Galvanic corrosion occurs when two different metals are electrically connected and immersed in an electrolyte (such as seawater or soil). This calculator estimates the corrosion rate of the less noble (anodic) metal based on Faraday's Law, utilizing corrosion current, material density, and equivalent weight.
Corrosion Rate Estimator
Please enter valid positive numbers for all fields.
Total measured current in microamps (µA).
Exposed surface area of the corroding metal in cm².
Density of the corroding metal in g/cm³.
Atomic weight divided by valence (g/eq). See reference below.
Current Density:–
Corrosion Rate (Metric):–
Corrosion Rate (Imperial):–
Mass Loss Rate:–
function calculateCorrosion() {
// Get input values
var currentUa = document.getElementById("corrCurrent").value;
var area = document.getElementById("area").value;
var density = document.getElementById("density").value;
var eqWeight = document.getElementById("eqWeight").value;
var errorMsg = document.getElementById("errorMsg");
var resultsDiv = document.getElementById("results");
// Validate inputs
if (currentUa === "" || area === "" || density === "" || eqWeight === "" ||
isNaN(currentUa) || isNaN(area) || isNaN(density) || isNaN(eqWeight) ||
parseFloat(area) <= 0 || parseFloat(density) <= 0) {
errorMsg.style.display = "block";
resultsDiv.style.display = "none";
return;
}
errorMsg.style.display = "none";
resultsDiv.style.display = "block";
// Parse Floats
var I = parseFloat(currentUa); // Microamps
var A = parseFloat(area); // cm^2
var D = parseFloat(density); // g/cm^3
var EW = parseFloat(eqWeight); // g/eq
// Calculations
// 1. Current Density (µA/cm²)
var i_corr = I / A;
// 2. Corrosion Rate in mm/year
// ASTM G102 Constant K1 for mm/yr = 0.003272
// Formula: CR = (K1 * i_corr * EW) / D
var rate_mmy = (0.003272 * i_corr * EW) / D;
// 3. Corrosion Rate in mils per year (mpy)
// 1 mm = 39.3701 mils
var rate_mpy = rate_mmy * 39.3701;
// 4. Mass Loss (grams per year)
// Mass Loss (g) = (I_amps * t_seconds * EW) / Faraday(96485)
// I_amps = I * 1e-6
// t_seconds (1 year) approx 31,536,000
// Simplified: Mass Loss approx (rate_cm_yr * A * D)
// Let's use the Faraday calculation directly for accuracy
var secondsInYear = 31536000;
var faraday = 96485;
var massLoss = ((I / 1000000) * secondsInYear * EW) / faraday;
// Display Results
document.getElementById("resCurrentDensity").innerHTML = i_corr.toFixed(2) + " µA/cm²";
document.getElementById("resMmy").innerHTML = rate_mmy.toFixed(4) + " mm/yr";
document.getElementById("resMpy").innerHTML = rate_mpy.toFixed(2) + " mpy";
document.getElementById("resMassLoss").innerHTML = massLoss.toFixed(4) + " g/yr";
}
Understanding the Variables
This calculator relies on Faraday's Law of Electrolysis, adapted for corrosion engineering (specifically ASTM G102). Accurate prediction requires precise inputs:
Galvanic Current (Icorr): This is the flow of electrons measured between the anode and cathode. It is typically measured in microamps (µA) using a zero-resistance ammeter.
Anode Surface Area: The specific area of the anodic metal that is exposed to the electrolyte. If the area is smaller while the current remains constant, the penetration rate increases significantly.
Equivalent Weight (EW): A dimensionless constant specific to the metal element or alloy. It represents the mass of metal lost for one mole of electrons transferred.
Common Equivalent Weights & Densities
Use the following table to find approximate values for common engineering metals if you do not have specific alloy data:
Material
Density (g/cm³)
Equivalent Weight (EW)
Carbon Steel (Fe)
7.86
27.92
Aluminum 6061
2.70
9.25
Copper
8.96
31.77
Zinc
7.13
32.69
Stainless Steel 304
7.90
25.12
Calculation Formula
The corrosion rate (CR) is calculated using the following equation defined in ASTM G102:
CR (mm/yr) = (0.003272 × icorr × EW) / ρ
Where:
0.003272: Conversion constant for Metric units (mm/yr).
icorr: Current Density (µA/cm²), calculated as Total Current / Area.
EW: Equivalent Weight.
ρ: Density (g/cm³).
Why is this important?
In galvanic couples, such as a stainless steel screw in an aluminum sheet, the aluminum acts as the anode. Calculating the corrosion rate allows engineers to predict the service life of the component and size the sacrificial anodes accordingly in cathodic protection systems.