NACE SP0775)

# 1. Overview & Theoretical Principles

In chemical synthesis and API manufacturing, reactor vessels, heat exchangers, and distillation columns operate under aggressive corrosive environments containing halogenated acids ( $\text{HCl}$ , $\text{HBr}$ ), thionyl chloride ( $\text{SOCl}_2$ ), organic solvents, and caustic scrubbers. Selecting the correct Material of Construction (MOC) (such as SS316L, Hastelloy C-276, Titanium Grade 2, Inconel 600, or Glass-Lined Steel) and evaluating corrosion rates via weight-loss coupon testing is critical for process safety, vessel mechanical integrity, and preventing catastrophic containment failure.

# 2. Mathematical Equations

# 2.1 Weight-Loss Corrosion Rate Calculation

# 1. Corrosion Rate in Millimeters per Year ( $CR_{mm/yr}$ ):

CR_{mm/yr} = \frac{87.6 \times \Delta W}{A \times t \times \rho}

# 2. Corrosion Rate in Mils per Year ( $CR_{mpy}$ ):

CR_{mpy} = \frac{534 \times \Delta W}{A \times t \times \rho}

Where:

$\Delta W$ = Coupon mass weight loss ( $W_{initial} - W_{final}$ ) in milligrams ( $\text{mg}$ )
$A$ = Total exposed surface area of the test coupon in square centimeters ( $\text{cm}^2$ ) for $CR_{mm/yr}$ , or square inches ( $\text{in}^2$ ) for $CR_{mpy}$
$t$ = Total exposure time in hours ( $\text{hr}$ )
$\rho$ $ρ$ = Metal alloy density in grams per cubic centimeter ( $\text{g/cm}^3$ $g/cm^{3}$ )
- SS304 / SS316L: $7.98\text{ g/cm}^3$
- Hastelloy C-276: $8.89\text{ g/cm}^3$
- Titanium Grade 2: $4.51\text{ g/cm}^3$
- Nickel 200 / Monel 400: $8.80\text{ g/cm}^3$
- Carbon Steel (SA-516 Gr 70): $7.85\text{ g/cm}^3$

\text{Unit Conversion}: 1\text{ mm/yr} = 39.37\text{ mpy} \quad \text{or} \quad 1\text{ mpy} = 0.0254\text{ mm/yr}

# 2.2 Estimated Remaining Equipment Service Life ( $SL$ )

SL = \frac{t_{actual} - t_{required}}{CR_{mm/yr}} \quad [\text{Years}]

Where:

$t_{actual}$ = Measured current wall thickness of shell/head via ultrasonic thickness gauge ( $\text{mm}$ )
$t_{required}$ = Minimum acceptable wall thickness per ASME VIII Div 1 design calculations ( $\text{mm}$ )

# 3. Corrosion Resistance Rating Categories (ASTM G31)

Corrosion Rate ( $\text{mm/yr}$ )	Corrosion Rate ( $\text{mpy}$ )	ASTM Resistance Rating	Process Equipment Suitability
$< 0.025$	$< 1.0$	Outstanding / Excellent	Highly recommended; long service life ( $> 20\text{ years}$ ).
$0.025 - 0.10$	$1.0 - 4.0$	Good	Suitable for pressure vessels with standard $1.5 - 3.0\text{ mm}$ corrosion allowance.
$0.10 - 0.50$	$4.0 - 20.0$	Fair / Moderate	Acceptable only with frequent wall thickness monitoring and planned replacement.
$> 0.50$	$> 20.0$	Poor / Unacceptable	Not recommended; rapid thinning and risk of leakage/failure.

# 4. Governing Standards

ASTM G1-03(2017): Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens.
ASTM G4-01(2014): Standard Guide for Conducting Corrosion Tests in Field Applications.
ASTM G31-21: Standard Guide for Laboratory Immersion Corrosion Testing of Metals.
NACE SP0775: Preparation, Installation, Analysis, and Interpretation of Corrosion Coupons in Oilfield Operations.