Using NACE standards to protect against corrosion
Compliance with standards is a necessary first step to mitigate corrosion risks.
Corrosion is the scourge of the oil and gas industry worldwide, eating up maintenance budgets and often causing incidents. The term corrosion comes from the Latin word corrodere, which means “to gnaw to pieces.”
American Petroleum Institute (API) 571 details more than 25 different industrial corrosion mechanisms, many of which pertain to upstream oil and gas facilities. A combination of pressure, temperature, flow, and process media creates an environment which can gnaw a system’s components and equipment to pieces.
The safety risks of corrosion are substantial. In a European Commission study of refining accidents around the world traced to corrosion, 76% resulted in fires or explosions1. While not surprising based on the flammable and explosive nature of the substances used in refining, the consequences of a loss of containment cannot be understated. One such incident could result in significant loss of equipment, personnel casualties, and bankruptcy.
The economic impact of corrosion is quite significant across industries. A NACE International study estimated the global cost of corrosion to be $2.5 trillion, or just over 3% of global GDP, with the industrial sector representing about half of this total cost2 (Figure 1).
Costs are incurred by inspections during outages, replacement of corroded piping and components, installation of corrosion-resistant components in new systems, and the damage caused from corrosion-related incidents.
How can these facilities design around this significant risk to their bottom line and system integrity? To answer this question, it’s helpful to look at problems caused by recent industry developments.
Industry changes exacerbates issues
The oil and gas industry has shifted more of its production to fields where sulfur and other corrosive agents are more abundant. A 4 parts per million (PPM) concentration of hydrogen sulfide in air under standard temperature and pressure is often referred to as sour, and crude oil with more than 0.5% sulfur is called sour crude. According to the International Energy Agency (IEA), about 43% of the world’s natural gas reserves are sour. The gas reserves in the Middle East, for example, are estimated to be about 60% sour.
The presence of sulfides in oil and gas creates additional challenges down the value chain until the sweetening process, which is used to sulfides from sour gas or oil. Hydrogen sulfide (H2S) in process media presents significant risks to hydrocarbon facilities in terms of cost, equipment damage, and harm to personnel (Figure 2).
Elevated levels of sulfur are not only relevant to process media but can also manifest during enhanced oil recovery operations. Equipment used in support systems for these processes also could be exposed to hydrogen sulfide, presenting additional risk.
Fortunately, standards are available to address these and other types of corrosion issues. While adherence to these standards won’t guarantee trouble free operation, these documents are a good starting point for corrosion mitigation programs.
What are NACE standards?
The document referred to as “NACE” was first issued in 1975 by the National Association of Corrosion Engineers, now known as NACE International, as NACE MR0175. This document was first revised in 1978, with another revision in 1984. The International Standards Organization (ISO) released a similar document in 2001 (ISO 15156), and the last revision of the NACE MR0175 standard was released in 2002. Until recently, most corrosion applications referred to the 2002 version.
NACE and ISO released a combined document in 2009 called NACE MR0175/ISO 15156, now the current standard, with a revision subsequently published in 2015. This document is titled “Petroleum and Gas Industries – Materials for Use in H2S-Containing Environments in Oil & Gas Production.” The standard provides metallurgical requirements for carbon steels, low alloys, and corrosion-resistant alloys with respect to chemistry, hardness, heat treatment, and hydrogen induced cracking resistance.
NACE compliance is needed in applications where the risks of corrosion are high as these conditions can lead to material failure and could pose a risk to the public, personnel, and equipment. This situation is most common in areas where sour gas is present.
The NACE MR0175/ISO 15156 standard is divided into three parts:
- General principles for selection of cracking-resistant materials.
- Cracking-resistant carbon and low-alloy steels, and the use of cast irons.
- Cracking-resistant CRAs (corrosion-resistant alloys) and other alloys.
Products or parts which are in contact with the process fluid meeting the criteria detailed in the standard. For a system to be “NACE compliant,” all elements of the system must comply with the requirements of the standard. This includes, but is not limited to, valve bodies, flanges, pipe nipples, springs, and other wetted components.
There are no overarching compliant materials for NACE/ISO because the process conditions always matter. Any combination of pressure, temperature, hardness, concentration of corrosive agents, and other factors can limit the use of certain alloys.
The current standard places “environmental restrictions” or “environmental limits” on almost all corrosion resistant alloys, which are essentially everything other than carbon steels and low-alloy steels. These limits are typically expressed in terms of partial pressure of the corrosive agent, maximum temperature, ppm chlorides, and the presence of free sulfur.
Figure 3, adapted from Part 2 of the NACE standard, shows the different regions of severity for stress corrosion cracking. The Y-axis is the local pH and the X-axis is the partial pressure of H2S. Depending on these environmental conditions, different precautions must be taken.
To the left of the vertical line at 0.05 psi, no precautions need be taken for the selection of steels, although the user may choose to be more conservative with material selection. In Regions B, C, and D of the graph, the user would refer to the standard to select materials appropriate for those process conditions.
The standard covers commonly used alloys in the industry and the requirements of each to comply with the standard. This includes material characteristics such as hardness levels, mechanical work needed, and heat treatment. One must refer to each specific material section to determine the requirements.
The NACE document has a table listing specific equipment excluded from the requirements. One exclusion of note is for crude oil storage and handling facilities operating at an absolute pressure below 65 psi. This means products in storage tanks like blanketing regulators or vent valves need not comply with the requirements of the standard, but users are certainly free to specify special materials to ensure safe operation.
Differences between standards
NACE/ISO was released in 2009, but even after so much time has passed, there are still many customers who still use the 2002 version of the standard and may not understand the differences between the MR0175 2002 version and the 2009 NACE/ISO standard.
In short, NACE MR0175/ISO 15156 provides a broader look at corrosion, covering a wide range of cracking methods as opposed to only sulfide stress cracking, the primary focus on previous standards. If a material is compliant with MR0175-2002, it may or may not be compliant with NACE/ISO. An additional review is needed to determine if the material meets the requirements of the NACE/ISO standard under the given process conditions.
There are too many differences between the NACE MR0175-2002 and NACE MR0175/ISO 15156 standards to compare in this document, but here are some highlights:
The 2002 version was not specific with welding procedures, whereas NACE/ISO has specific guidance for different types of welds and locations of the welds that must be qualified.
The NACE/ISO standard requires weld procedures for most materials to be qualified using a special hardness survey method to ensure proper hardness is achieved in the weld deposit and the base metal heat-affected zone. Guidance regarding overlay welds also are included in the standard. If an overlay weld is used for corrosion or wear protection, it needs to be qualified.
Stainless steel alloys
Free-machining steel alloys, such as 416 stainless steel, are prohibited in the standard. These alloys may have sulfur, selenium, and lead added to improve their machining characteristics. Conversely, these elements, namely sulfur, degrade the corrosion resistance of the metal. For specific 300 series stainless steel alloys, the standard lists acceptable elemental ranges and environmental restrictions such as pressure/temperature limits, sulfur content, and chloride content. For example, 316 is allowed in MR0175/ISO 15156, but with environmental restrictions.
Nickel-based alloys typically perform well in corrosive applications, and generally divided into two categories:
- Solid-solution nickel-based alloys: Hastelloy C-276, Inconel 625, Incoloy 825
- Precipitation-hardenable alloys: Incoloy 925, Inconel 718 and X750
- N04400 (Monel 400) is now included in the standard in wrought and cast forms.
- N05500 (Monel K500) and N07750 (Inconel X750) are prohibited as pressure retaining parts including bolting, shafts, and stems because they do not have sufficient ductility to be used in these applications. However, they can be used without environmental restrictions for internal parts like springs.
- The Alloy C (Hastelloy C) family, N06625 (Inconel 625), N08825 (Incoloy 825), have no environmental restrictions in the solution heat-treated condition.
Bolting is subject to NACE requirements if it is wetted by a sour process fluid, buried, insulated, equipped with flange protectors, or not exposed directly to the atmosphere. For wetted applications, ASTM A193 grade B7M or ASTM A320 grade L7M bolting with ASTM A194 grades 2HM and 7M nuts should be used. S17400 (or 17-4) bolting is no longer allowed. Coatings for bolting are allowed is they are not intended to prevent cracking of the material.
In the past, materials not listed in the document are considered not compliant. However, MR0175/ISO 15156 does contain provisions for using unlisted materials based upon lab qualification testing.
Using the NACE/ISO standard to mitigate corrosion
The standard states the end user is responsible for selecting suitable materials for the application and documenting the desired process media parameters. There is a warning in bold type at the beginning of each part of the standard:
WARNING – Metallic materials selected using ANSI/NACE MR0175/ISO 15156 are resistant to cracking in defined H2S-containing environments in oil and gas production but not necessarily immune to cracking under all service conditions. It is the equipment user’s responsibility to select materials suitable for the intended service.
The manufacturer or vendor, on the other hand, is responsible for making sure the user’s chosen material meets the metallurgical and manufacturing requirements and complies with any applicable testing requirements, and for complying with marking and documentation requirements.
Compliance with NACE/ISO will likely require special alloys, especially when dealing with process media containing higher levels of sulfides and chlorides. These alloys come with a much higher price tag than standard materials, but these costs must be weighed against the risk of using unsuitable materials.
Corrosion presents significant risks, much of which can be mitigated with proper material selection. The standards developed by NACE International and ISO, specifically NACE MR0175/ISO 15156, provide guidance to users when selecting materials appropriate to corrosive process conditions.
The end user is responsible for selecting the proper material for the application, and manufacturers are responsible to ensure the materials they provide have the appropriate characteristics and documentation. During the selection process, end users and vendors can work together to manage corrosion risks and prevent excess maintenance costs and equipment damage for a given application.
- Corrosion‐Related Accidents in Petroleum: https://publications.jrc.ec.europa.eu/repository/bitstream/JRC84661/lbna26331enn.pdf
- NACE International, Assessment of the Global Cost of Corrosion: http://impact.nace.org/economic-impact.aspx
- NACE International: ANSI/NACE MR0175/ISO 15156
- ASTM International: https://www.asminternational.org/documents/10192/1849770/06691G_Chapter_1.pdf
- Daniel Canty, The Middle East Sour Oil and gas Upstream Profile: https://www.oilandgasmiddleeast.com/article-10245-the-middle-east-sour-oil–gas-upstream-profile
- ADNOC, Occidental Petroleum Open World’s Largest Ultra Sour Gas Project: https://www.oilandgas360.com/adnoc-opens-worlds-largest-ultra-sour-gas-project/
- Da Huo, Stanford Energy Journal: https://sej.stanford.edu/global-sour-gas-problem
- What is a NACE MR0175 / ISO 15156? Thanh Nguyen Le: https://shipbuildingknowledge.wordpress.com/2019/07/17/what-is-a-nace-mr0175-iso-15156/
- The History of NACE MR-0175, ISO 15156, and the 1% nickel limit – Timeline, Mariano Iannuzzi: https://www.aboutcorrosion.com/2017/05/21/mr0175-timeline/
- Wood, Vetere Arellano, and Van Wijk. Corrosion-Related Accidents in Petroleum Refineries: https://publications.jrc.ec.europa.eu/repository/bitstream/JRC84661/lbna26331enn.pdf
- MR95: https://www.emerson.com/en-us/catalog/fisher-mr95
- ACE95: https://www.emerson.com/en-us/catalog/fisher-ace95
- Sulfide Stress Cracking: https://www.emerson.com/documents/automation/manual-sulfide-stress-craking-fisher-en-140706.pdf