Brian Dalder Forum Moderator 177 Posts
Re: Can you define salinity in practical terms?4 November 2011 at 1:29pm
Let's answer the easier parts of your question first. Salinity has a very specific definition. The ASM Handbook Volume 13C "Corrosion: Environments and Industries" defines salinity as the total weight, in grams, of inorganic salts in 1 kg seawater when all bromides and iodides are replaced by an equivalent amount of chlorides and all carbonates are replaced by an equivalent amount of oxides. Salinity, S, is expressed in units of parts per thousand (ppt). Chlorinity, Cl, is defined as the mass, in grams, of silver required to precipitate the halogens in 0.3285234 kg of seawater. It turns out this is about equal to the mass of chloride in a seawater sample. Chlorinity is related to Salinity by the equation S = 1.80655 * Cl where S and Cl are measured in ppt. For reference, seawater with a salinity of 35 ppt has a chloride content of about 19 g/kg seawater.
The specific chloride content of 1.5 g/kg that you asked about is quite low but its corrosivity will depend on many things including the dissolved oxygen content, pH, temperature, velocity, and what other chemical species are present. As a point of reference, corrosion rates of carbon steel in seawater with a salinity ranging from 32 to 37 ppt have been reported to average about 4.6 mpy (mils per year) but corrosion rates as low as 0.5 mpy and as high as 375 mpy have been observed. As a rule of thumb, < 2 mpy is considered excellent corrosion resistance, between 2 and 20 mpy good corrosion resistance, between 20 and 50 mpy fair corrosion resistance, and > 50 mpy poor resistance and not recommended. Of course, what corrosion rate is acceptable in your case will ultimately depend on the specifics of your application.
I am a mechanical engineer and am currently working on a project where I need to understand the salinity as chlorides in the water. It is measured in mg/l, but I can't picture if this is corrosive or not? Is my understanding correct in that for arguments sake if I have 1500 mg/l salinity as chlorides I essentially have 1.5 g of salt for every 1 liter of water? Therefore, depending on densities, that's approximately 1.5 g per 1 kg - am I correct? My next question, would this be highly corrosive to the likes of carbon steel pipes? I try to equate everything I do to practical examples - such as a normal hit with a sledge hammer equates to about a ton of force, give or take, therefore if I need to know quickly if a structure can withstand some force I use this test to gauge it quickly. Are you able to give me an example of salinity as chlorides and how corrosive they can be to carbon steel so I can gauge this quickly just by looking at the number and then I can investigate further if needed or not worry about at all. Sorry to go on a bit, but your help would be very much appreciated.
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