Introduction
Dew point plays a role both internally and externally for pipelines. Depending on your role in corrosion control, it can have a different meaning.
This article explains the definition of dew point in the context of pipeline coating inspection and application, and separately, its significance inside pipelines regarding internal corrosion and moisture management. By understanding these different contexts, technicians can better appreciate why dew point matters and how it impacts corrosion control.
Understanding Dew Point: The Basics
The dew point is the temperature at which air becomes saturated with moisture and water vapor begins to condense into liquid water. When the temperature of a surface falls to or below the dew point temperature of the surrounding air, condensation (dew) forms on that surface. This process is called condensation.
In pipeline contexts, dew point is critical because it signals the temperature below which condensation will form on the pipe surface or inside the pipeline, creating a thin electrolyte layer that can initiate or accelerate corrosion.
Externally, dew point governs the environmental window for things like coating application, where even minor condensation can cause huge issues.
Internally, it determines when transported fluids or gases release moisture, creating localized corrosive environments.
While both contexts involve condensation, the drivers (ambient air versus internal gas composition) and implications differ significantly.
Dew Point - Outside
Knowing the dew point is important because condensation on steel surfaces during or after abrasive blasting can trap moisture under coatings, leading to adhesion failure, blistering, and accelerated corrosion.
Relationship Between Dew Point and Relative Humidity
Relative humidity and dew point are interdependent but distinct concepts:
Relative Humidity (RH): Indicates how close the air is to saturation at the current temperature. For example, 50% RH means the air holds half the moisture it could at that temperature.
Dew Point: The temperature at which the air would reach 100% RH (saturation) if cooled without changing moisture content.
In practical terms, a high RH means the dew point is close to the air temperature, increasing the risk of condensation if the surface temperature drops. Conversely, low RH means a lower dew point, and condensation is less likely unless the surface is very cold.
For coating inspectors, monitoring both RH and dew point helps predict when condensation might occur on pipeline surfaces, especially after abrasive blasting when the steel surface temperature can drop.
Why Dew Point Matters in Pipeline Coating Inspection
Dew Point and Surface Temperature Control
Industry standards such as NACE SP0105-2024 and NACE SP0394-2023 emphasize the importance of maintaining the steel surface temperature at least 5°F (about 3°C) above the dew point during and after abrasive blasting and before coating application. This practice prevents condensation on the steel surface, which can:
Compromise coating adhesion.
Cause coating defects such as holidays, blistering, and delamination.
Accelerate external corrosion under the coating.
Maintaining this temperature differential ensures the surface is dry and ready for coating, meeting quality control requirements and extending coating service life.
Dew Point Monitoring as a Quality Control Step
Coating inspectors must:
Continuously monitor dew point and surface temperature during surface preparation and coating application.
Compare steel surface temperature readings (using infrared thermometers or contact pyrometers) to dew point measurements.
Halt coating application if the surface temperature approaches or falls below the specification until conditions improve.
Document dew point and temperature data as part of inspection records to demonstrate compliance with specifications.
This is especially important in environments with fluctuating ambient conditions, such as early morning, nighttime, or cold weather operations.
Dew Point in Internal Pipeline Processes and Corrosion
Shifting to the internal environment, dew point takes on a different but equally important role in managing corrosion within pipelines. Here, it pertains to the conditions inside the pipe, where transported hydrocarbons or gases may carry water vapor that can condense under certain circumstances.
Pressure Influence: The dew point temperature is pressure dependent. Higher pressures increase the partial pressure of water vapor, raising the dew point temperature. This means that at elevated pressures typical in transmission pipelines (often hundreds to thousands of psi), water vapor can condense at higher temperatures than at atmospheric pressure.
Moisture Dropout: When the pipeline temperature falls below the dew point at the operating pressure, water vapor condenses on the pipe wall or within the fluid. This moisture dropout creates a thin liquid film or droplets that serve as an electrolyte, enabling electrochemical corrosion reactions on the steel surface.
How Dew Point Drives Internal Corrosion
Internal corrosion in pipelines requires the presence of an electrolyte, typically liquid water. Even in gas pipelines that appear dry, moisture condensation at or below the dew point can create localized wet spots. These wet spots are often the initiation sites for corrosion, especially when corrosive gases like carbon dioxide (CO₂) or hydrogen sulfide (H₂S) are present.
Corrosive Environment Formation: Condensed water can dissolve CO₂ and H₂S, forming carbonic and sulfuric acids, which aggressively attack carbon steel. The presence of these acids lowers the pH of the condensed water, accelerating metal loss.
Pressure and Temperature Variations: Changes in pressure (e.g., pressure drops at valves or regulators) and temperature fluctuations along the pipeline can cause cyclic condensation and evaporation, increasing the time-of-wetness (TOW) and corrosion risk.
Internal Moisture and Dew Point
Inside pipelines, dew point relates to the temperature at which water vapor in the transported gas or fluid condenses into liquid water. This internal condensation can cause moisture dropout, creating localized wet environments that promote internal corrosion. Unlike external dew point, which involves ambient air, internal dew point is influenced by the gas's composition, pressure, and temperature profile along the pipeline.
For gas pipelines, dew point is the point where the partial pressure of water vapor equals the saturation vapor pressure, leading to liquid formation. In multiphase flows, this can result in water accumulating at the pipe bottom, forming an electrolyte that supports corrosion. Dew point in this context defines the operational boundary where dry gas becomes wet, directly affecting the pipeline's internal integrity.
Moisture Dropout and Corrosion Risk
Moisture dropout increases internal corrosion risk by providing the aqueous phase necessary for electrochemical reactions. In sweet corrosion (CO₂ dominated), carbonic acid forms, dissolving iron and creating protective or non-protective scales depending on conditions. In sour service (H₂S present), hydrogen sulfide can lead to sulfide stress cracking or blistering in the presence of condensed water. Dew point, therefore, signals the onset of these risks, as even trace moisture can concentrate corrosion.
The importance lies in how dew point influences the formation of these corrosive environments; higher water content in the gas raises the dew point, making condensation more likely during temperature drops, such as in uninsulated sections or during shutdowns. This underscores dew point's role in predicting internal corrosion hotspots, where moisture accumulation can lead to under-deposit corrosion or microbial activity if bacteria are present.
Distinction from External Dew Point
Unlike external dew point, which concerns ambient air and surface temperature, internal dew point depends on gas composition, pressure, and temperature inside the pipeline. Both are critical but require different monitoring and control approaches. Externally, dew point is about preventing surface wetting during above ground activities, while internally, it's about managing fluid dynamics to avoid liquid holdup.
Pressure effects are more pronounced internally; as pressure increases, dew point can shift due to changes in vapor-liquid equilibrium, a factor absent in external scenarios. Additionally, internal dew point can be hydrocarbon specific, where heavier components condense first, forming a complex mixture that enhances corrosivity. This distinction highlights why strategies for external and internal dew point differ, with internal focus on gas dehydration or temperature maintenance to stay above the dew point.
Why Dew Point Matters: Key Takeaways
Dew point defines the threshold for moisture condensation, a precursor to corrosion.
Externally, dew point guides coating application and inspection timing to ensure coating integrity.
Internally, dew point governs moisture dropout, influencing internal corrosion risk and mitigation strategies.
Relative humidity and gas composition directly affect dew point values and corrosion potential.
Understanding both external and internal dew point contexts enables technicians to optimize corrosion control measures.
Referenced Standards and Guidelines
NACE SP0169 — Control of External Corrosion on Underground or Submerged Metallic Piping Systems
NACE SP0105-2024 — Field-Applied Liquid Coatings for Weld Joints and External Repair and Rehabilitation of Buried Steel Pipelines
NACE SP0394-2023 — Application, Performance, and Quality Control of Plant-Applied Fusion-Bonded Epoxy External Pipe Coating
NACE SP0106 — Control of Internal Corrosion in Steel Pipelines and Piping Systems
NACE SP0110 — Wet Gas Internal Corrosion Direct Assessment Methodology for Pipelines
AMPP Guide 21569-2024 — Guidance on Implementing Corrosion Control Methodologies to Align with New PHMSA Regulatory Procedures
Conclusion
Dew point is a fundamental concept bridging environmental science and corrosion control in pipeline systems. Externally, it ensures coatings are applied and inspected under conditions that prevent moisture related failures. Internally, it signals when moisture may condense inside the pipeline, triggering corrosion risks that require active monitoring and mitigation. By clearly distinguishing these contexts and understanding their implications, corrosion field technicians and inspectors can make informed decisions that enhance pipeline integrity and longevity.
Roberts Corrosion Services, LLC
Established in 2011, Roberts Corrosion Services, LLC delivers comprehensive, turn-key cathodic protection and corrosion control solutions nationwide. Our end-to-end expertise encompasses design and inspection, installation and repair, surveys and remedial work. We provide drilling services for deep anode installations and a full laboratory for analysis of samples and corrosion coupons, as well as custom CP Rectifier manufacturing.
While our initial focus was on the Appalachian Basin area, we complete field work all over the US. We are a licensed contractor in many states and can complete a wide range of services.
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