Tuesday, May 17, 2011
Tests of fiberglass fuel tanks from a 1967 and 1970 Bertram:
To date, the testing done by IMS, LLC indicates that the two fuel tank samples have undergone some aggressive degradation (40% of their strength) from gas - ethanol fuel blends. The bottoms of both tanks have lost more strength than the tops. The older tank (1967) was laminated to a much higher level of quality in terms of entrapped air and fiber roll out. The mode of property reduction in the newer tank (1970) appears to be both resin softening and loss of adhesion between fiber and resin. This is evidenced by a moderate loss in both strength and stiffness. The older tank has lost nearly a similar amount of strength but has retained all its original stiffness. This indicates some resin degradation has occurred but no loss of the fiber/resin interface’s integrity has occurred.
Should check for presence of ethanol using the following methods -
To check for ethanol in gasoline - use two bottles, with calibrations in ounces.
In one bottle put ten ounces of gas from the suspect tank, in the second bottle - put two ounces of tap water.
Pour the water in to the bottle with the gas, cap off and shake.
Let set and the water will settle to the bottom and if ethanol is present it will absorb the water and the fill line will change from twelve ounces to something else. If this is the case - then the tank should be viewed as suspect - replacing tanks can be a trick.
Tuesday, March 15, 2011
This entire corrosion thing is one of the many dark arts of boat maintenance faced by mariners today. Nothing really happens overnight – and in the case of corrosion related wastage of metals – the deterioration process is usually slow with ample but subtle tell tales along the way to give the alert mariner time to take corrective actions. One of the most insidious forms of corrosion is found as small patches or blooms of white powder-like deposits on aluminum. Poultice corrosion can happen on unprotected aluminum or under bubbling paint. In either case – the amount of wastage can be severe if not discovered and properly treated. Before looking at the cure – let’s discuss the cause.
Here’s a classic example of poultice corrosion – a bare aluminum window frame on a Hatteras which has been damaged by corrosion. Over a period of time, water trapped by debris and dirt against the aluminum frame formed an acidity material call aluminum hydroxide which has perforated the frame leaving white deposits scattered across the sill. Poultice or under deposit attack corrosion may occur when bare, unprotected aluminum surfaces are covered by absorbent materials, dirt, and debris trapping moisture against the unprotected metal surface. In this instance, corrosion attack can continue even when the other surfaces are dry due to the retention of moisture in the poultice. The corrosion mechanism is similar to crevice corrosion in that the covering materials or deposits act to limit the migration of oxygen to the covered area. This leads to acidic shifts in pH, concentration of Clˉ ions in the shielded area, and a more active corrosion potential under the deposit. Negatively charged chloride ions tend to migrate under the deposit to balance the positively charged aluminum ions produced there. The high concentration of chloride ions causes the area under the deposit to become more acidic compared to the bulk solution, further enhancing the corrosion under the deposit with a white, poultice-like material produced (aluminum hydroxide).
Corrosion attack on aluminum surfaces is usually quite obvious, since the products of corrosion are white and generally more voluminous than the original base metal. Even in its early stage, aluminum corrosion is evident as general etching, pitting, or roughness of the surface. Aluminum alloys commonly form a layer of smooth surface oxidation (0.001” to 0.025 thick). This is not considered detrimental as it provides a barrier against corrosion. But when this protective layer is removed – damage can and will happen.
Another real site for problematic poultice corrosion is aluminum fuel tanks. Don’t store cardboard boxes, equipment, or other items on top of tanks. Don’t use moisture wicking materials to cushion tanks against framing structures. ABYC recommends tankage to be installed in accordance with H-24 (gasoline fuel systems) which states that all non-integral tank supports, chocks, or hangers shall be separated from metallic tank surfaces by a non-metallic, non-moisture absorbent and non-abrasive material suitable for the purpose (e.g., neoprene, Teflon, and high density plastics) permanently bonded to the tank surface with impermeable, non-hydroscopic adhesive. Self-wicking material, such as carpet pile, shall not be in contact with a metallic tank.
Another common refuse for poultice corrosion attack is in the deck boxes of aluminum sport boats where stowed fishing gear, nets, towels, and life jacket can trap moisture against bare aluminum.
Poultice corrosion can also occur under chipped paint – the mechanism is the same – trapped moisture against unprotected aluminum. Look at paint bubbles – such as around beauty rings on portholes (don’t be confused by galvanic corrosion). Chipped paint or coatings will promote poultice corrosion – allowing moisture to be trapped against bare metal. The best form of prevention is to properly coat all bare metal surfaces. If not possible, bare aluminum must be kept clean and dry – free of wicking materials. Think about inside airplane wings – aluminium is always coated to prevent corrosion. Maintain regular cleaning and proper coating of all exposed surfaces to prevent corrosion.
Unprotected aluminium should be maintained within a pH range of 6 to 8 where the alloy is stable. In the event of poultice attack, a base solution should be applied to control acidic corrosion. Protective steps would include, but not limited to the use of Alodine®. This material is one step in a multi-stage protection scheme and does not provide sacrificial protection to aluminum alloys. Instead, it acts as a passivating inorganic thin coating over which a primer can be applied. In the same manner, a zinc chromate primer does not provide sacrificial protection but its corrosion inhibiting properties retard but does not prevent corrosion.
Thursday, January 13, 2011
Galvanic Corrosion Thoughts…To avoid or reduce wastage -
• Thoroughly and carefully coat or paint metals especially in galvanic cells.
• If a coating is used, then use it on the cathode (the metal which is not going to corrode) because coating the zinc anodes will reduce their surface area. Reduce the area of the cathode not the anode.
• 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 series.
• Change the potential between metals (anodes – impressed current systems).
• Properly wire vessels to ABYC standards – no ground and neutral lines connected.
• Use galvanic isolator – transformer.
• Zinc – Salt Water (make sure to use Military Spec M 18001J zinc anodes).
• Activated Aluminum – an alternative to zinc in sea or brackish waters. Both zinc and activated aluminum do not work well in fresh water unless cleaned monthly.
• Magnesium – Freshwater, potential excessive protection in seawater (short life).
Friday, January 7, 2011
Saturday, January 1, 2011
Sail Rig inspection guidance from the USCG Sector Honolulu Inspection Note Number 13
- Monthly - a thorough inspection of all rigging equipment.
- Annual – a comprehensive (mast standing) inspection of mast and rigging system with rigging slacked off, turnbuckles opened and lubricated with rigging properly tension and re-tuned.
- Every six (6) years - mast removal and disassembly of all components for comprehensive inspection.
- Replacement of stainless fittings every five (5) to ten (10) years for vessels operating in the tropics.
- The USCG Sector Honolulu Inspection Note Number 13 refers to a six (6) year “cycle” – Replace wire every 6 years, terminal fittings every twelve (12) years, and chain plates every eighteen (18) years. This may be as a result of the “tropics” and the beating it gives to stainless steel rigging. Mariners have all seen rigs that are well over ten (10) years old that appear to be in serviceable condition. A conservative estimate may be to use the “six year cycle” for those vessels operating in the tropics. Then move to possibly an “eight year cycle for those operating on the “coasts”. With a “ten year cycle” for those vessels that are used in the northern climates.