I’ve talked about rusting metal a lot recently. As a metal detectorist, finding little treasures with signs of corrosion is par for the course. And while galvanized steel isn’t necessarily a sought-after find, it’s worth discussing how this metal reacts in its environment.
Even though galvanized steel is highly resistant to corrosion, rusting will eventually occur if presented with unsuitable pH levels, high chloride exposure intensified by moisture, and extreme humidity.
But don’t get me wrong. This zinc-coated steel alloy is as tough as they come and doesn’t submit to the degradation of rust without a fight.
What Is Galvanized Steel?
What makes galvanized steel near impenetrable is its carefully selected anode layer. By itself, steel is made strong because of the carbon content added to its composition.
Making steel galvanized involves hot-dipping it in a molten zinc bath heated to roughly 842 degrees Fahrenheit. This metallurgical process coats the steel, allowing it to maintain its thickness and durable properties but adding a passive layer meant to sacrifice itself to corrosive elements.
Another method of galvanizing steel is known as electroplating, but the traditional method is specific to applying zinc as the protective layer.
Galvanized Steel vs. Steel
It’s no surprise to you if you’ve been following along with my rusting series coverage that steel is strong and takes on the thickness of the manufacturer’s goal. It’s an alloy of iron with a small amount of carbon, making it one of the two main ferrous metal categories from which all others derive.
The main differences between steel and galvanized steel come from the passive zinc layer and its ability to extend the life of metal in harsher surroundings.
- Galvanized steel can last up to 50 years longer than steel on average.
- Because of its superior durability, it requires less maintenance, especially in more hostile areas with acidic exposure.
- Galvanized steel is more cost-effective than steel due to its more prolonged use before treating or completely replacing it.
What Is Galvanized Steel Used For?
With all the benefits of galvanized steel, it’s no wonder so many industries rely on it.
According to the U.S. General Services Administration, galvanized steel is applied to sheet metal and other building materials for everyday construction and utility work.
But if you look closer at our economy and each industry on its own, you’ll find that galvanized steel is used in applications that we see daily but don’t often acknowledge.
|Industry||Uses for Galvanized Steel|
|Agriculture||Farm Equipment, Irrigation Systems|
|Appliances||Refrigerators, Washers, Dryers, Dishwashers|
|Automotive||Automotive Parts (Fuel Tanks, Body Panels, Exhaust|
|Construction||Nails, Beams, Bridges, Bolts|
|Electrical||Wiring, Equipment Casings/Enclosures|
|Industrial||Storage Tanks, Sheet Metal, Shipping Containers|
|Real Estate||Playsets, Rails, Light Poles, Pipes, Fencing, HVAC|
Galvanized Steel Properties
To double back for a second, the beneficiary of galvanized steel is the reinforced steel by the zinc barrier coating. The passive layer offers many properties that steel doesn’t possess individually. But the zinc can be applied thicker in some instances than others. The thinner the coating, the faster rust will eventually begin to appear.
One of the premier properties of galvanized steel is the way zinc when attacked by corrosion, will reinforce the galvanization. You see, zinc does rust, but it rusts much slower than most metals. So the idea behind galvanized steel is while corrosive elements attack the passive layer, the zinc will slowly corrode and form a patina. In contrast, the steel underneath is left intact and unexposed.
And according to the GAA, galvanizing steel will:
- Increase its tensile elongation by 40%
- Give immunity to ultraviolet radiation
- Protect even small areas of exposed steel from rust
- Provide a superior barrier than alternative paint coatings
Furthermore, galvanized steel is manipulated easily. Whether the manufacturer intends to weld it, punch holes in it, bend it, or roll it, its strength and yield will remain the same.
How to Tell if Metal Is Galvanized
There are several ways to tell if a metal is galvanized, but the easiest is to look for spangling along the surface. Spangle appears on galvanized steel during the hot-dipping process and is easily identifiable by the spongy metallic look instead of the consistent sheen of most metals.
The other methods are a bit more tricky for the average person as they require lab testing, magnetic coating thickness measurements, or visually inspecting for signs of rust, only if you know the age of the steel.
While lab testing for the zinc content is the most accurate way to know, I doubt most of you have the time or need to do this.
What Corrodes Galvanized Steel?
If the zinc coating of galvanized steel is damaged or the underlying steel is uncovered in a particularly corrosive environment, rust can form on the surface of the steel. Additionally, If the zinc coating is thin, it may corrode faster, allowing the steel to rust.
Some specific conditions that can cause rust in galvanized steel include exposure to saltwater, high humidity, scratches or damage to the zinc coating, acidic or alkaline substances, environmental factors, and time. It’s important to note that galvanized steel has a longer life span than regular steel, but it’s not rust-proof.
But let’s talk specifics.
A study authorized by the NCSPA and the AISI aimed to accurately predict the average service life of galvanized steel in varying soil conditions. Now, we’ll get to life expectancy in a minute. But this study determined that three factors directly impacted the corrosion rate of their samples: chlorides, moisture, pH levels, and resistivity.
Here are the conclusions they settled on.
- Chloride ions in the soil negatively impacted the zinc layers’ rust resistivity. And the inclusion of increased moisture sped up the corrosive process.
- Moisture content above 17.5% led to a significant corrosive effect.
- When the pH levels tested were lower than 7.0, corrosion rates of the zinc layer were consistently higher.
How Long Does Galvanized Steel Last?
Generally, galvanized steel will last up to 75 years due to the successful galvanization of steel with a molten zinc layer. But this life span can change dramatically depending on the galvanized steel’s environment.
In the same study we were discussing, the samples of galvanized steel were examined from 122 US cities to gauge the reaction from varying influences. Below, you’ll see the chart created by the AGA based on the results, but here’s a quick rundown of how different factors can impact the longevity of galvanized steel.
|Longevity||Conditions Present||Years||Side Note|
|Best||Moisture: < 17.5%|
Chloride: > 20 PPM
pH: < 7.0
|Worst||Moisture: > 17.5%|
Chloride: > 20 PPM
|50 Years *||If pH < 7.4 & Moisture > 26.8 = 28 Years|
|Average||Moisture: < 19.3%|
pH: > 7.8
|100 Years *||If pH < 7.8 & Moisture > 19.3% = 76 Years|
|Volatile||Moisture: < 19.4%|
pH: > 6.0
|50 Years *||If pH > 7.0 & Moisture is 5% = up to 150 Years|
How to Protect Galvanized Steel from Rusting
The best way to protect galvanized steel from rusting is to keep it out of harmful environments where moisture or air is high in destructive properties.
Sounds easy, right? Well, it’s not always possible, which is why steel is commonly galvanized to begin with. Short of that suggestion, you could apply paint specifically made to add an extra barrier for the passive layer.
To sum up, galvanized steel, typically known as resistant to corrosion, will rust in time.
Will it take 50 years? 150 years? It all depends on what’s around it, how thick the zinc coating is, and other physical impacts it sees in its life span.