Marine Corrosion Coupon Exposure Study of Hull and Seawater-System Metals by Weight Loss

Marine Corrosion Coupon Exposure Study is a standards-driven experimental study that measures how four workhorse ship materials — mild steel (hull plating), stainless steel 304 (fittings and fasteners), copper (seawater piping representative) and an aluminium alloy (superstructure sheet) — degrade in real coastal conditions over an eight-week exposure.

The method is the one corrosion laboratories have used for a century, executed to standard: flat coupons are cut, surface-finished, degreased and weighed to 0.1 mg; they are then mounted on an inert rack in two environments — an atmospheric splash-zone rack at a fishing harbour (with written permission from the harbour authority) and static immersion tanks of natural seawater refreshed weekly — following the exposure practice of ASTM G31 and the cleaning and evaluation practice of ASTM G1. After retrieval, coupons are chemically cleaned with the solutions the standard specifies, re-weighed, and their mass loss converted to corrosion rates in mm/year; stainless steel gets a separate pitting assessment under the microscope because its weight loss alone tells almost nothing.

The study produces exactly what a marine engineering report should: a ranked, statistically summarised comparison of materials, an environmental log (temperature, pH, salinity) that explains the results, morphology photographs, and a discussion that connects the numbers to shipboard reality — why hull steel needs coatings and cathodic protection, why 304 fails in crevices, and why copper alloys foul and corrode differently. Everything is built from locally purchased stock, needing no equipment beyond a balance, a microscope and the chemistry lab's glassware.

Indian ship operators and boat builders select hull, piping and superstructure metals from corrosion tables that come from other climates and longer exposures, and then face premature failures — wasted hull plate, perforated seawater lines, pitted 304 fittings — that the tables did not predict. At the student level, corrosion is taught from beaker-scale NaCl experiments that strip out the wet–dry cycling, biology and particulate effects of a real harbour, so the numbers students compute never resemble service reality.

There is a need for a compact, standards-compliant coupon exposure study on the Karnataka coast that (a) exposes the four most common shipboard metals in both harbour atmosphere and natural seawater for eight weeks, (b) evaluates them strictly per ASTM G1/G31 — cleaning blanks, mass-loss corrosion rates, pitting statistics — and (c) produces a ranked, reproducible comparison with a full environmental log, giving the department a repeatable experiment and students a defensible bridge between textbook electrochemistry and shipboard corrosion practice.

Abstract

Corrosion is the single largest recurring maintenance cost on Indian vessels — hull plate wastage, pitted seawater piping and seized deck fittings all trace back to electrochemical attack in chloride-rich environments. This project quantifies that attack directly: twenty-four coupons of four shipboard metals are exposed for eight weeks to two marine environments, cleaned and evaluated per ASTM G1, and ranked by corrosion rate and pitting severity.

Introduction

A ship is four or five different materials welded and bolted into one aggressive electrolyte. Designers choose between them using published corrosion data, but those tables are mostly temperate-climate, long-exposure figures; Indian coastal conditions — warm water, high salinity, monsoon humidity — are less documented at student-accessible scale. A coupon exposure study is the classical, standards-backed way to generate such data, and it exercises the full marine materials syllabus: electrochemistry, metallurgy, measurement and data analysis.

Existing System / Literature Gap

• Textbook and handbook data (Fontana; Revie & Uhlig; ASM Handbook Vol. 13A) gives order-of-magnitude rates, mostly from temperate or tropical-but-non-Indian sites.
• ISO 9223 classifies atmospheric corrosivity (C1–CX) using first-year corrosion of standard specimens — but a first-year exposure is impossible inside an academic semester.
• Most student projects stop at beaker experiments with NaCl solution, which misses the biological, particulate and wet–dry cycling effects that make harbour exposure unique.
• Gap: an eight-week, two-environment, four-material study using natural seawater and harbour atmosphere, executed to ASTM practice, with full statistical treatment — feasible within one semester and rarely done at this level.

Proposed System

• Materials under test: mild steel, SS 304, commercial copper, aluminium alloy — 3 duplicate coupons each per environment (24 coupons total), plus unexposed control and cleaning blanks.
• Environments: (A) splash-zone atmospheric rack at a fishing harbour; (B) static immersion tanks with natural harbour seawater, refreshed weekly and chemistry-logged (temperature, pH, salinity).
• Measurements: mass loss → corrosion rate via CR = 87.6·W/(D·A·T); pit density, depth and morphology under microscope; photographic record before/after.
• Outputs: material × environment corrosion-rate table with mean ± standard deviation, ranking against published data, and a shipboard interpretation (coating, cathodic protection, material-substitution recommendations).

Feasibility Study

• Technical: all procedures are described step-by-step in ASTM G1 and G31; the chemistry lab provides balance, pH meter, oven and fume hood; the only field dependency is the harbour rack, covered by a permission letter.
• Economic: coupon stock ~₹1,200; rack from HDPE/FRP offcuts and nylon hardware ~₹900; chemicals and consumables ~₹800; total under ₹3,000.
• Operational and safety: immersion tanks live in the lab (no diving or boating); harbour visits are daylight-only with faculty escort; acid cleaning runs in the fume hood with PPE and neutralisation disposal per lab rules; no hazardous discharge into the harbour.

In scope

• Four metals × three coupons × two environments (harbour splash-zone atmosphere; lab immersion in natural seawater), eight weeks.
• Gravimetric weight-loss evaluation per ASTM G1 with cleaning blanks; corrosion-rate and pitting analysis; weekly seawater chemistry log.
• Statistical summarisation (mean ± SD), comparison with published values, and shipboard interpretation.

Out of scope

• Electrochemical techniques (polarisation, EIS) — listed as future scope; the study is purely gravimetric and optical.
• Flowing-water, tide-cycling or crevice-specific test rigs; long-term (1-year) exposures needed for formal ISO 9223 classification.
• Coating or inhibitor evaluation; biofouling quantification beyond photographic record.
• Testing on real vessel hulls or sampling ship ballast systems.

This is an experimental research design with fixed exposure, triplicate replication and standards-anchored evaluation — the phases below fit the 14-week VTU major-project calendar.

Quality assurance: triplicate coupons per condition so every reported rate is a mean with spread; cleaning blanks (unexposed coupons through the same chemical cycle) bound the base-metal loss caused by cleaning itself; balance calibration checked with a reference weight; a second analyst re-measures 25 % of coupons to bound operator variation. Deviations from the written procedure (a missed weekly refresh, a storm day) are recorded, never silently corrected.

As a research bundle, the architecture is the study design — how specimens, environments, standards and analysis connect.

1. Specimen layer — 24 numbered coupons (4 metals × 3 replicates × 2 environments) plus 2 cleaning blanks and 2 unexposed controls; each coupon has a register entry: material, heat/composition claim from the supplier, dimensions, area, finish, baseline mass.
2. Exposure layer — environment A is a splash-zone rack: HDPE frame, nylon bolts and spacers, 30° tilt facing sea, mounted above high-tide level at the harbour wall; environment B is two glass/HDPE immersion tanks in the lab holding natural seawater, refreshed weekly from a fresh harbour collection, partially supported by the chemistry lab.
3. Standards layer — ASTM G31 governs exposure geometry (minimum solution volume per area, specimen support, avoiding galvanic contact); ASTM G1 governs post-exposure cleaning per metal and the cleaning-blank correction; ISO 9223's corrosivity-category framework is used to discuss (not formally certify) the atmospheric site.
4. Analysis layer — mass loss → CR = 87.6·W/(D·A·T) (mm/yr), replicate statistics, pit-depth distribution, and comparison charts against published coastal data, all scripted in Python with the raw CSV committed to the project appendix.

flowchart TD
  M["Materials: MS, SS 304, Cu, Al alloy"] --> P["Coupon prep per ASTM G1:<br/>finish, degrease, weigh 0.1 mg"]
  P --> REG["Coupon register:<br/>ID, area, baseline mass"]
  REG --> A["Env A: harbour splash-zone rack<br/>(HDPE + nylon, permission letter)"]
  REG --> B["Env B: immersion tanks,<br/>natural seawater, weekly refresh"]
  A --> LOG["Weekly log: T, pH, salinity, photos"]
  B --> LOG
  A --> R["Retrieval after 8 weeks"]
  B --> R
  R --> C["ASTM G1 chemical cleaning<br/>+ cleaning blanks"]
  C --> W["Re-weigh to constant mass"]
  W --> CR["Corrosion rate 87.6W/(DAT)<br/>mean ± SD, triplicates"]
  W --> PIT["Pit density, depth, morphology,<br/>microscopy + photos"]
  CR --> CMP["Rank vs published data +<br/>shipboard recommendations"]
  PIT --> CMP

The design's central discipline is isolation: coupons never touch each other or any metal (nylon hardware only), because a stray steel-to-copper contact would convert a uniform-attack study into an uncontrolled galvanic one. Every number in the report traces back through the register, the logbook and the raw CSV — that traceability is what makes the study defensible in the viva.

• Electrochemical extension — potentiodynamic polarisation and open-circuit potential logging on fresh coupons to complement the gravimetric rates.
• Cathodic-protection demonstration — a zinc-annealed protected steel coupon group showing the rate drop quantitatively.
• Seasonal replication — a second exposure across the monsoon to test how wet–dry cycling changes the atmospheric ranking.
• Coated-coupon arm — a commercially painted mild-steel group with scribe-line underfilm corrosion assessment.
• Crevice rigs — ASTM-style crevice fixtures on 304 to study the washdown-waterline pitting common on deck fittings.
• A formal one-year exposure aligned with ISO 9223's first-year corrosivity classification, run by next year's batch as a continuing study.