What is NACE?
In piping systems, we frequently encounter standards for pipes and fittings that satisfy NACE MR0175/ISO 15156. Do you know the primary differences between NACE-compliant carbon steel pipes and standard carbon steel pipes? Furthermore, what are the cost implications of NACE materials?
NACE (National Association of Corrosion Engineers) was founded in 1943 by 11 corrosion control experts from the pipeline industry. Its mission is to disseminate knowledge and enhance skills in corrosion prevention. Key NACE standards include MR0175, which outlines material requirements for oil and gas production and equipment in H2S-containing environments. NACE-compliant piping is commonly used for the transportation of natural gas and oil containing hydrogen sulfide ($H_2S$).
NACE MR0175/ISO 15156 provides guidelines for selecting qualified steels used in oil and gas production equipment and gas dehydration plants operating in $H_2S$ environments. This standard aims to prevent equipment failure, which can pose risks to human health and safety while causing environmental damage. In the United States, ISO 15156 is regarded as the federal standard for materials used in $H_2S$-containing environments within the oil and gas industries.
Applicable Grades:
- API 5L PSL2 NACE MR0175 Line Pipe: Grade B, X42, X46, X52, X56, X60, X65, X70.
How to Select NACE MR0175 Materials?
When selecting or qualifying steels for $H_2S$ environments in oil and gas production, it must be noted that materials qualified under NACE MR0175/ISO 15156 are resistant to cracking under specified conditions, but not necessarily under all service conditions.
Therefore, the following instructions should be followed when selecting qualified materials, including carbon steel, low-alloy steel, corrosion-resistant alloys (CRA), and other materials resistant to $H_2S$ environments. This standard applies to specifications such as API, ASTM, ASME, and ANSI. In many contexts, NACE piping is synonymous with Sour Service Piping applied in $H_2S$ environments.
NACE Testing Methods
For Sour Service steel pipes, plates, and fittings, two common testing methods are utilized under NACE standards: Hydrogen-Induced Cracking (HIC) and Sulfide Stress Cracking (SSC).
- HIC Testing: This is a mandatory requirement for NACE pipes and fittings. However, if a certificate proves that the raw materials used meet NACE MR0175, repeated testing may not be necessary.
- SSC Testing: This is a long-term experiment specifically for alloy steels. Material is placed in a corrosive environment while subjected to a constant tensile force, simulating working stress and internal forces. The combined effect of corrosion and force can lead to brittle fracture, known as SSC cracking.
SSC is considered one of the most hazardous types of corrosion. The rapid propagation of cracks caused by SSC poses a significant safety risk and can lead to catastrophic failures in structures including bridges, aircraft, and boiler tanks. These incidents can result in severe casualties and property damage. SSC accidents have occurred across power stations, marine vessels, boiler systems, and the petroleum industry.
Chemical Composition
For carbon and alloy steel pipes, NACE MR0175 imposes strict limits on the content of Sulfur (S ≤ 0.002%), Phosphorus (P ≤ 0.020%), and Carbon (C ≤ 0.10%).
Cost Comparison: NACE MR0175 vs. General Piping
The price of the NACE pipe itself is not significantly higher than standard steel pipe (specifically for API 5L or ASTM A106 pipes, the price premium typically does not exceed $100 per ton).
However, HIC and SSC testing is expensive and challenging. If the required quantity is small and no existing stock is available, it may be difficult to procure the necessary materials due to production minimums.
NACE MR0175 Steel Pipe Specification
- API 5L: API 5L B NACE MR0175; API 5L X42/X46/X52/X56/X60/X65/X70 PSL2 NACE MR0175; API 5L LSAW NACE MR0175.
API 5L NACE MR0175 is applicable to oil and gas extraction, production, and transportation pipeline systems in $H_2S$-containing environments. These environments can lead to material corrosion, cracking, or failure, requiring specialized material selection and processing.
The standard specifies requirements for chemical composition, resistance to SSC and HIC, as well as hardness, macrostructure, and microstructure. These requirements ensure that the piping materials maintain excellent corrosion resistance and environmental stress cracking resistance in $H_2S$ environments.