Executive Ship News Bulletin

Effects Of Pollution
- Continuation of Febuary 2007 article

The explosion of harmful algae has caused toxins to move through the food chain and concentrate in the dietary staples of marine mammals causing standings and mass die-offs of whales, dolphins and other ocean mammals — barometers of the sea's health.

The surge in mortality of these mammals has coincided with the algae and bacteria blooms. Although some of the deaths defy easy explanation, telltale bio-toxins have turned up in urine, blood, brains and other tissue. Sometimes the toxins kill animals outright. In other cases, they kill slowly by promoting tumour growth or compromising immune systems, leaving marine mammals vulnerable to parasites, viruses or bacteria.

Some of the California sea lions were found emaciated, disoriented and suffering from seizures, exhibiting the classic symptoms of domoic acid poisoning, a condition that scrambles the brains of marine mammals and causes them to wash ashore off the California coast. They pick up the acid by eating anchovies and sardines that have fed on toxic algae.

Although the algae have been around for eons, they have bloomed with extraordinary intensity along the Pacific coast for the last few years.

Scientists believe the episodic die-offs of bottlenose dolphins along the Atlantic and Gulf coasts that began in the late 1980s may stem from toxic algae that weaken the animals and enable a virus related to canine distemper to attack the lungs and brain.

Sea turtles in Hawaii have been found with fist-sized tumours growing out of their eyes and mouths and behind their flippers. Scientists say the growths are the result of a papilloma virus and an ancient microorganism called Lyngbya majuscula, which appears as a hairy weed that has been spreading in tropical and subtropical waters. The tumours doom the turtles by inhibiting their ability to see, eat or swim.

These blooms (of algae, bacteria, etc) are part of a worldwide pattern of oceanic changes that scientists attribute to warming waters, excessive fishing, and a torrent of nutrients unleashed by farming, deforestation and urban development.

As they watch the oceans disgorge more dead and dying creatures, scientists have come to a disquieting realization: The proliferation of algae, bacteria and other microbes is making the oceans less hospitable to advanced forms of life — those animals most like humans. Marine mammals share our waters, eat some of the food we eat and get some of the same diseases we get. If environmental conditions are not good for these sea creatures, then it won't be good for us either. What we allow to flow into the sea is coming back to hurt us.

- Article compiled by Capt. Vijay Cherukuri, Marine Superintendent.

The Chemistry Of Corrosion in Stainless Steel Tanks (Part 1)

Crevice Corrosion

	The following encompasses a description of the steel, an explanation of how typical corrosion problems happen on stainless steel surfaces, how they can be avoided and remedied. Steel typically used on board ships is AISI 316l. This may be different depending on the trade pattern of the vessel. Chosen
for this application due to rich Molybdenum and low carbon content. This alloy is widely used in the marine environment due to its pitting resistance in low temperature seawater. It is, however, susceptible to crevice attack.

An area of the steel surface incorporating a crevice is surrounded by an electrolyte. We can use seawater as the electrolyte here and as a crevice, for example, the tiny gap between a nut and a flange, or, more seriously, the small gap or hole sometimes seen in a shoddily repaired welding seam joining the tank top to, for example, a bulkhead.
Initially, the composition of the electrolyte inside and outside of the crevice is the same. The steel will begin with an equal corrosion resistance inside and outside of the crevice.
A basic wet corrosion cell has 4 components :
a)	The anode. In this area electrons are removed from the neutral metal atoms and the charged atoms enter the electrolyte as ions. This happens inside the crevice.
b)	The cathode. Here the reaction depends upon the pH of the electrolyte.
	-	The electrolyte is acidic (pH is less than 7 ). The electrons travelling from the anode will combine with the hydrogen ions in the acidic electrolyte producing hydrogen gas.
	-	The electrolyte is alkaline ( pH is greater than 7 ). The electrons travelling from the anode will combine with water and oxygen present in the electrolyte to produce hydroxyl ions. In both cases consumption of electrons produced at the anode will provide a stimulus to the anode to produce even more. The cathode is the steel surface outside the crevice.
c)	The electrolyte. In our case this is seawater or fresh water.( Ionic solution)
d)	An electrical connection between the anode and the cathode. In our case, this is the fact that the anode and the cathode are part of the same tank.

Corrosion will begin in the crevice if there is a difference in the free energies between the anode and the cathode. This means that an electrical potential difference exists between the anode and the cathode, and that the electrons produced at the anode can move through the steel to the cathode, and from there into the surrounding electrolyte. The greater this potential difference (measured in mV ) , the greater the potential for corrosion.

The question here, therefore, is how does the reaction start, which initiates corrosion by setting up the potential difference between the anode and the cathode? The answer, in the case of stainless steel, is found by analyzing the influence of oxygen.

The pH of seawater is slightly alkaline i.e. greater than 7. This gives rise to the reaction described in b.ii) above. The electrolyte outside the crevice dissolves more oxygen from the air with which it has contact as long as the oxygen already dissolved in the electrolyte is used up to make hydroxyl ions. Inside the crevice this cannot happen ( lack of oxygen ) and the generation of hydroxyl ions will cease. We now have an imbalanced situation. A distinct anode and cathode emerge. More positive ions inside the crevice will build up. These positive ions are metal ions from the steel. Negative ions in the electrolyte outside the crevice will be attracted by the existence of the positive ions inside the crevice and move into the crevice. This would not be so bad if they were only hydroxyl ions, but our electrolyte is seawater. The most influential negative ion in seawater when discussing stainless steel is the chloride ion. The effect of this diffusion of negative ions into the crevice is to raise the pH of electrolyte inside the crevice. Hydrogen ions are produced. Together with the presence of the chloride ions there is a hydrochloric acid solution produced in the crevice.To make matters worse, it has been shown experimentally that it is the dissolution of chromium atoms which leads to the most significant drop in pH inside the crevice.

We can avoid crevice corrosion by removing any one of the 4 elements necessary to produce it.
a)	The anode or the cathode. (2 elements inextricably intertwined). To do this we have to remove the crevice.
b)	The electrolyte. To do this we have to remove the seawater.
c)	The electrical connection. We cannot do this without scrapping the vessel.

As a consequence of the above, we can conclude that crevice corrosion is preventable in stainless steel AISI 316L by removing crevices, seawater or preferably both. - Part 2 to be continued in the April issue.

- Article compiled by Capt. Thomas T. Varghese,
Superintendent Vetting & Operations.