April 2007 | Page - 8
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Effects Of Pollution: Domoic Acid
- Continuation of Mar 2007 article.

Marine algae, or phytoplankton, occur naturally and make up the first link in the oceanic food chain. A litre of seawater typically contains hundreds of thousands of phytoplankton and millions of bacteria, viruses and protozoans, all in concentrations that keep each other in check. That equilibrium can be upset when certain types of algae overwhelm their competitors, this change is most pronounced in coastal waters, and scientists believe it is tied to nutrient pollution from a variety of human activities.

Toxic algae thrive on the same elements that turn lawns green and make crops grow — nitrogen, phosphorus and iron.

Many a cities the world over with millions of people, sends billions of litres of partially treated human waste into the ocean every day. Sewage treatment cuts down on disease-causing bacteria but does little to remove nutrients. Seasonal rains carry enormous loads of urban and agricultural runoff into the ocean, much of it down drainage canals and rivers from the dairies, orchards and farms. The destruction of coastal wetlands, which filter nitrogen and other nutrients, also plays a role, as does over-harvesting of shellfish and sardines, menhaden and other algae-eating fish. With climate change - warmer seawater speeds up microbial growth and allows aggressive algae and bacteria to move into areas once too cold for them. Further commercial ships help the spread by transporting the algae in ballast water.

Researchers studying one such toxic algae ‘Pseudo-nitzschia’ off the mouth of the Mississippi River have unearthed evidence in the seafloor that agricultural runoff triggers the outbreaks. Scrutinizing core samples they found evidence that shows that this algae started proliferating after the 1950s, when grain farmers began widespread use of chemical

fertilizers. In contrast to the Mississippi Delta, such telltale clues cannot be seen in marine sediments off the Pacific coast or other coasts with less agricultural activity because the seafloor is constantly being churned up.

One such toxin produced by these algae is Domoic acid. Scientists first became aware of Domoic acid and its toxicity in 1987, when three people died and at least 100 others were sickened after eating contaminated mussels from an Island in Canada. Nineteen people were hospitalised with seizures, comas and unstable blood pressure.

Many of the patients never recovered gaps in their memory, lending this malady a new name – ‘Amnesic Shellfish Poisoning’. An examination of brain tissue from the three people who died showed severe loss of nerve cells, mostly in the hippocampus, a part of the temporal lobe that resembles a seahorse and plays a key role in memory and navigation.

Reported cases of the illness are rare in developed countries of North America, Europe, etc because health authorities closely monitor shellfish for toxins and because such seafood makes up a tiny fraction of most people's diets. But for animals that consume little else, Domoic acid is a recurring danger.

The acid mimics a neurotransmitter, over stimulating neurons that retain memory. The acid prompts nerve cells to fire continuously until they swell and die.This acid has the same effect on the sea mammals like Sea lion, etc. The Sea lions gorge on anchovies and sardines that have fed on toxic algae. Domoic acid doesn't appear to affect the fish, but sea lions eat anchovies in such quantities that they accumulate a toxic load, causing them to suffer seizures which results in them losing their memory and sense of direction, eventually leading to a painful death.

A California sea lion has as warm and strong a maternal instinct with a newborn as in any animal, however Sea lions suffering from neurotoxin poisoning usually show no interest in their young. Some that previously cared for their pups shun them after suffering seizures or even attack them when they try to suckle or in worse case kill them.

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

The Chemistry Of Corrosion in Stainless Steel Tanks (Part 2)
Pitting Corrosion

The mechanism of pitting is essentially the same as crevice corrosion. The difference is that whereas in crevice corrosion the phenomenon is derived from the fact that a crevice already exists, pitting needs to be initiated - take for example cases of pitting in the tank tops in the centre of a steel plate, nowhere near a weld seam or a crevice.

Pitting is avoidable. Like crevice corrosion, only one of the factors leading to it has to be removed to stop the reaction.

When a clean tank is inspected and pitting is observed, then it is a case of locking the stable door after the horse has bolted. We are seeing the result of a previous corrosion cell. When we measure the passivity of the steel, even inside a corrosion pit, it almost invariably shows a passive area. Passivity is the unlikelihood of a particular area to corrode. Inside a pit under inspection conditions the electrolyte has been removed, as far as is observable. Inside the pitting the steel has access to oxygen and the passive chromium oxide layer has been restored. This is no reason, however, to ignore the pitting. The chromium concentration on the surface of the pit will be less than that of the surface surrounding it. The pit can trap seawater again in the future and reactivate a corrosion cell.

The pitting has to be removed mechanically and filled by the correct metal. The resulting repair must be devoid of crevices. In other words, the repair must have the same characteristics as the surrounding steel. This is achieved by careful welding followed by the minimum removal of new material by mechanical means to achieve a smooth finish. This is followed by firstly activating the steel in the area by pickling to remove unwanted oxides and contamination, followed by passivation to restore the passivating chromium oxide layer. Weld spatter must be similarly removed from the surrounding area and the area similarly pickled and passivated.
Pitting is often associated with areas of uniform corrosion. For example, an area of steel about 1m2 is rougher than the area surrounding it. Inside
this area are examples of even worse corrosion in the form of pitting, either shallow, open pitting or deep pinpoint pitting.

It is necessary to look at the causes of pitting to avoid it in the future.

Pitting is caused by starving a surface of the steel of oxygen relative to the surrounding surfaces, followed by exposing both to an electrolyte.

To starve a steel surface of oxygen you need to contaminate it.
Some examples of contamination are :

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Free iron. If iron from tools or boots imbeds itself into the stainless steel surface, then the surface will rust and the rust will form a porous layer on the stainless steel surface. The steel surface is deprived of oxygen but an electrolyte can penetrate through to the steel surface. We have created an 'artificial crevice'.

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Organic deposits
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Mud or silt
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Remains of impure cargoes such as wet phosphoric acid. This is usually the cause of the uniform deterioration of the surface texture of the steel, i.e. it becomes rough or abrasive.

Following the discharging of wet phosphoric acid, a thorough tank cleaning is required. This is needed to remove the tightly adhering sediments present in the acid. If this is not done, the large areas of the tank surface are deprived of oxygen. Subsequent immersion in an electrolyte will set up a gigantic corrosion cell between the 'clean' areas and the 'dirty' areas. These sediments, as far as corrosion is concerned, have the same properties as rust - they are adherent and porous to electrolytes.

The result is, that although through repeated use of the tank, eventually it is clean, a uniform corrosion has occurred during the time it was dirty and exposed to electrolytes. The dirty areas having sacrificed themselves as anodes to the clean cathodes.


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