Rate this article:  
Silver nitrate testing (AgNO3)
By Albert Weatherill, Director, McAusland & Turner Ltd., Marine & Cargo Surveyors, Hull


The Gard Services website has recently published an article about salt water wetting on steel products being caused inland as a result of salting icy roads.1 It is worth noting that this is not the only way in which steel products can become contaminated with chlorides prior to shipment. It is also relevant to explain the workings and purpose of the silver nitrate test.

1See article "Steel - sea water wetting inland?" dated November 2002 under "News from correspondents" in www.gard.no.

As mentioned in the article, one source of chloride contamination prior to shipment is from rock salt used on icy roads. Steel products are carried around the country on road transport with many of them being unsheeted or only partly sheeted. If that transport is travelling on a road that has been treated with rock salt and is also wet there is the distinct possibility that the steel will become contaminated. Anyone who has found themselves driving behind a truck on a wet road will be aware of how much water spray is thrown up. That spray will be carried along with the truck in its vortex and as such will wet cargo being carried. If that spray is also contaminated with road salt it is easy to see how the steel would become contaminated prior to its arrival at a vessel.

Another source of chloride contamination could be from chemical dust deposited on to steel prior to loading. Some chemicals could have a high chloride content, which if wetted could produce a severely corrosive effect on the steel. These may originate from shore-based facilities or indeed from bulk cargoes being worked adjacent to vessels either loading or discharging steel. In the worst example seen by us we found over 3,000 tonnes of steel coils contaminated by a chloride-rich cargo prior to shipment. Inadequate clausing of the bills of lading could have exposed the owner to horrendous "sea water" damage claims.

Perhaps one of the more serious sources of chloride contamination prior to shipment would be from sea water itself. In some instances vessels load steel cargoes very close to the open sea or adjacent to breakwaters. It is not uncommon for sea spray to be blown over cargoes awaiting shipment on exposed quaysides. Even in enclosed docks high winds can whip dock water up and over a quayside contaminating cargo before shipment. On a number of occasions we have had to request masters to close their hatch covers as dock water spray was blowing over the decks and into the holds.



Unsheeted steel coil travelling on wet road treated with rock salt.
 

Unsheeted steel slabs travelling on wet road treated with rock salt.

Extensive testing with silver nitrate is therefore recommended whenever it is considered a possibility that steel products could be contaminated by chlorides prior to shipment. On discovering the presence of chloride it is again important that a shipper's representative be made aware of its presence. If that cargo is then subsequently shipped the following remark should be applied to the relevant bills of lading: "… (number) pieces/bundles reacting to silver nitrate solution tests".

The long-standing test for salt contamination (chloride ions) has traditionally been the "silver nitrate test", where a milky white response is indicative of chloride ions.

Numerous sources are available that give the chemical make-up of silver nitrate. The solution being two parts AgNO3, two parts nitric acid (chloride-free) and 96 parts distilled water. The solution should be stored in a dark bottle having a dropper-type applicator. Some surveyors have adapted atomiser-type sprays which are useful for testing the undersides of products or deckheads, etc. The principal aim of the AgNO3 test is to detect the presence of chloride ions, and it must be recognised that:
(i) if the test is carried out correctly it is an acceptable method of detecting the presence of chloride ions.
(ii) the test can be carried out incorrectly, i.e., if the dropper comes into contact with the product hence contaminating the contents of the bottle.
(iii) the test can not be indicative of the source of the chloride ions, and on its own does not suffice to warrant rejection.

The Lloyd's Survey Handbook recognises that the test may give a salt reaction, which is not necessarily indicative of the cause of the damage and should not be the basis of rejection. The AgNO3 test will simply be positive or negative. It should also be noted that chloride detection alone is not sufficient to identify sea water - sodium (Na) and magnesium (Mg) being of equal significance in confirming "sea water".

However, a positive result to a silver nitrate test at discharge, even where sodium and magnesium have also been detected, does not mean that the cargo has suffered sea water damage during carriage. The positive result can also be obtained if condensation occurs and picks up salt from the atmosphere and/or vessel's structure.

Many surveyors acting for receivers and cargo interests place a great deal of emphasis on the results of the silver nitrate test. Indeed many cargo underwriters have stated that a positive silver nitrate test was all that was required for them to seek recovery against a shipowner. Clearly if a vessel has suffered from sea water ingress through the hatch covers during a voyage the test would be carried out to confirm the presence of that sea water. However, if cargoes are loaded in a condition that, if tested with silver nitrate, would produce a positive result, it is essential that the bill of lading reflect this condition. If not remarked upon, a vessel could be held responsible for "sea water" damage when no ingress has actually occurred.

The silver nitrate test should be seen only as a means of establishing the presence of chlorides. If there is any doubt as to the origins of those chlorides then further chemical analysis will be required.

Any comments to this article can be e-mailed to the Gard News Editorial Team.

Gard News is published quarterly by Gard Services AS, Arendal, Norway.