How much hydrogen is produced in underwater cutting.
We have all learned in our manuals that hydrogen can be produced by two ways when cutting under water – by electrolysis and by heat. What does that mean exactly?
If we look at the many videos available on the web dealing with the electrolysis of water, we can without entering too much into technical details, see that when a DC electric current passes between two electrodes (anode and cathode) of inert metal it will then “crack” (dissociate) the hydrogen and the oxygen atoms present in the water molecules, and we will then see a flow of bubbles appear around each electrode.
Hydrogen bubbles will appear around the cathode (electrode connected to the negative pole), while the oxygen bubbles will be generated around the anode (electrode connected to the positive pole).
When we are cutting under water with electricity, our torch which is equipped with a cutting electrode made of copper and/or other alloys, is connected to the negative pole and thus it will react by producing bubbles of hydrogen around the non-isolated areas of the rod (tip & backside) as soon as the electric current is sent into the circuit.
On the other hand, and as contrary to the tests carried out in a laboratory, under water the earth clamp is generally connected to an oxidizable metal (steel) and in this case very little or no oxygen bubbles at all will appear on the anode side because the oxygen will tend to oxidize and dissolve that metal.
What greatly influences the flow of hydrogen created by electrolysis is for one part the nature of the water in which the diver operates but also the intensity of the electrical current and the (moving) distance between the electrode backside non-isolated part and the piece to be cut.
The two mediums in which a commercial diver is generally cutting is fresh water and sea water. Fresh water generally contains less than one percent of salt and because of this is significantly less conductive of current than sea water; therefore, it greatly reduces the electrolysis and hence the volume of H2.
For what concerns the current intensity, in contrary to the very low current used in a laboratory which generates a very little flow of bubbles, the one we use to burn our rods is much higher. It depends on the type of rod the divers use, but it is generally situated between 110 and 400 Amps.
How much hydrogen is then produced by electrolysis?
As not much has been published on that subject and as the scarce information available seems to be based on assumptions, we decided to do a number of tests in fresh as well as in salt water to get more objective values.
To calculate that flow, we have realized a series of tests in both fresh and salt water with a specific intensity of 50, 100, 150, 200 Amps.
For the fresh water, we have chosen to make the tests in two very different environments, namely quarry water and potable water because apparently the water turbidity has an influence on the electrolysis.
For the tests in quarry water that were carried out by divers, we couldn’t see any remarkable change in the H2 bubbles flow versus the intensity and therefore the maximum volume of gas obtained in that type of water at 150 Amps turned around 0.12 cc (0.007 cu in) / sec or 7.2 cc (0.44 cu in) / minute. As for the second type of water (potable), the hydrogen flow at this same intensity was still much lower and turned to be around 1 cc (0.06 cu in) / minute.
As far as the salt water tests are concerned, as we were too far from the sea we have not been able to do them by diving. They were instead made in a 70 l (2.47 ft3) volume basin that was filled with sea salt until the water reached a density of 1024 kg/m3 (64 lbs/ft3).
To calculate the production of H2, we used three different brands of Ø 9 mm (0.35 in) exothermic rods with again that specific intensity going from 50 to 200 Amps. During a first test at 150 Amps, 50 cc (3.05 in cu) of electrolysis gas was collected and submitted to a flame test, which confirmed that it was made of pure hydrogen. Then for each rod and at each programmed intensity, a video was made at both side of the electrode to compile the exact times that were necessary to fill 30 cc (1.83 in cu) of gas in the test tube.
During these tests we have observed that all 3 rods presented nearly the same flow of bubbles, and therefore for clarity only an average curve is drawn in the following graph.
For what concerns the non-isolated backside part, the flow was calculated at the constant distance of 30 cm (12 in) but in reality, it will tend to come closer to the tip line as it approaches the work.
As can also be seen on the graph, we have extrapolated the curve to the 400 Amps value line which permit to also have a rough estimation of the H2 generated during oxy-arc cutting.
The other way, which produces hydrogen during cutting takes place by thermolysis or, more simply said, by the vaporization of water.
Indeed, if water is brought into contact with a very high heat source greater than 2200°C (3992 °F), it will then cause the breaking (cracking) of the water molecules into oxygen and hydrogen atoms.
Apparently, here too, no description about how this is going on is found in our divers’ technical documentation, but the process kerfis relatively simple and can be explained as follows: when the liquid slag is ejected from the kerf and the tip of the rod by the jet of oxygen, it will as soon as it comes into contact with the water be almost instantly wrapped by a film of water vapour (Leidenfrost effect).
Inside theses gas bubbles, the water molecules trapped between the incandescent nucleus and the vapor membrane will be cracked within a few milliseconds, which will have the effect of separating the H2 and O2 atoms.
The oxygen will then immediately begin to oxidize the slag and form a crust around it while the hydrogen will dilate the membrane to its breaking point and thus be able to escape to the surface in the form of a small gas bubble.
During a few microseconds, the surface of the slag will then again be in contact with water, until a new film is created. Then, the same cycle is resumed for a few seconds until the temperature of the slag no longer allows the chemical reaction.
To measure the hydrogen gas produced during the water vaporization, we have recovered the molten slag in an adapted basin equipped with a calibrated gas recovery receptacle to which a little hose was connected to make the flame tests.
With each brand of rods, we have then made 3 × 3 accurate cuts of 10 cm (3.93 in) long in a 10 mm (0.39 in) plate in order to each time burn 10 cc (0.61 cu in) of steel. After each cutting sequence the produced volume of gas was noted (and increased by 25% to compensate for the loss of sparks that were blown outside the guiding box) and then sent to a little bag for a flame test.
As can be seen on the graph n° 2, there is an H2 production difference between the rod A and the two others. This is probably due to the fact that the external tube of that rod has another composition, which may influence the temperature of the slag droplets.
Based on the above graphs, we can see that amount of hydrogen that could be produced by electrolysis and heat during one minute for the cutting of about 35 cc (2.13 cu in) of steel with one exothermic rod could be equal to more or less: 450 cc (27.45 cu in).
In reality, it will be less than that because as soon as the diver starts cutting, the current will pass through the arc and the electrolysis at the tip of the rod will then cease and in this case only the backside of the rod will continue to produce hydrogen (the electrolysis flow can also be entirely interrupted by cutting cold).
As for the H2 bubbles that are produced by heat, some are immediately burned by passing close to the flame.