Using a tube made from stainless steel, a faucet made from brass and the weekend we run into a problem that arises from wondering whether it is possible to combine the two alloys or not. The answer to this question would be both yes and no depending on the kind of water we are using and that is the reason why the Internet has its two completely contradicting answers to the problem. The difficulty here is that no one tells you what the conditions to belong to group one or group two would be and this is the knowledge one gets by unpacking the installation.
Further we would provide the essential principles in electrochemistry because the recommendations we can give can only be valid provided we know what kind of metal profile would be corroded.
To summarize: Indeed, brass can be used with stainless steel fittings, and this is a widely practiced method in potable water piping systems. Galvanic couple is available between brass and stainless steel, with a very low electrical voltage, ranging from 0.15V to 0.25V, with brass acting as a cathode and stainless steel becoming an anode. The level of electrical voltage between brass and stainless steel fittings is definitely lower than electrical voltage between copper and stainless steel fittings and aluminum and stainless steel fittings. However, the presence of chlorides in water, high temperature (over 60°C) and increases of total dissolved salts concentration lead to higher risk. Such failures that take place due to the galvanic action can be attributed to dezincification of brass and galling of threads, which are two types of failures that may occur.
Table of Contents
- What Happens When the Two Metals Touch
- Where Mixing Is Routine and Safe
- Where the Joint Will Fail
- Dezincification: The Brass Problem That Looks Like Corrosion
- Galling: Why the Thread Seizes Before It Corrodes
- Four Ways to Isolate the Joint
- How to Tell It Is Happening
- What to Specify So It Does Not Happen
- Where Eagle Fits
What Happens When the Two Metals Touch
A galvanic cell is created when two metals that are different are in contact with each other within an electrolyte. In plumbing, water is the specific electrolyte. The more noble metal forms the cathode while the less noble metal is the anode and gets dissolved. The rate at which both metals are corroded can depend on the potential difference, nature of the water, and on the size of metals used.
Brass and stainless steel are found near each other on the galvanic series with only about 0.15 to 0.25 V of difference depending on the alloy and water used. The same cannot be said about zinc and stainless steel where the potential difference is close to a single volt while aluminum and stainless steel also show a difference close to 0.6 V. If comparing all joints together, it becomes clear why the particular pair of metals survivable – small voltage means small electric current; small current compared to the large area of anode leads to long-lasting metals.
What makes an electrochemical element a problem is the area ratio. Corrosion on the anode amounts directly to the current density; current density amounts to the cathode area divided by the anode area. For example, if there is a huge stainless tank attached to a small brass nipple, it maximizes the corrosion process and diminishes the time needed for perforation.
Water chemistry also matters in this respect. The best conditions for corrosion exist if pure water is being used for cooling of metals. For instance, using the condensating machine means using less harmful water than using brackish water.
Where Mixing Is Routine and Safe
In municipal drinking water, using a brass ball valve mounted on a stainless steel pipe has been standard practice for years. Chloride concentrations in most municipal water supplies are significantly lower than the point of vulnerability for 316 stainless, there are few dissolved solids present, the temperature is low (below 60 °C) and the joint is usually just a small brass part in a big stainless steel pipe, which is good from the standpoint of area ratio.
The same goes for stainless tubing having brass compression fittings in the instrument air and clean water equipment as well as for stainless steel manifolds having brass drain valves which are used twice a year. In both cases the joint is dry most of the time or the water is harmless and the lifespan of the assembly is determined by the elastomers and the threads rather than electrochemistry.
Using stainless steel in a brass valve body is also a standard practice that has a good rationale in terms of corrosion protection. The stainless steel ball and stem in the brass body is the special design choice made for wear resistance, while the area of stainless steel exposed to water is small compared to that of brass which is a good sign.
In simple words, the principle is that mixing is safe when the water is clean, operation conditions are moderate, stainless steel is the dominant part and the joint does not require the current from the whole system to go through it.The comparison of brass and stainless steel fittings covers the material properties behind that judgement, and it is a useful reference before a specification is fixed, not after a failure.
Where the Joint Will Fail
There are four reasons for the transition from regular to hazardous operation of the same joint. They usually occur together.
- Presence of high chlorides. Any water source located close to the coast, any water from brackish wells or swimming pool systems belongs to this group of sources. The key effect in these cases is a simultaneous increase in conductivity and pitting of stainless steel. Simply put, the cathode is getting more cumbersome at the same time as the anode is corroding faster. After chloride content exceeds 200 mg/L the discussion is not about whether the joint will last, but rather about how often it will need inspections.
- High temperatures above 60 °C with cyclical variations in temperature. In many solutions the corrosion rate doubles for each increase in temperature of 10 °C. Heating cycles create conditions for mechanical stress that result in fracture in protective layers. A water circulation system that operates at 70 °C with brass inserted in stainless steel is the best illustration of the point.
- Stagnation followed by flow. A dead leg or a seldom-used segment at the end of the pipe creates a possibility for the buildup of corrosion products, and the first flow afterwards carries aggressive water through the joint. Seasonal systems, fire mains and standby lines will show this, when a fitting is good for two years and then fails in a month.
- Unequal surface areas. A small brass fitting joined to a large stainless steel tank, a brass drain attached to a stainless drain, or a brass instrument connected to a stainless header. All of them result in concentrations of electric charge passed through a small section of brass, and instead of the damage being visible as thinning it is a small hole in brass.
| Service condition | Galvanic risk at a brass to stainless joint | What the job usually needs |
|---|---|---|
| Municipal cold water, under 100 mg/L chloride | Low | Standard brass valve, no isolation |
| Hot water above 60 °C with cycling | Moderate | DZR brass, dielectric union considered |
| Coastal, brackish or chlorinated, over 200 mg/L chloride | High | Dielectric union, 316 or duplex stainless |
| Small brass drain on a large stainless vessel | High, from area ratio | Brass to brass transition or non-metallic spool |
| Dead leg or seasonal branch | Moderate to high | Flushing schedule, isolate where practical |
| Demineralised water or condensate | Very low | No special measures, poor electrolyte |
Dezincification: The Brass Problem That Looks Like Corrosion
Many people think that any pitfalls associated with galvanic action must be signs of galvanic corrosion; however, the truth is that most failures are actually cases of dezincification. The fact that it is so important to differentiate between the two problems is due to the fact that the solutions to these two cases are quite different.
Dezincification leaves characteristic signs on the failed material. The outer layer of the product will become pinkish or reddish due to zinc loss, and the item will look intact until it touches something, when it falls apart completely. The cross-section of the object indicates the leached layer and a proper core and the way a product fails suggests that it suffers from brittle rather than ductile failure.
Dezincification is possible in two chemistries of brass, aluminum brass and tin brass, which both have small additives, inhibiting the leaching process. Standard brass, on the other hand, is not inhibited and suffers from higher chances of failure. The label that should be looked for is DZR, which confirms that a particular material is resistant to dezincification, as well as galvanic actions.
The problem with diagnosing the case as galvanic is that when a brass component is connected to a cathode, the dezincification occurs at a higher rate, which plays into the hands of those who claim that it is the galvanic corrosion that has taken place rather than dezincification.The comparison of 304 and 316 stainless steel is worth reading alongside this, because the same chloride threshold that makes 316 necessary also makes DZR brass necessary, and specifying one without the other is half a decision.
Galling: Why the Thread Seizes Before It Corrodes
The second incorrect diagnosis is galling, and it has nothing to do with the chemistry of the water. Galling happens when the cold welding takes place between two surfaces composed of metal being subjected to pressure, the process is activated by the local peaks that penetrate the oxidized surface and reveal the metal for bonding. The process is common for stainless steel because it does not have an effective oxide surface, which is exactly what makes it resistant to corrosion.
Galling may occur between stainless bolts, and it may also occur between a stainless steel bolt and a brass nut, which plays the role of weaker material. The process will be evident when the fitting of two fasteners alternates between an easy turn for maybe three external threads and then jamming that does not allow any further turn in either direction without destroying the threads. In this instance, it is not corrosion but seizing which occurs during the assembly process, not over the course of time.
The existence of pipe threads makes it even more difficult because it is intended for interference. NPT threads engage through wedging, and the last two turns are when the actual sealing takes place, resulting in the increasing torque. The value of wrong threads increases dramatically if an NPT male goes into a BSP female since different angles at the threads are involved, making the discrepancy critical.
The good practices are the following. First of all, use a lubricant or a thread sealant, as it will reduce friction and prevent the access of oxygen to the fresh metal. Make sure that you turn slowly and in the event that an unexpected resistance is experienced, go back with a quarter of the turn rather than forcing it. Finally, do not mix the threads as it must be checked beforehand.The comparison of stainless and brass fittings in service covers the thread and torque differences that matter at the bench.
Four Ways to Isolate the Joint
When the conditions necessitate breaking the electric circuit, there are four successful methods available, each having different costs and includes a different type of failure.
- A dielectric union. A factory-made union consisting of an insulating sleeve and washer separating two metallic parts. It is the cleanest example of the method available, as it is rated in accordance with both temperature and pressure, and it becomes the standard solution used in HVAC and water treatment practices. Its failure mechanism is mechanical rather than electrical, as it degrades under high temperatures, therefore, check the rating according to the conditions rather than assume the union to be universal.
- A plastic-lined nipple or a non-metallic spool piece. It contains a short segment of either CPVC, PVC, PP, or plastic-lined pipe connecting two metals. This method has the lowest price and works well when temperature and pressure are low. However, the problems arise when the conditions go beyond 60 degrees Celsius or 10 bars or when plastic becomes the weakest part of the design.
- A brass-to-brass connection. If the pipe is stainless steel and the accessory is brass, insert a brass segment type so the fluid flow at the critical point does not form a pair of metals. It seems like a trick, but it is standard practice that uses couples in a location inconvenient so the ratio of area, as well as the water flow, are lower.
- A non-metallic pipe section. A piece of PEX, PVDF, or PP is placed between the two metal sections of the construction. In reality, this is a common occurrence of actual buildings, where a piece of plastic was introduced for other purposes by coincidence.
Want for the better part of total isolation of the connection is worth saying for the sake of clarity, seeing as there is a misunderstanding surrounding the case due to numerous misconceptions.
The sanitary pipeline standards used in hygienic service are relevant here for a different reason: in sanitary systems the joint is usually a clamp with an elastomer gasket, which isolates the metals by design and is one of the reasons hygienic lines rarely have a galvanic problem despite mixing materials freely.
How to Tell It Is Happening
The indications of galvanic corrosion are visible for years until the point of the occurrence of leaks, and those indications can be seen without any instruments if the targeted areas are known.
First of all, go to the brass side of the connection and look for a circle of green or white stains around the stainless steel, which is either copper or zinc salts leaving the alloy behind. A light or white crust appearing after being cleaned off is the earliest sign of galvano-corrosion. On the stainless side, the roughness and pitting at the contact point can be the evidence of the fact that the passive film has been damaged locally.
Next, check the water. Whenever there is dezincification occurring, copper will be added to the water, thus resulting in either blue-green stains on the pipes, copper taste, or copper in the water tests. In a drinking water system, it is a matter of safety, since too high amount of copper means reaching a limit allowed.
Mere physical checks require some disassembly. A brass part that seems lighter, has lost weight, or shows a pink surface area where processed is the thumbnail in case of dezincification. The fitting stuck or broken while unscrewing is galled, not corroded. Punctured leak on the brass part in case of a big stainless steel vessel being the case proves that the classic area/volume failure has occurred.
What to Specify So It Does Not Happen
Most of these failures can be specified but not implemented, which means they are virtually free of cost on paper.
- Do not just specify "brass"; name the exact alloy. "Brass" is a term for many alloys, and the difference between DZR and brass is the difference in the lifetime of a fitting in aggressive water. Indicate the alloy on the purchasing order.
- Specify the water chemistry. These include the values of chloride, TDS, pH, and temperature at the place where the system will be installed. A supplier who knows these parameters will have no doubt whether the standard product will work, while a supplier who does know them will be doing guesswork.
- Define what isolation philosophy you are going to use before finalizing the layout of the system. If dielectric unions are to be used in the project, they will need their place in the system and will increase the project cost. If you want to insert these unions into the system when it is already fabricated, it will become an additional work that will automatically increase the cost.
- The choice of stainless steel grade must match the environment. For instance, 304 is ok for "clean" municipal water used indoors, while 316 or super duplex steel must be used for coastal areas and salt water.
- It is important to indicate the torque and the type of sealant. Galling happens because of improper assembly, therefore, assembly instructions should be indicated on the drawing.
The stainless steel ball valve range used in potable and process water is a fair illustration of how the specification conversation works in practice: the body grade, the seat elastomer and the end connection each answer a different question, and the material pairing is only one of the three.
Where Eagle Fits
Our company produces fittings, valves, and sanitary parts made of stainless steel grade 304 and 316L. We work with different types of threads used for drinking and other water transmission. Our clients are usually manufacturers and companies specializing in water purification, who want to have the same fitting for a particular project instead of one specification.
One of the best ways to add value during our conversation about different materials is to tell you what combination is typical and what is not. For instance, using brass valves in stainless steel piping for municipal water is fine, and we will simply communicate this without pushing any unnecessary product on you. However, having a brass drain on a stainless steel tank in coastal waters is problematic, and, in most cases, the solution is to use a different material instead of using some tricky connector.
If you are designing a system rather than repairing one, send us the water chemistry data and the diagram of the system. We will tell you where to change materials, and you will spend less money this way than doing the inspection and finding the leak two years down the line.
FAQ
Is it okay to use brass and stainless steel fittings together?
As a rule, yes, in clean drinking water at a moderate temperature that is where usually these connections are fitted. The galvanic potential between the two is only about 0.15V – 0.25V, and thus the rate of corrosion is low when the water is a poor electrolyte and the stainless component is a bigger one. ACS becomes a problem when the water is high in chlorides, above 60 °C with water circulation, or when the system remains stagnant for long periods.
How to prevent galvanic corrosion between brass and stainless steel?
Interrupt the electrical circuit or adjust the environment. A dielectric coupling, plastic-lined joint, metal connection, or plastic pipe all work and are priced differently and have different temperature limits. What does not work is PTFE tape because it is a sealer, not an insulator, because even if the pipe connection is tight, there is still metal touching metal on the joint.
Are brass and 316 stainless steel compatible?
It can generally be said that 316 is the more preferred stainless choice when it comes to a mixed joint due to its higher molybdenum content which makes it more resistant to chlorides than 304. With compatibility being determined as galvanic compatibility, it must be said that this factor is not an issue. The main problem with brass would be dezincification, while with 316 we are talking about chloride pitting at the contact and both of the issues fall under the description of water chemistry not pairing issues.
Can I put stainless steel with brass?
The two are always together when it comes to valves, pumps, and various fittings. A lot of valves will have a stainless steel ball and stem fitted into a brass valve body. The theory is basically where size and environment come into play rather than permission: a small brass fitting placed inside a big stainless steel system seems risky while a small stainless part fits a big brass system safely; both will be ok in clean cold water.
References
- Copper Development Association — Brass Alloys and Dezincification Resistance
- Swagelok — Dissimilar Metal Connections and Galling Prevention
- ASME — B1.20 Pipe Threads and B31 Piping Codes
- ASTM International — Stainless Steel and Copper Alloy Specifications
- Alfa Laval — Hygienic Fittings and Material Compatibility
Conclusion
Both brass and stainless steel are fairly close together on the galvanic scale so that the joining of these metals often works. However, they can be far enough apart for the combination to fail at times. The above-mentioned variables include water chemistry, temperature, stagnation, and the ratio of the surface area of the cathode to that of the anode, and the only variable that can be controlled by the designer is the latter.
The above-mentioned pair can produce two types of failures. The first one is dezincification, which can literally eat brass from inside out. The second one is galling, which blocks screws during assembly and makes them look defective.
