Hot tub heater corrosion

[JoyfulNoise]

Does this increase with acidic water?

pH definitely drives some forms of corrosion and the passivation layer on any steel surface can be compromised by changes in pH. Low pH is typically needed.

What standoffs? The heater surface is grounded at the mounting bolts, the element is isolated within a (ceramic?) coating and protrudes from the ends where the terminals are welded on. The whole mess is sealed with epoxy beneath the terminal.

Where the wire connector and nut screws on to the threaded connection. That should have a good coating of dielectric grease on it ... kind of like how you grease the terminals on your car battery.

This is, quite literally, the standard for spa heaters. They are nearly all like this, and have been for decades. If the heater design were at fault, it would be a universal problem. This is not to say that they do not short out at this location, usually from a leak/seep at the epoxy seal. The same epoxy seal that can be damaged during installation if you don't hold the bottom bolt head on the terminal while tightening the nut.

And this is where a thousand things can go wrong ... just because it was built at the factory doesn't mean Gomer on the assembly line wasn't having a bad day and torqued down on the nut a little too hard or when Junior was installing the assembly (while listening to loud music with his AirPods in and texting his girlfriend) that he didn't bang into connection with his power driver ... I'd pay YOU $200/hr to do the job because I'm pretty sure you'd do it right ... as for some of the bozo's that get sent out to my home to do repairs or installs, uhhhh, I'm not sure they shouldn't being paying ME for telling them which end of the screwdriver to use ....

 

[JamesW]

The element will look something like this.

If the casing is split open, that is usually due to overheating.

If there is scale buildup, then that will cause overheating.

If split and no scale, it is usually a dry fire issue.

If there is corrosion of the element jacket that exposed the heating element, then that could be a type of corrosion.

 

[RDspaguy]

Where the wire connector and nut screws on to the threaded connection. That should have a good coating of dielectric grease on it ... kind of like how you grease the terminals on your car battery.

Not one spa heater has that, nor do residential electric water heaters. I hear what you're saying in general terms, but can't attribute this unusual circumstance to an issue that every spa has.

And this is where a thousand things can go wrong

Ok, ok. You make a good point. Bad parts do happen, and it could be a complete coincidence that it's in a salt spa. I don't think that's the case, but it is possible. All I can say is that this is typical for the few salt spas I have seen. I don't know why, nor do I know why I don't see similar corrosion issues in salt pools, but in my opinion salt systems tear up spas whatever the exact mechanism.

 

[JoyfulNoise]

Not one spa heater has that, nor do residential electric water heaters. I hear what you're saying in general terms, but can't attribute this unusual circumstance to an issue that every spa has.


Ok, ok. You make a good point. Bad parts do happen, and it could be a complete coincidence that it's in a salt spa. I don't think that's the case, but it is possible. All I can say is that this is typical for the few salt spas I have seen. I don't know why, nor do I know why I don't see similar corrosion issues in salt pools, but in my opinion salt systems tear up spas whatever the exact mechanism.

Well, let’s face it, I could certainly conceive of and build a heater that would never have any of these issues …. But it would also cost $10,000 per unit and require a PhD in astrophysics to become a certified installer … and no tub manufacturer would ever use one.

These are, for better or worse, the consumable parts of a hot tub. When you buy the hot tub, just like any other appliance, you’re buying a depreciating asset with a finite life and lots of potential problems to fix in the future. So you just have to look at it as the price you pay for all that enjoyment you receive while soaking in a warm bubbly tub of human-stew …

 

[RDspaguy]

But it would also cost $10,000 per unit and require a PhD in astrophysics to become a certified installer … and

Yeah, I'm not getting a PhD to change a heater.

 

[JamesW]

What is an Incoloy® Heating Element - Rheem Asia

Some type of alloys includes Incoloy®, which is a type of superalloy or high-performance alloy). But what really are they? What is their use? Read more here!

www.rheemasia.com

 

[RDspaguy]

holes show corrosion, which points to heat induced steel damage during the cutting process maybe from a bad drill bit.

Is that a thing? What I see is everyplace with an o-ring. Looks like lots of rust inside the tube too.

The same thing happens on the stainless steel band in a Pentair filter.

I am familiar with this, and in my mind attributed it to the rivet being a different material and the juncture often soaked with chlorine treated water of questionable ph.

Remove the two nuts and pull out the element.

Remove the wires and left sensor too.
If heavily corroded, the element fitting will twist off the element and just spin in the hole. In this case, the nut must be cut off, carefully as it touches the heater tube.

 

[JamesW]

Heat and machining can damage the molecular structure of stainless steel and make it susceptible to corrosion.

The steel is heated so that it can be bent into a circle.

The color indicated that the steel probably got too hot.

I think that the joint is welded.

If it was a chemistry issue, the steel would be uniformly corroded.

 

[JamesW]

1. Galling

This corrosion occurs when stainless steel parts are forced together. Commonly seen with nuts, bolts, and other fasteners, corrosion isn’t visible until the parts are separated or disassembled.

The process uses repeated passes to adjust the dimensions of the steel slowly. In most cases, this will involve rolling through multiple mills over time to achieve the desired thickness.

5. Weld Decay

Also known as intergranular corrosion, this corrosion results from heating stainless steel to a specific temperature range—often between 550C and 850C.

This causes chromium to precipitate from the stainless steel and reduces the ability for the passivation layer to regenerate.

Understanding Stainless Steel’s Corrosion-Resistant Properties - Unified Alloys

Whether for the consumer market or industrial use, one of the most popular reasons for choosing stainless steel is its corrosion-resistant properties. However, the exact resistance levels and weaknesses of a stainless product, sheet...

www.unifiedalloys.com

 

[JamesW]

A less common form of rusting in stainless steel is after the stainless has been exposed to very high temperatures, often in the 750-1550°F range (400-850°C).

This type of corrosion is often seen in welding applications in which stainless is heated and then cooled.

If this happens, “sensitization” can occur which is where the carbon and the chromium bond together in the stainless steel and form carbides.

These carbides situate themselves at the stainless steel grain boundaries, and the grain boundaries become deficient of chromium.

With lower chromium concentrations at the grain boundaries, the chromium oxide protective layer can become discontinuous and rusting becomes possible.

https://www.polymersolutions.com/blog/why-does-stainless-steel-rust/

More than 0.03% C in Type 304 and 316 makes them vulnerable on welding to “sensitization”, which reduces the corrosion resistance of the grain boundaries, making intergranular corrosion possible.

Heat of welding causes chromium to combine with carbon and precipitate chromium carbide at grain boundaries, depleting chromium from the general microstructure in the weld’s heat affected zone (HAZ).

This makes the affected zone susceptible to intergranular corrosion, or intergranular attack (IGA).

https://nickelinstitute.org/media/1810/stainlesssteelsandspecialtyalloysformodernpulpandpapermills_11025_.pdf

 

[JamesW]

 

[JamesW]

Once you get the parts disconnected from the tube, can you show pictures of the inside of the tube from both ends so that we can see if the corrosion is only at the holes or if it is evenly distributed on the inside of the tube?

Do you have an ozonator?

Can you show a picture of the whole equipment bay?

Has the chemistry always been good?

 

[RDspaguy]

Thanks @JamesW , that's good to know.
And I thought you meant the outer clamp ring, where the clamp is riveted onto the band. Though I guess this could apply there as well since they drilled (or punched) it for the rivet.
Speaking of which, would those holes be drilled or punch pressed do you think? Would a press heat it that much during a shear cut? Or maybe a waterjet? That's how stainless spa manufacturers cut their panels, and it leaves a more rounded edge than a shear or drill.
I know, you're not a stainless fabricator (are you?), but you seem to know about this stuff so I figured I'd ask while we're on the subject.