This article is based on a white paper written by John Carissimi and Jeremy Payne in 2015 and rewritten on May 6, 2024.

Sour gas and crude oil operations involve harsh conditions with a high risk of corrosion. Metals and alloys used in environments with hydrogen sulfide (H2S) are especially prone to corrosion and stress cracking, which can lead to material failure and dangerous fluid leaks in your system. To enhance safety and prevent equipment failures, the National Association of Corrosion Engineers (NACE) established control standards for any instrumentation used in these types of applications.

As the Product Marketing Leader for process gauges at Ashcroft, a global leader in pressure and temperature instrumentation, I have spent the past 15 years answering questions and guiding customers on the importance of ensuring compatibility between your instruments and process media. 

In this article, you will get an overview of the NACE standards that specifically address corrosion that can occur with exposure to sour gas or sour crude. You will also learn what to look for when selecting pressure and temperature instruments for your sour gas and crude applications. When you are done reading, you will be able to access additional resources to help make informed decisions about selecting instrumentation for corrosive operations.

What are NACE specifications? 

The National Association of Corrosion Engineers (NACE) was established in 1943 by engineers in the pipeline industry. NACE engineers set out to address metal corrosion by creating control standards and guidelines for selecting appropriate materials used in harsh applications, including oil and gas. This led to the creation of more than 150 standards that manufacturers follow to combat corrosion in numerous industries. 

Two NACE standards specifically address reducing stress corrosion cracking from sour gas or sour crude exposure:

• MR0103 for sulfide corrosion in petroleum refining. Refineries often have lower concentrations of chloride and higher pH levels, therefore these environments tend to be less severe than oil and gas production. As a result, NACE MR0103 tends to be less stringent than those required for oil and gas production.

• MR0175 (ISO 15156) for H2S corrosion in oil and gas production. This NACE standard has been adopted as a global standard by the International Organization for Standardization (ISO) and the American National Standards Institute (ANSI). Among other things, it specifically addresses ferrous alloys and corrosion-resistant alloys used in field applications for sour oil and gas. 
  

Note: These NACE specifications only consider stress corrosion cracking due to exposure to H2 S and other corrosives and do not address general or localized corrosion. Additionally, the standards restrict the usage of many alloys by limiting the temperature, pH, and/or composition to which these alloys may be exposed.  

Each of these standards applies to metals only, not plastics or elastomers.  Following NACE standards ensures that industry professionals select appropriate materials and components that can prevent or reduce corrosion-related processes and equipment failure. Read the full whitepaper, NACE Standards measuring the pressure of sour gas and crude to learn more. 

Why pressure instruments follow NACE standards in sour gas and crude oil applications.

Getting accurate and reliable pressure measurements in any application is critical for monitoring system performance and ensuring the safety of system operators. This is especially true with applications involving corrosive elements that can contribute to a greater risk of system failure. 

Sour gas, for example, is a natural gas that contains Hydrogen Sulfide (H₂S). This odorless, toxic and highly flammable gas can cause corrosion in pipeline applications and be extremely dangerous to anyone exposed to it. H2S can attack containment materials, causing stress corrosion cracking, which can lead to the gas or oil escaping into the environment, putting workers, and equipment in danger. 

To get reliable and consistent pressure measurements to keep your system running safely and efficiently, you need to ensure that the chemical composition of the process medium is compatible with your pressure instrument's wetted materials. For example, the Ashcroft® 1209 and 1279 Pressure Gauges are offered with Stainless steel and Monel® wetted materials. Alternatively, you can also isolate the pressure instrument from the corrosive media by installing a diaphragm seal such as the Ashcroft® 100 Flanged Series and 200 Threaded Series options in between.  Download our Assembly Guide to learn more. 

Four factors to consider for material compatibility.

As you search for compatible materials, consider these four factors. 

1. Temperature:
The temperature of the process medium significantly impacts a material's corrosion resistance. Stainless steels are particularly affected by temperature variations, especially in the presence of chlorides.

2. Chemistry:
Corrosion can be influenced by the presence of hydrogen sulfide, chlorides (from salt deposits), and carbon dioxide. The types and amounts of these compounds play a role in determining the suitability of specific metals.

3. pH:
The acidity or alkalinity of the pressure media is crucial. Low pH levels (acids) can accelerate corrosion rates and lead to hydrogen-induced metal embrittlement.

4. Metallurgical Condition:
Hardened metals are more prone to stress fractures. Annealed metals, on the other hand, are softer and less likely to crack, but have lower tensile strength.

How to ensure your pressure gauge is NACE compliant. 

The best determination of a NACE-compliant instrument is by checking the manufacturer's datasheet. Beyond that, you can:

• Ensure that the instrument meets the conditions specified in the NACE standard. If you are uncertain, use materials that are compliant with the worst-case scenario.
• Conduct laboratory testing to replicate the NACE test standard variables. Certified testing labs with expertise in this type of testing can establish and execute an appropriate test protocol to verify material and component compatibility. 
• Draw on field experience. Components that have been in service without succumbing to corrosion over a suitable timeframe can serve as documented proof of their suitability for the specific application.

Choosing the right material. 

When selecting materials for corrosive conditions, the choice depends on the severity of the environment and cost considerations. For example, Hastelloy® C22/C276 or titanium offer higher compatibility than other options, but they come at a higher price. Here's a quick guide for your reference:

Stainless steel, including 316 and Alloy 20: Suitable for minimally harsh conditions.

Super Duplex Steels and Alloy 400 (Monel®). Suitable in moderate conditions.

Hastelloy® C22/C276, Alloy 400 (Monel®), and titanium. Recommended for their resistance to stress cracking in extreme conditions with high concentrations of corrosive elements.

Ready to learn more?

Now that you know why it is important to use NACE-compliant instruments in sour gas and crude oil applications and ensure process components are compatible with wetted materials, you may have more questions. 

Feel free to talk to one of our industry experts for help selecting the best instruments for your specific needs. In the meantime, here are a few more articles that may be of interest to you. 

• 4 Important Benefits of Critical Application of Instrument Audits
• How To Identify Your Pressure Gauge and Get the Proper Replacement
• How Does Temperature Affect Pressure Gauge Performance?
• How Do I Select the Right Pressure Gauge Range?
• How Often Should I Check the Calibration of My Pressure Gauge?

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