There are many choices available when it comes to heat exchanger material in a condensing boiler. In today’s current offering of commercial condensing boilers you can find copper, carbon steel, 304 stainless, 316L stainless, 439 stainless, duplex steel alloys, and aluminum. A boiler manufacturer evaluates a number of different factors when selecting their material for the heat exchanger. Cost, workability and corrosion resistance all play a part in the final decision.

In this paper, we will examine the differences between the most popular choice, 316L stainless steel and a new choice in condensing boiler heat exchangers, 439 stainless steel.

What does 439 and 316L mean?

The words “stainless steel” are often thrown about with abandon. There are over 70 different grades of stainless steel in five different categories. The particular grade of choice depends on the application you are manufacturing for. The common factor in all these grades is the presence of Chromium (Cr) in some variable percentage. This ranges from a low of 4% in some 500 series to a high of 28% in 329 grade.

We are looking at 316L and 439 as 316L is the most common material in condensing boiler heat exchangers and 439 has now made some inroads into condensing boiler heat exchanger manufacturing. Below is a table outlining the physical properties between the two.

Category

These two types of stainless steel are categorized by their microstructure. 316L is an Austenitic Stainless steel and 439 is a Ferritic stainless steel.
This designation is related to the microstructure of the metal The differences can be seen under an electron microscope in this picture A is austenitic (304) and B is ferritic (430).

Despite the popular myth of stainless steels not being magnetic, ferritic stainless steels are magnetic. This has an impact for many of the uses of 439. Stainless steel refrigerators that will hold a fridge magnet, stainless steel cookware for induction cooking tops. Knives that will stick to magnetic cutlery holders. All are prime uses for 439.

Differences

Apart from the molecular structure, there are two main alloying differences between 316L and 439. The most significant difference is the lack of nickel (Ni) in the 439 stainless.

Nickel

Nickel is added to alloys to increase resistance to corrosion. In percentages above 8%, Nickel increases resistance to oxidation, nitriding, thermal fatigue and strong acids. It adds significant toughness and strength to the alloy along with the improved resistance to oxidization and corrosion. For this reason, it is used extensively in the food, healthcare, pharmaceutical, and chemical industries. It is also the most significant cost driver in the pricing of stainless steel. As per above 316L contains 12% Ni while 439 has none.
The primary purpose of substituting 439 for 316L in a condensing boiler heat exchanger is to eliminate the cost nickel adds to the product.

Molybdenum

Molybdenum is added to alloys to improve resistance to pitting corrosion, especially by chlorides and sulfur chemicals. It greatly diminishes the tendency of steels to decay in service or in heat treatment. 316L has a 2% content of Mo while 439 has none.

Titanium

439 ss was developed as an alternative to 409 ss to provide better performance in the automobile exhaust industry for headers and exhaust piping. To improve upon 409’s corrosion resistance Ti was added in trace amounts. 316L does not contain Ti.

Corrosion Resistance

Types of Corrosion

Pitting

Is perhaps the most frequently seen type of corrosion. Pitting is a function of chloride molecules dissolving the chromium oxide on the passive layer leaving corrosion susceptible Fe. Chlorides combine with the Fe and form ferric chloride and the result is spherical pits that bore into the metal surface. Choosing the wrong alloy can mean catastrophic failure.

PREN number

PREN is Pitting Resistance Equivalent Number. The larger the number the more resistant to pitting the material is. Increasing Molybdenum in the alloy increases resistance to pitting, therefore, there is a direct relationship between the %Mo and a higher PREN.

Below you can see the influence Mo has on pitting resistance in alloys. This chart shows the relationship between pitting, pH, and Mo content. Pitting is not an issue below the line but above the line pitting can be severe.

Flue gas condensate can have pH content between 3.2 and 5.5. Chloride content can be between .13ppm to 1.10 ppm, even higher depending on environmental factors.
At 0% Mo, as found in 439 grade, any chloride content at pH levels of 4.0 or lower could cause serious pitting. 316L remains, for all practical purposes, unaffected by the residual amounts of chlorides found in flue gas condensate. It would take drastically higher concentrations of chlorides at considerably lower pH levels than will be found in natural gas flue condensate to cause a pitting issue.

The use of ferritic stainless steel alloys such as 439 in a condensing boiler application relies on acid pH levels always staying above 4 to prevent pitting corrosion. The literature suggests that the normal range for condensate is as low as 3.2.

Crevice Corrosion

Crevice corrosion prevention is directly linked to the presence of Mo and Cr. Microstructure, austenitic or ferritic, seems not to be a deciding factor in susceptibility. As in PREN it is the Mo content is most important for corrosion prevention.
However, in ferritic structures, when the ph level drops below depassivication, crevice corrosion propagates very quickly once initiated. Ferritic alloys rarely repassivate in acidic solutions so acidic conditions and crevice like configurations are definitely contra-indicated when using 439 stainless steel. Optimum design of equipment is imperative so conditions encouraging crevice corrosion are avoided.

The above chart shows the difference in pit depth over time between 3 grades of ferritic stainless including 439 against the performance of the austenitic 304ss. 316L contains 4% more Ni and 2% Mo more than 304ss making its resistance even greater. Pitting corrosion resistance is substantially improved with 316L.

316L performs well in crevice corrosion tests due to the presence of Mo. Of course, care still needs to be taken in the manufacturing process to prevent crevice corrosion conditions.

Stress corrosion cracking

Ferritic stainless steels generally perform well in stress corrosion cracking tests. Caveats are the poor capacity to repassivate especially in acidic environments which can lead to failure when combined with their poor performance in deformation tests. The addition of Ti to 439 helps in its stress corrosion cracking performance.

If under tensile stress Austenitics may be subject to stress corrosion cracking in the presence of chlorides. Mo is added to austenitics for prevention. With its high Ni content and the addition of Mo, 316L performs equally as well as 439 in stress corrosion tests albeit at a considerable cost disadvantage.

Work-ability

439

Typically, ferritic SS are subject to grain growth when welding and 439 is no exception. The tensile, fatigue and toughness properties in welded areas are relatively poor. Generally, 439 is limited to a combined thickness of 3mm in a welded condition, i.e. a lap joint between two 1.5mm thicknesses.

It is possible to use 316L as a filler material to improve this condition to some extent but all welding procedures should endeavor to maintain minimum heat inputs. Care must be taken to remove discoloration via pickling and passivation to restore maximum corrosion resistance after material handling. This may or may not be possible depending on the actual design of the heat exchanger.

Inter-granular corrosion, especially at HAZ of welded structures, is a potential problem with all ferritic alloys. Ti and Nb are added to stabilize the HAZ zone and prevent the precipitation of carbides in the welding process. Nonetheless, care must be taken when welding 439 even with its addition of Ti.

439 is much harder than 316L and therefore heat exchangers are manufactured using a straight tube design. The brittleness of the alloy precludes a bent tube design.

316L

Austenitic SS are typically much more workable than their ferritic counterparts. 316L is not as hard a metal as 439. Elongation at break is 22% for 439 and 60% for 316L. This is a definite advantage in forming. In addition, thermal stresses are much easier to manage due to better ductibility and drawing 316L has a low carbon content to prevent carbide precipitation during welding which drastically improves its corrosion resistance. However, as with 439 correct welding procedures are mandatory to ensure the long life of the weld. The skill of the welder has a direct impact on the quality of the final product.

While both have low carbon content welding 316L is much easier due to the additions of Ni and Mo.

Conclusion

It is clear from the literature there is a considerable difference between the corrosion resistance of 316L and 439 stainless steel. 316L outperforms 439 in the two critical areas of pitting and crevice corrosion and is equal, due to the addition of Mo, to 439 in stress corrosion cracking.

In work-ability 316L is much easier to weld and while it is easy for a manufacturing plant to have highly skilled welders on the shop floor it may be much more difficult to replicate those skills in the field with service personnel who don’t spend every day with difficult to weld 439 stainless steel. This can have a direct impact on lifetime servicing costs down the road.

Of course, costs are a factor but in the lifetime of a boiler purchase price can be less than 3% of the total spent. The use of corrosion-susceptible materials based on the initial cost seems foolhardy at best. Incumbent upon the specifying engineer is the careful consideration of all aspects of a boiler including construction materials. We hope this comparison helps make that decision a little easier.