You've hit on a common and frustrating situation. While it might seem odd to mix zinc hardware with galvanized strut outside, it's more common than you might think, although not necessarily the ideal practice. Here's a breakdown of why this happens and what the implications are

Why You See Zinc with Galvanized Strut:
Cost

Zinc plating is generally cheaper than galvanizing. For many applications, the difference in longevity isn't perceived as significant enough to justify the higher cost of fully galvanized hardware, especially for lower-stress components like washers and nuts.

Availability

As you've discovered, zinc-plated hardware is much more readily available. Distribution networks often focus on what sells in the highest volume, and zinc meets the needs of many indoor applications.

Perception of "Good Enough"

For some installations, the longevity of the zinc hardware is considered "good enough" for the lifespan of the installation. They might anticipate needing to replace the strut system before the zinc hardware degrades excessively.

Lack of Awareness or Concern

Sadly, some installers and suppliers may not fully understand the implications of mixing zinc and galvanized materials in outdoor environments, or they prioritize short-term cost savings over long-term durability.

The Problem with Mixing Zinc and Galvanized

The key issue is galvanic corrosion. Here's how it works:

Galvanic Series

Metals have different electrical potentials. Zinc is more active than galvanized steel (which is steel coated with zinc).

Sacrificial Anode

When two dissimilar metals are electrically connected in the presence of an electrolyte (like rainwater), the more active metal (zinc in this case) will corrode preferentially, sacrificing itself to protect the less active metal (the galvanized steel).

Accelerated Corrosion

The zinc hardware will corrode at a faster rate than it would if it were used with other zinc-plated components. While the zinc is corroding, it is protecting the galvanized strut to some extent, but it's shortening the life of the hardware itself.

Potential for Weakness

As the zinc corrodes, the washers and nuts can weaken, potentially compromising the structural integrity of the strut system over time.

Cosmetic Issues

The zinc corrosion can create unsightly rust streaks on the surrounding galvanized surfaces.

Is it "Regularly" Done? Yes. Is it "Ideal"? No.

Mixing zinc and galvanized is a common practice, but it's not considered best practice for long-term outdoor durability. If you're concerned about the lifespan of your outdoor strut system, using fully galvanized hardware is the better option.

Recommendations



Source Galvanized Hardware

As you're planning to do, order galvanized strut washers and nuts online from a reputable supplier. Look for specifications like ASTM A153 for hot-dip galvanizing.

Consider Stainless Steel

For harsh environments (coastal areas, industrial settings), stainless steel hardware might be a worthwhile investment for even greater corrosion resistance. It's more expensive, but it offers superior longevity.

Use Isolation Materials (If Mixing is Unavoidable)

If you absolutely have to use zinc hardware in contact with galvanized strut, you can try to minimize galvanic corrosion by using non-conductive washers (e.g., nylon or plastic) between the zinc hardware and the galvanized strut to break the electrical connection. However, this is only a partial solution and doesn't completely eliminate the risk of corrosion.*

Apply Protective Coatings

You could also consider applying a protective coating to the zinc hardware, such as a zinc-rich paint or a corrosion inhibitor, to slow down the corrosion process. Again, this is not a perfect solution, but it can provide some additional protection.

In conclusion, while you'll see zinc hardware used with galvanized strut, especially in less demanding environments, using fully galvanized hardware is the best approach for long-term durability in outdoor applications. You're right to be proactive in sourcing the correct materials. Flag for review