What do we know about salty water?

I was carrying out a little research on my local area to find that my nearest river, the tidal Blackwater in the county of Essex is supposedly the saltiest river in England! 

This is due to it’s length (23km), width & relatively shallow depth, when the tide drops, a large expanse of estuary bed is exposed & the evaporation effect increases salinity, add the low rainfall in this area, along with the very low freshwater input and I can easily understand why it’s so salty.

No wonder all my wet kit gets so much salt damage!

My local river is however not that different from most estuaries, they all suffer from exposed beds when the tide drops and due to the evaporation at this time, a greater increase in saltiness when the tide flows back in.

When salt water evaporates it’s only the water and not the salt that dissipates, the salty taste when you stand on the sea cliffs or beach results only from water droplets (aerosols) being carried by the wind.

We all know that salt is a huge equipment killer, but simply drying off the salt water doesn’t stop the problem, you are probably familiar with drying off after being in the sea, the towel does the job but you just don’t feel dry, if your lucky enough for the sun to be shining, then you may feel dry, but you are always very salty. 

The same is true for your U/W camera kit, if dried or left to dry the salt still remains. 

The salt in a dry state is not a major problem, but as soon as any moisture is in the air the salt will absorb it, as it is hygroscopic (attracts moisture from the air or moist objects)

This is why salt is such a good food preservative, it absorbs any moisture and without access to moisture bacteria cannot form.

This is also why the story of dried salt on your U/W kit doesn’t stop here, any moisture will reactivate the salts ability to ‘attack’ your equipment……. 

But how does salt do so much damage?

The damage is caused by electrochemical corrosion as saltwater conducts electricity, this allows the ions within a metal object to attract the electrons from within the water, this creates a cycle of attack & slowly corrodes the metal.

Once removed from the wet stuff and dried, corrosion will continue where any dried salt is left on your kit, absorbing any moisture in the air and continuing the attack 

Corrosion-What is it?

Simply put corrosion is oxidation of the metal usually on the surface, but it can also occur as pitting, crevice or flaking.

Oxidation occurs on any metal surface when exposed to air & moisture

In the case of underwater camera equipment, there are a number of causes other than just air & moisture, the most common being galvanic corrosion, this is when two metals of different electrical potential are joined physically or by a conductive solution and are subjected to moisture, like salt water.

Each type of metal has a potential to conduct electrical energy, and the galvanic table list all metals in order of their potential rating. Any two metals joined with dissimilar ratings are likely to react with each other. The greater the potential difference between metals the faster the rate of corrosion. 

Most metals react with with moisture & most metals The more noble metal will become the cathode while the less noble will become the anode, the greater the potential difference between the metals, the faster the anode will suffer corrosion.

As you can tell there are many variables with galvanic corrosion, and the best protection found so far to reduce the effect is the sacrificial anode. This works best when the anode is sized correctly to have the biggest effect. 

So the physical size of the anode compared to the cathode, basically means the smaller the anode in relation to the cathode the higher the rate of corrosion of the anode. 

This anode needs to give enough protection over a long enough period for our kit, preferably protecting between services. Obviously we all have different dive logs so most anodes overshoot the average divers experience, but they are always available as a spare for you to change.

However with worth noting that any sulfate’s or bacterial deposits left on your kit can also cause a continued cycle of attack even when out of the water.

This is another reason why metals corrode much faster in salty water than fresh water, but even faster when they are continually immersed & removed.

This is the most difficult environment to stop damage from occurring, which is why it’s so important to understand how to protect your valuable kit from corrosion or be resigned to renew it.

I have a routine of testing and replace failing surface sport kit that has been damaged by salt corrosion at least once a year! 

The piece of kit I find most vulnerable and is critical to safety, is the carabineer this is not easy to dismantle or clean thoroughly and sometimes I am on the water for several days, getting wet/dried repeatedly, any surface wet sports will suffer the same problems with critical kit and often replacement is the only option.

Obviously most manufacturers try to limit this corrosion so it’s worth thinking about the efficiency and relevance of the presence of any protective coatings, barriers, inhibitors and of course the efficiency of the electrolytic, what contaminants the water has that you are diving in, is it fresh but in an area with harmful deposits e.g. iron ore, chemicals, acidic or alkaline and of course wether it is fresh water or salty.

I think you will agree understanding the problem is probably enough, finding out what contaminants are effecting your housing on any trip, is probably way beyond most mortals!

So how can we stop corrosion of our kit?

We now know how our kit is attack and that the manufacturers do a sterling job of protecting our kit, so why should we have to stop corrosion surely the manufactures have done this during the build.

Manufactures have of course taken into account the protective properties of the materials they use and many, choose to build products from metals that if damaged, can to a certain degree protect themselves. 

Corrosion is where the oxidation of metal that forms a layer, that is protecting the metal beneath, so in some circumstances is it better to leave the oxidation on than remove it?

Stainless steel 300 series produces a very thin layer of chromium oxide that protects the main block of steel and if not broken is extremely efficient and long lasting.

Aluminium anodising produces a thin layer of oxide that will protect the metal block from further attack.

When the surface oxide is attacked, scratched, chipped or rubbed off, the exposed metal will attempt to re-oxidise, however when it suffers repeated action, or if it has a rough surface, other contaminants can stop the protection process and form pitting (a small surface hole) or crevassing, a larger and deeper area that continues to expand, this is significant with aluminium, as the crevice creates a ‘micro climate’ that due to the high acidity in this localised area, creates it’s own battery cell structure with cathode and anode in the metal.

This accelerates the corrosion in the crevice or pit and can severely compromise the integrity of your underwater housings, by forming relatively large 'caverns' within the material, severely weakening the structure, especially on control entry/exit points or edges of the casing that are routinely damaged.

Stainless Steel can also suffer similar problems but rarely as severe. The main problem is where the control shaft wears against another surface, such as the casing or even a sand laden 'O'-ring, a crevice will eventually cause a leak path, but it is unusual to weaken the strength of the steel material significantly to cause it to fracture or break.

Unfortunately this isn’t just theory, in the environment we use our equipment, varying water P.H. values, physical wear, along with the continued cycle of soaking & drying, will all degrade the protective coatings and then the oxidation layermarine graded metals form and the result we see time after time is a breakdown in the housings integrity, often to the point that is uneconomical to repair.

So back to the question how do we protect our kit from the scourge of salts?

The simple answer is to understand the risks, of not removing the salt & other contaminants from your equipment, as soon as possible.

To ensure you equipment does not dry out until you have access to fresh clean water, in order to flush any contaminants out / off of your system.

Then to carefully dry, inspect & cover any mark, scratch or ‘dink’ that might compromise the integrity of your housing.

The failure to carry out this simple process results in;

• Early failure of your equipment (good for repair shops)
• More frequent replacement (good for equipment retailers)
• Blown budgets (good for loan companies)
• Ruined dives, trips or holidays (Not good for anyone) 

Hopefully you have found this information informative if you would like to know more about best practice on looking after your equipment click on the button below.