3.1 Corrosion Rates (Short-Term, Long-Term, and Initial Estimates)

3.1 Corrosion Rates (Short-Term, Long-Term, and Initial Estimates)

In-service piping inspection under the API 570 standard is fundamentally driven by the need to understand how fast a piping system's metallic pressure boundary is thinning. The rate at which corrosion occurs directly determines when the piping system must be inspected next, whether it needs to be repaired or replaced, and how long it can safely remain in service. Consequently, calculating and selecting the correct corrosion rates are amongst the most critical responsibilities of the API 570 authorized piping inspector.

Long-Term and Short-Term Corrosion Rates

The API 570 code defines two primary corrosion rate calculations: the Long-Term (LT) corrosion rate and the Short-Term (ST) corrosion rate. Both are calculated as linear rates expressed in inches per year (in./yr) or millimeters per year (mm/yr).

Long-Term Corrosion Rate (LT)

The Long-Term corrosion rate averages the loss of wall thickness over the entire documented operating history of the piping system. It represents the overall steady-state thinning rate and is less sensitive to brief operational upsets or temporary seasonal changes. The formula for the Long-Term corrosion rate is:

$CR_{LT} = \\frac{t_{initial} - t_{actual}}{t_{time}}$

Where:

• $t_{initial}$ is the thickness (in inches or millimeters) at the initial baseline inspection (typically when the pipe was first placed in service or first measured).
• $t_{actual}$ is the actual thickness measured at the most recent inspection at the same Condition Monitoring Location (CML).
• $t_{time}$ is the time period in operating years between the initial and actual inspections.

Short-Term Corrosion Rate (ST)

The Short-Term corrosion rate calculates the wall thickness loss over the most recent inspection interval. This rate is highly sensitive to recent changes in process conditions, fluid chemistry, catalyst degradation, or chemical injection rates. The formula for the Short-Term corrosion rate is:

$CR_{ST} = \\frac{t_{previous} - t_{actual}}{t_{time}}$

Where:

• $t_{previous}$ is the thickness measured at the inspection immediately preceding the current inspection at the same CML.
• $t_{actual}$ is the actual thickness measured at the most recent inspection.
• $t_{time}$ is the time period in operating years between the previous and actual inspections.

Operating Years vs. Calendar Years

A critical parameter in both formulas is the time ($t_{time}$). The inspector must use operating years (time in service) rather than calendar years. If a piping system has been idle, blinded, or decommissioned for several years, using calendar years would artificially depress the calculated corrosion rate, creating a dangerous underestimation of the thinning rate. For example, if a pipe has been in place for 10 calendar years but was shut down and nitrogen-purged for 4 of those years, the time variable in the denominator must be 6 operating years, assuming no corrosion occurred during standby.

Selecting the Controlling Corrosion Rate

API 570 Section 7.1.1 states that the authorized inspector, in consultation with a piping engineer, must select the corrosion rate that best reflects the current behavior of the system.

1. Standard Default: In most steady-state applications, the inspector will compare the calculated LT and ST rates and select the larger (more conservative) value as the controlling rate for remaining life and inspection interval calculations.
2. Accelerated Corrosion: If a process change, fluid velocity increase, or temperature elevation has occurred, the ST rate will typically be much higher than the LT rate. In this scenario, the ST rate must be selected as the controlling rate because the historical LT rate no longer represents the current process aggressiveness.
3. Operational Upsets: If a temporary process upset (e.g., a desalter bypass or acid carryover) occurred during the last interval, the ST rate might show a massive, non-representative spike. If the upset has been resolved and the system has returned to normal baseline conditions, the inspector, with proper engineering justification, may choose not to use the spiked ST rate, or may use a rate determined by a corrosion specialist.

Initial Rates for New or Changed Services

When a new piping system is placed in service, or when an existing piping system's service conditions are modified, there is no historical thickness data to calculate a rate. API 570 establishes a strict protocol for establishing the initial corrosion rate:

• Step 1: Similar Service Data: The inspector should obtain corrosion rate data from existing piping systems in similar service at the same facility or in similar industry operations.
• Step 2: Published Data & Software: If local data is unavailable, the inspector can consult published corrosion databases, corrosion curves (such as McConomy curves for sulfidation or Couper-Gorman curves), or proprietary modeling software.
• Step 3: Specialist Consultation: A corrosion specialist or piping engineer should review the process conditions (temperature, pressure, velocity, chemical concentrations) to estimate the rate.
• Step 4: Initial Verification (The 3-to-6-Month Rule): If an estimated corrosion rate is used, the inspector must verify the actual rate by taking thickness measurements (typically using non-destructive on-stream ultrasonic testing) after a short period of operation, generally 3 to 6 months (or up to 1000 hours of operating service). This physical reading is used to validate or adjust the initial estimate.

Localized and Pitting Corrosion Considerations

While the standard formulas assume uniform thinning across the pipe wall, many damage mechanisms, such as acid attack, microbiologically influenced corrosion (MIC), or erosion-corrosion at elbows, result in localized thinning. For localized corrosion, the inspector must use the minimum localized thickness reading at a given CML to calculate the rates. If pitting is present, standard ultrasonic grid measurements or profile radiography are employed, and the deepest pit within a designated area is tracked. Under API 570, localized corrosion rates must be treated with extreme caution, and the highest local rate must be selected as the controlling rate for that specific circuit. The inspector cannot average high and low thickness readings across different CMLs to artificially lower the corrosion rate; each CML stands alone in its corrosion rate determination.

Worked Calculation Example

Consider a piping circuit fabricated from carbon steel (NPS 8, Schedule 40) in a refinery hydroprocessing unit. An inspector reviews the following CML historical thickness measurements:

• Baseline (Initial - Year 2016): $0.322\ \text{in.}$
• Previous Inspection (Year 2021): $0.297\ \text{in.}$
• Actual Inspection (Year 2026): $0.262\ \text{in.}$

The piping system operates continuously (100% duty cycle, so calendar years equal operating years).

1. Calculate Long-Term Corrosion Rate ($CR_{LT}$): $CR_{LT} = \\frac{t_{initial} - t_{actual}}{Time} = \\frac{0.322\\ \\text{in.} - 0.262\\ \\text{in.}}{10\\ \\text{years}} = \\frac{0.060\\ \\text{in.}}{10\\ \\text{years}} = 0.006\\ \\text{in./yr}\\ (6.0\\ \\text{mils/yr})$

2. Calculate Short-Term Corrosion Rate ($CR_{ST}$): $CR_{ST} = \\frac{t_{previous} - t_{actual}}{Time} = \\frac{0.297\\ \\text{in.} - 0.262\\ \\text{in.}}{5\\ \\text{years}} = \\frac{0.035\\ \\text{in.}}{5\\ \\text{years}} = 0.007\\ \\text{in./yr}\\ (7.0\\ \\text{mils/yr})$

3. Selecting the Controlling Rate: Since the short-term rate ($0.007\ \text{in./yr}$) is higher than the long-term rate ($0.006\ \text{in./yr}$), the controlling rate for this CML must be set to $0.007\ \text{in./yr}$ to maintain conservative safety margins.

Common Exam Traps

• Calendar vs. Operating Years: The exam may state that a pipe was installed 12 years ago but was out of service for 2 years. Ensure you divide the thickness loss by 10 operating years, not 12.
• Units Confusion: Be prepared to convert between inches and mils (where $1\ \text{mil} = 0.001\ \text{in.}$). A rate of $0.005\ \text{in./yr}$ is $5\ \text{mils/yr}$. The exam may mix these units in the question and options.
• Initial Rate Timing: Remember the 3-to-6-month verification rule. Questions frequently ask when thickness measurements must be taken to verify an estimated rate.