancies like this are common in the petroleum refining industry and can lead to confusion. The affect of NAC on the different alloy systems is a function of two completely different protective passive films, namely FeS and Cr2O3

The API 58111 G.7 High Temperature Sulfidic and Naphthenic Acid Corrosion tables have been generated from industry data and include qualifiers that the corrosion rates are an order of magnitude, only. Austenitic stainless steel Tables G-23--Estimated Corrosion Rates for Austenitic SS without Mo, G-24Estimated Corrosion Rates for 316 SS with 2.5% Mo and 317 SS provide corrosion rates in mpy with Sulfur and TAN versus temperature. A multiplier of 5 is used for velocity > 100 fps. While condensing streams are known to be one of the worst conditions, it is not described or accounted for specifically in the text or tables. The tables should not be used for material selection. They are generated for inspection purposes and should only be used for modeling purposes in a Risk Based Inspection (RBI) program and, then, only if data are not available from inspection history or other predictive models.. Comparison of the corrosion rate

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information in the three tables could lead to a different set of rules that many would not agree with. For instance, a stream with a TAN up to 2.0 at any temperature could use 304/304L SS. Increase the TAN up to 4.0 and 316/316L with 2.0% Mo may be satisfactory at any temperature. Only at TAN greater than 4.0 and temperature greater than 650F is 2.5% Mo required. Because the tables qualify the corrosion rates as an order of magnitude, material selection should not be generated from the data.

Computer Based Corrosion Programs

There is a computer model for crude units marketed under the name SULTAN(1) which calculates corrosion rates based on local stream properties, material temperature, and flow regime. This approach was described more fully in a recent paper by Kapusta, et al12. Briefly, crude assay, operating conditions, and materials of construction serve as inputs to the model. The approach has been most fully developed for predicting corrosion rates as a function of Cr content for CS through 18-8 stainless steels. The predicted corrosion rate equation is:

CR = (CRNAP + CRS + CRRSH) x ff x fsc x fal (1)

where CRNAP, CRS and CRRSH represent the corrosion induced by naphthenic acids, sulfur and mercaptans, respectively. Factor ff is the flow enhancement factor, fsc is a scale factor that reflects the protectiveness of iron sulfide, and fal is the alloy factor, i.e., the ratio of the corrosion rate of an alloy to that of carbon steel. These factors are not independent: for example, the effect of flow, scale protection and alloy composition depend on the relative importance of the naphthenic and sulfidic components of the corrosion rates. The effect of Mo is reflected in the alloy factor and is not independent of the other alloying elements.

Honeywell is in the 2nd stage of a Joint Industry Project Assessing Refinery Crude Corrosivity again the emphasis is on corrosion prediction and limits for chrome containing alloys. This approach could be extended to develop corrosion curves for Mo grades of stainless steel. The curves would be used to develop a computer program Predict Crude(2), similar to Predict SW(3).

Material Selection Methodology- New Units

A good start to material selection would be to follow the work process outlined in NACE SP407-2007 Format, Content, and Guidelines for Developing a Materials Selection Diagram.

Next acquire the necessary process flow information for a given unit. A heat and material balance will almost always include pressures, temperatures, liquid/vapor phase and general compositional properties that are developed by the process engineer. While it may be difficult to find or generate, the materials and corrosion engineer needs to have valid sulfur and TAN as a function of distillation temperature and for each stream. Do not settle for bulk crude TAN alone. It is nice to have but is not sufficient. Usually assays for the premised crude slate are available. If not, distillation, analysis, and process simulation to determine the individual cut

(1) SULTAN is a trademark of the affiliated companies of the Royal Dutch/Shell Group of Companies

(2) Predict Crude is a trademark of Honeywell International, Inc.

(3) Predict SW is a trademark of the Honeywell International, Inc.

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properties are necessary for proper material selection. Historical TAN data with corrosion history, published corrosion data, and operator confidential data can be used to help in material selection. For new crudes, a complete assay is needed to determine the relevant contaminant levels. Predictive corrosion models are frequently used in these situations where operating history does not exist.

It is necessary to consider all potential crude combinations and their impacts on each cuts properties. The combination of high TAN with low sulfur concentrations can result in conditions where non-Mo alloys are not acceptable.

To determine where SS Type I corrosion, caused by a high velocity or vaporizing/condensing condition, occurs can be a difficult task. Every effort should be made to extract the information during flow diagram reviews with the process engineer. Velocity can be calculated for individual process lines but the level of detail required may not be available at the stage material selection is required. New construction sets the material selection phase just after process flow diagrams are issued and before P&ID are issued. The pipe size needed to calculate velocity is set during P&ID development. An alternative to requesting velocity is to set a maximum velocity for the heaters, transfer lines and piping as part of the material selection. A fallback to identifying these areas is to use industry data that focuses on the crude and vacuum heaters, transfer lines, and the vacuum tower above 400F. Turbulent areas can cause localized, high velocity effects at elbows, tees, reducers, welds, and thermowells. Corrosion allowance for stainless steel piping and equipment should not be overlooked. Zero corrosion allowance will not provide sufficient design life for a new facility. Corrosion allowance in mils divided by corrosion rate in mpy will determine the design life. Most importantly, Mo content of the specified alloy should be carefully considered in these areas against inspection and replacement costs.

SS Type II corrosion will occur in liquid systems and is controlled by determining the correct TAN level versus selection of the material of construction. Changes in TAN will not have as great an impact on corrosion as in SS Type I systems. Industry data is more available for comparison of common crude slates versus TAN, temperature and corrosion rate when velocity is removed from the equation.

After material selection is complete, a set of maximum operating limits are generated from the process and environmental variables used to select alloys in the unit. Limits are monitored on a regular basis through crude purchase specifications, lab testing, instrumentation, and other available tools. Common parameters are Total Sulfur (weight %) and TAN for the crude and various cuts, flow rate or velocity, and temperature. The limits are set during unit reviews and are used in conjunction with the inspection data like thickness monitoring and corrosion coupons to pro-actively locate problem areas.

Austenitic Stainless Steel Failures

There are many references to failures of austenitic stainless steel in published papers and NACE RefinCor. Some of the failures are included in Table 2. The information provided with most failures is missing critical parameters to allow extrapolation of the whole to an industry rule of thumb much less an empirical corrosion prediction model. More empirical data needs to be shared across the industry to improve material selection and prevent failures from NAC.

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Table 2

SUMMARY OF AUSTENITIC STAINLESS STEEL NAC FAILURES

Location/Unit Year Equip/Piping Material & Description

%Mo Feed TAN Temp. Condensing (Yes/No)

Velocity Corrosion Rate Corrosion Description

1 CA/Vacuum Flasher

316L Vacuum transfer line

SJV 5.0 750F Yes 100 ft/s 5 mpy

2 CA/Vacuum Flasher

316 liner in Vacuum column

SJV 4.7 560F Yes

CONCLUSIONS

Austenitic stainless steels have a different passive film and protective mechanism than carbon steel and low alloys. Because the mechanism is so different future work should try to separate the discussion of the two groups of materials with respect to NAC.

Corrosion curves that address TAN, velocity, condensation and temperature need to be developed for different grades of austenitic stainless steel and Mo contents.

Insufficient corrosion data is available to set a minimum Mo content versus TAN. Specifying 316L 2.5% Mo minimum is a rule of thumb and not supported by corrosion data.

API 581 Tables G-23, G-24, and G-25 should not be used for NAC material selection. They are for defining inspection programs, only.

STG 34 should consider forming a task group to collect data on the failures of Mo bearing stainless steels to help define empirical corrosion rate models.

RECOMMENDED READING

The paper by Xinquiang Wu, Erosion-corrosion of various oil-refining materials in naphthenic acid2 contains some excellent discussions, observations, and conclusions with respect to NAC and NAEC.

Digby Macdonald3, 4 has been recognized across multiple industries and organizations for his work on passivation and passivity breakdown. Both papers provide theoretical insight into the formation and growth of a passive layer and the defect growth that leads to breakdown of a passive layer. The ability of Mo to slow or prevent breakdown is also discussed.

NACE 95 Paper 333 by H. L. Craig, Naphthenic Acid Corrosion in the Refinery presents some of the first detailed testing on Mo content versus TAN, temperature and velocity.

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REFERENCES

1 Uhlig, H. H. Corrosion Handbook (John Wiley and Sons)

2 Wu, Xinquiang. Erosion-corrosion of various oil-refining materials in naphthenic acid, (Elsevier) May 2003

3 Urquidi-Macdonald, M. and D. D. Macdonald. Theoretical Analysis of the Effects of Alloying Elements on Distribution Functions of Passivity Breakdown, J. Electrochem Soc. Vol 136, No 4, (1989)

4 Macdonald, D. D. Passivity-the key to our metals-based civilization, J. Electrochem Soc. Vol 139, 3434 (1992)

5 Metals Handbook 9th Ed, Vol 3 Pg 56 (ASM)

6 Derungs, W. A., Naphthenic Acid Corrosion An Old Enemy of the Petroleum Industry. Corrosion, 12(2), 41(1956)

7 Craig, H. L. Temperature and Velocity Effects in Naphthenic Acid Corrosion, CORROSION/96, Paper No. #96603, NACE, Houston, Texas, 1996

8 Piehl, R.; Correlation of Corrosion in a Crude Distillation Unit with Chemistry of the Crudes, Corrosion National Association of Corrosion Engineers, Vol. 16, pp. 305 t-307t, (1960)

9 R.L. Piehl Naphthenic Acid Corrosion in Crude distillation Units Corrosion 87, Paper No 196 (San Francisco, CA 1987)

10 API RP 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry-First Edition, 2003

11 API PUBL 581, Risk-Based Inspection Base Resource Document-First Edition, 2000

12 Kapusta, Sergio, etal, Safe Processing of Acid Crudes, CORROSION/2004, Paper No. 04637, NACE International, Houston, TX, 2004.

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Effect of Molybednum on Ss and Nac Resistance

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