Introduction to Managing Internal Corrosion in Process Vessels
Process vessels in the oil and gas industry typically operate at elevated temperature levels and pressures, and are constantly subjected to a wide variety of aggressive, corrosive chemicals produced or added to the process, as well as to erosive media in some cases.
Managing anticipated internal erosion and corrosion in arduous service environments is a challenging task. The majority of these process vessels are constructed from readily available and relatively inexpensive carbon steel. Interaction of carbon steel and process service environments can cause corrosion, erosion, or their combination, resulting in interrupted operations, costly shutdowns and high-risk safety and environmental hazards.
Everyone in the oil and gas industry – asset owners, operators, mechanical engineers, metallurgists, process engineers, vessel designers, and inspectors, among others – acknowledges that erosion-corrosion management is of paramount importance to safe and productive process operations. (Discover The 6 Corrosive Components That Can Be Found in Crude Oil.)
Material Selection Strategies
Historically, vessel designers have specified carbon steel as the material of construction for vessels operating at temperature levels below 800°F (426°C) when expected corrosion rates were less than 20 mpy.1 Vessel designers might otherwise calculate suitable corrosion allowance factors based upon anticipated corrosion rates and expected equipment service life.
However, experience has shown that operating upsets, downtime conditions, and a great variety of process streams and units can generate corrosion rates far higher than originally expected. Hence, other strategies must be implemented when approaching material selection from an erosion-corrosion standpoint. Some of the strategies implemented by vessel designers, metallurgists, and corrosion engineers include:
- Upgrading metallurgy
- Using cathodic protection (CP)
- Using epoxy lining technologies
Metallurgy upgrades can be accomplished by:
- Constructing the equipment from corrosion-resistant alloys such as stainless steels, nickel and copper alloys among others, rather than carbon steel
- Using alloy-clad or alloy-weld overlay onto carbon steel as effective forms of lined construction and is more economical than employing solid corrosion-resistant alloys throughout the vessel2
However, even with upgraded metallurgy, corrosion problems can occur, such as stress corrosion cracking in austenitic stainless steels when exposed to inorganic and organic chlorides, caustic solutions and sulfurous and polythionic acids, among other chemicals.3
Cathodic protection is, in simple terms, a technique to reduce the corrosion of a metal surface exposed to an electrolyte by making that surface the cathode of an electrochemical cell. Typical types of cathodic protection include galvanic or sacrificial anodes and impressed current anodes. Developing an effective cathodic protection system is a complex task that requires experience, knowledge, and appropriate maintenance.4
The lining strategy implies establishing a barrier between the carbon steel vessel and the environment by internal lining technologies that improve the erosion/corrosion performance of the substrate in aggressive environments. Several factors will influence the decision:
- Vulnerability of the substrate to corrosion attack
- Safety consequences of failure to personnel and associated equipment
- Availability of materials and relative ease of fabrication
- Operational reliability
- Maintainability, including frequency of routine internal inspections requiring process plant shutdowns
- Material and installation costs
There are a number of benefits in using suitable linings to isolate the vessel from its environment. Lining carbon steel fabricated vessels with fit-for-service coatings provides a cost-effective method to improve the corrosion resistance of carbon steel when compared to using corrosion-resistant alloys or cladding alternatives. (More information can be found in Choosing a Fit-for-Service Lining System for Process Vessels.)
Lining materials are readily available if needed, even on short notice. This is not the case with other technologies such as cladding. Lining materials do not add significant weight to the vessel and in some cases can facilitate a reduction in weight through a reduced metal corrosion allowance due to lower anticipated erosion and corrosion rates.
Efficient application methods allow for shorter project lead times and equipment downtime. While linings require more on-line monitoring, fewer internal inspections are needed compared to totally unprotected carbon steel vessels.
Initial Considerations for Lining Selection
When selecting a fit-for-service lining for a vessel, there are a number of criteria that must be properly addressed with the asset owner or operator to justify the long-term performance of such a solution. Certain questions should be asked and answered for due diligence.
Is there any data that can be used to predict the service life of the lining? This question is very difficult to address because the service life of a lining depends on many factors, mostly associated with application standards and maintainability. In the majority of the cases, the operator or asset owner would request case studies of similar or identical applications under the same operating conditions or chemical service as that of the vessel for which a specific lining is being recommended.
What testing data is available to corroborate the fitness of the lining for the intended service? This inquiry would commonly require the lining manufacturer to provide pre-qualification test results of the lining after being subjected to various laboratory tests in accordance with internationally recognized standards.
How can we ensure traceability of the lining after fabrication? The lining manufacturer would have to provide tangible evidence that the linings have been carefully manufactured to ensure the highest quality possible and tested in strict accordance with universally recognized standards.
How can we ensure that the lining application procedures meet the requirements of the supplier? This inquiry requires the lining manufacturer to provide details of off-job validated theoretical and practical instructions offered to selected application contracting partners, certification and in situ auditing to ensure that all linings are applied to the manufacturer’s best standards.
Many factors should be considered to arrive at the best strategy for managing internal corrosion in process vessels. These can include material acceptance, performance criteria, application quality control, non-conformance control and in-service experience. In a later article, we will delve into these and other factors that influence selection of a fit-for-service lining system.
- ASTM A27/A27M-13, “Standard Specification for Steel Casting, Carbon, for General Application” (West Conshohocken, PA: ASTM International).
- ASTM A240/A240M-15b, “Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and for General Applications” (West Conshohocken, PA: ASTM International).
- Hay, Jorge L., “Corrosion Inspection and Control in Refineries”, Shell Projects and Technology, Shell Global Solutions (2013).
- NACE SP0169-2013 (formerly RP0169), “Control of External Corrosion on Underground or Submerged Metallic Piping Systems” (Houston, TX: NACE).