Metal Boat Survey Notes
• Wastage – Erosion - Corrosion (physical - chemical – electrical) – stray current and galvanic.
• Hull – Framing Damage (dents, buckling between frames, notches)
• Weld Seams – decay and faying surface corrosion (between hull shell and frame) from incomplete welds (usually a concern in recreational vessels).
• Non-marine metal alloys
• Visual and tactile observations
• Percussion testing
• Gauging – UT or Borings
• Hull Potential
Steel Vessels -
• NVIC 7-86 Inspection and Repair of Steel Vessels
• Relative Inexpensive – good build strength – doesn’t fatigue easily
• Iron and Mild Steel (carbon steel) – rusts, wastes away in submerged in seawater at about the same rate. In still seawater, unprotected steel wastes at a rate of about 7 mils per year – so 1/8” (0.125”) hull plate will last about 24 years. In the splash zone wastes away at about 15 mils per year. An additional 2-knot current will increase wastage to 2X to 3X.
• Subject to galvanic corrosion, impingement (erosion), and weld decay.
• Protection Schemes – Coating (paints, epoxy, fiberglass), galvanizing, dry, Cor-Ten alloys, cathodic or impressed current protection. Remove all rust – don’t paint over flaking rust (galvanic cell – rust oxide is about 0.3 volts more noble).
Stainless Steel –
• Stainless – is just stainless not stain-proof. The addition of chromium (> 10%) increases resistance to corrosion due to the tight layer of surface oxidation (passive state). Marine grade stainless is austenitic (non-magnetic iron).
• Subject to corrosion in active state (no O2 acts like iron “crevice” - and w/o zincs in saltwater “pitting”), weld corrosion, stress and corrosion fatigue.
• Marine Grades 304 and 316 for deck fittings but suffer from pitting corrosion when submerged in still sea water.
• Questionable alloy content in foreign made fittings, stainless plated brass/bronze castings.
• Protection Schemes – expose to flowing water, air, cathodic protection.
• Unprotected Galvanic Potential – 316 (active) 570 to 680 mv DC, 930 to 1030 (passive) - (zinc reference cell).
Aluminum Alloys –
• ABYC Project T-1 and NVIC 11-80 deals with aluminum vessels.
• Marine grades are 5000 and 6000 series – hull plate is from the 5000 (magnesium) and extrusions are 6000 (magnesium and silicon).
• Hull plating - 5083 (higher weld strength) or 5086 H-116 (highest corrosion resistance) – for structural members 6061 T-6 (most common) or 6063 T-6.
• High strength to weight/thickness ratio.
• Fatigues easier than steel – prone to stress corrosion cracking (notches – sharp bends). Alkaline will cause aluminum corrosion – so better to protected using zinc anodes rather than magnesium (more electrically active).
• Prone to poultice corrosion.
• Aluminum is low on the galvanic scale – all other marine metals are a concern.
• Protection Schemes – coating (barrier coat), cathodic protection, avoid direct contact with dissimilar metals.
Copper Alloys -
• Brasses (Cu/Zn) – yellow, Admiralty (70/30), Naval Brass or Tobin Bronze (60/40) – all subject to alloy breakdown (dezincification – addition of tin/arsenic, helps to reduce galvanic protection), stress cracking.
• Manganese bronze commonly used in propellers, shafts, deck fittings is a high Zn brass (40%) with the addition of manganese to increase strength. Bronze and Cu/Ni Alloys (Zn free Cu alloys containing tin) – aluminum (90% Cu – 10% Al), silicon (3% silicon) – not subject to dezincification, Cu/Ni alloys (strong and resists corrosion).
• Present day bronzes can have questionable alloy content.
• Protection Schemes – selective use, cathodic protection.
Protection Strategies -
• Coat - paint metals especially in galvanic cells. Thoroughly coat weld seams.
• Good housekeeping – remove debris, tools, fasteners, etc - especially in bilges – spaces – voids – framing members.
• If dissimilar metals are causing unwanted corrosion – then one or more of the following should be done –
• Electrically isolate dissimilar metals.
• Select metals that are close to each other on the galvanic chart. If a coating is used, then use it on the cathode (the metal which is not going to corrode) because coating the zinc anode will reduce its surface area. Reduce the area of the cathode not the anode.
• Change the potential between metals (anodes – impressed current systems).
• Properly wire vessels to ABYC standards – no ground and neutral lines connected.
• Watch for stray current issues. Use galvanic isolator (double diode bridge) – transformer.
Cathodic Protection -
• Every boat is a battery – some larger, some smaller.
• In marine environments – we struggle everyday with galvanic corrosion and cathodic protection.
• The problem - the corrosion (and resultant damage) that occurs at the anode of a galvanic cell is caused by the flow of electronics (galvanic current) from the anode to the cathode through an electrolyte. Usually – the damaging galvanic potential is less than 1 volt DC.
• In 1824, Sir Humphrey Davy mounted iron anodes on the copper hull sheathing of the HMS Samarang to prevent the copper from corroding.
• The plan – reduction or prevention of corrosion of a metal by either coating and or making it cathodic by the use of sacrificial anodes or impressed current to change the protected metal’s potential voltage by at least 200mv DC.
ABYC Project E2, Table II
Recommended Range of Cathodic Protection
(AG/AGCL reference cell in sea water flowing at 8 to 13 ft/sec, temperature range 50 to 80˚F)
-550 to -1100mv
-550 to -600mv
-950 to -1100mv
-850 to -1100mv
Non-metallic Hulls with Aluminum Drives
-950 to – 1100mv