Hello Niekie!
Long time no read!!!
I was going to give Insp76 a "double"(yes and no) answer but, I decided to answer his question with an explanation...
In an attempt to avoid any possible confusion, I'm going to give what I did originally write as the answer to insp76's question by stating that prevention of galvanic corrosion must be first incorporated into the design of any system first!!! A multi-pronged approach will produce the best results... First of all, I mean no disrespect when I comment below, to anyone But, I must warn everybody that this will take some time to explain so prepare for a long reply...
By consolidating all of your bets on or in other words, "putting all of your eggs in one basket" as far as relying on the heat, and lack of oxygen in the fire box as the only means of protection or as the only reason for the absence of galvanic or any other form of corrosion would be oversimplification to say the least!!! What I'm trying to get at is that steam is still water only in a vaporous state, and even though in comparison to all natural types of water (whether it be from a river or the sea in it's liquid state) or ultra-pure water, steam is still an electrolyte because, until the elements that make up water, separate from each other, then we're still dealing with water only in a different state ...
Therefore, the steam is still water, which means that it's still an electrolyte!!!
The amount of galvanic corrosion would decrease as compared with water in it's liquid state and yet, Galvanic corrosion is still occuring, just on a smaller scale, and mainly in the interior of the tube!!! Sure, the exterior is protected by the heat, and lack of oxygen in the environment of the "fire box". However, one must consider what has already occured within the interior, and in the thickness of the tube prior to the weld overlay, and afterwards... This is where corrective measures to diminish this are important!!!
Dissolved oxygen refers to the volume of oxygen that is contained in water. Oxygen enters the water by photosynthesis of aquatic biota and by the transfer of oxygen across the air-water interface. The amount of oxygen that can be held depends on the water temperature, salinity, and pressure. Gas solubilty increases with decreasing temperature (colder water holds more oxygen). it also increases with decreasing salinity (fresh water holds more oxygen than saltwater). Both the partial pressure and the degree of saturation of oxygen will change with altitude. Finally, gas solubility decreases as pressure decreases. Thus, the amount of oxygen absorbed in water decreases as altitude increases because of the decrease in relative pressure.
A key point to remember is that in modern boiler systems, dissolved oxygen (DO) is handled by first mechanically removing most of the dissolved oxygen, and then chemically scavenging the remainder. The mechanical degasification is typically carried out with vacuum degasifiers that reduce oxygen levels to less than0.5-1.0 mg/L or with deaerating heaters that reduce oxygen concentration to the range of 0.005-0.010 mg/L. Even this small amount of oxygen is corrosive at boiler system temperatures and pressures. Removal of the last traces of oxygen is accomplished by treating the water with a reducing agent that serves as an oxygen scavenger. Hydrzine and sulfite have been widely used for this purpose, but they have shortcomings. Sodium sulfite, although an effective scavenger, is not recommended for use in systems operating above 1,000 psi because breakdown occurs to form corrosive hydrogen sulfite and sulfer dioxide. Also, sodium sulfite increases the amount of dissolved solids, as well as the conductivity, in the water.
Hydrazine efficiently eliminates the residual oxygen by reacting with the oxygen to give water and gaseous nitrogen. Unfortunately, hydrazine is an extremely toxic. This is where Erythorbic acid and it's sodium salt are replacing sulfite and hydrazine as oxygen scavengers in boiler water treatment. Based upon the Stoichiometric relationship, it should take about 13 parts of sodium erythorbate to react with one part of dissolved oxygen. However, actual lab and field test data shows that much less erythorbate is needed than what the theoretical results to scavenge oxygen (DO). Further scavenging occurs from the breakdown products of the erythorbic acid. Field trials in large utility boilers show the intermediate breakdown products to be lactic and glycolic acids. The ultimate breakdown product is carbon dioxide... Also, a thin film of hydrogen will form to protect the interior surface initially...
Erosion corrosion can also wear out the interior's protective coating, applied intially by the manufacturer/supplier of the tubes, and some of the older tubes have coatings that actually did more harm than what they were intended to protect!!!
How does all of the above relate to galvanic corrosion or any other form of corrosion? Well - just look at all of the different types of water treatments that were and are currently either being used or are being tested... If you look closer, you'll see that this is another consideration that must be taken into account when designing, by using a multi-pronged approach. Thermal cycling (coefficients of thermal expansion differ when comparing the filler metals used as the overlay) results in accelerating certain forms of localized corrosion as do other environmental factors. The entire circumference, wall thickness, surface (both internal and external) finish, and length of each tube will not be uniform, metallugically speaking. Furthermore, no two tubes are exactly alike as far as metallurgical (microstructural) uniformity is concerned so because of this, various forms of localized corrosion has already occured prior to applying the weld overlays on existing boiler tubes, and there are a variety of reasons for this which I wo'nt get into as it's becoming apparent that I've spent too much time on this attempt to explain that initially, galvanic corrosion may not occur where one might expect, and that environmental factors that are preventing this from happening when one looks at the exterior, can be decieving. One must also look at what forms of corrosion has occured at the interior of the tubes... Finally, if different forms of corrosion are already present, and originating from the interior surface, the potential for galvanic corrosion to also occur as a result of previous localized corrosion is a real possibility!!! Erosion corrosion is an example of this. If the thin protective hydrogen layer or film which forms between the steam and the tubes interior surface erodes then, the potential for the electrochemical reaction which results in a form of galvanic corrosion increases (erosion of the protective coating or laquer applied to the tube prior to installation is also a factor), especially in carbon or low alloy steels!!!
For instance, if one localized area of a previous form of corrosion has formed in a small area of the interior surface of the tube, and another one has also formed on the opposite location on circumference of the interior surface of the tube or tubes then, because of this relative close proximity to each other, and yet not actually making contact with each other, the end result will be that galvanic corrosion can occur!!! Now this may not be important in the short term but, we must look at this from a long term perspective in order to decide whether or not to either replace a section of tube or tubes instead of just blindly applying weld overlays only, wherever without looking at the root causes of this phenomenon in order to formulate a more effective strategy that results in reduced repetitive maintenance...
I can go on, and on but, I think I'll stop here so, if anybody wants a further explaination, you can go to the corrosion doctors website which I had posted in my previous reply on this topic...
Respectfully,
SSBN727 Run Silent... Run Deep!!!