Mild steel (MS) rods are a cheap and common choice for framing, fixtures, and light structural work. MS rods, on the other hand, will rust if they are not treated when they come into contact with oxygen, moisture, and other substances. Knowing how corrosion starts and spreads, as well as how long different environments can expect service lives, helps designers, fabricators, and owners plan for protection, inspection, and maintenance.
How corrosion starts and gets worse on MS rods
When metal, moisture, and an electrolyte (like salt-laden water) are present, iron oxidizes to make iron oxides (rust). This is how MS rods corrode. The condition of the surface is important. Mill scale, scratches, welding heat-affected zones, and debris that gets stuck can all speed up local attack. Geometry also affects how things behave. For example, crevices, tight bundles, and threaded connections can hold moisture and salts, which speeds up localized corrosion (pitting). Pollutants like SO₂ and NOₓ can make acidic films in some places that speed up the loss of metal.
Normal ways that corrosion happens
- Uniform corrosion: the surface gets thinner evenly, which makes it easier to predict and keep an eye on.
- Pitting: small, deep pits that can make something fail quickly even if it loses very little mass overall.
- Crevice corrosion happens in gaps and between surfaces that touch.
- Galvanic corrosion happens when MS rods touch a more noble metal. In this case, the steel becomes the anodic (corroding) partner.
Expected lifespans (untreated MS rods) — useful ranges
The climate, pollution, and detailing all have a big effect on how long things last. The ranges below are rough estimates that assume normal fabrication, storage, and no active corrosion protection:
- Indoor, dry, and climate-controlled (like in warehouses or buildings): 50 years or more. Low humidity and no contaminants make corrosion happen very slowly.
- Under canopies and covered walkways, sheltered outdoor areas last 20 to 40 years. Less wetting greatly increases life.
- Exposed in cities and suburbs (with occasional rain and moderate pollution): 10–20 years. Corrosion rates go up because of road salts, SO₂/NOₓ in the air, and wet-dry cycles.
- Industrial/chemical sites (high pollutant levels): 5–15 years. Acidic deposits and strong chemicals make things last much shorter.
- Exposure to salt spray and splash zones on the coast or in the ocean lasts from 1 to 10 years. Salt speeds up corrosion a lot; splash and tidal zones are the worst.
These estimates are directional — design details (drainage, crevices), contact with dissimilar metals, frequency of wetting, and maintenance can shift lifespans substantially.
Strategies for inspection and protection
- Check: plan visual inspections every 6 to 12 months in places that are exposed; measure thickness where failure is most likely. Check for rust under coatings, pitting, and attacks in crevices.
- Design: stay away from cracks, make sure there is drainage, and let air flow to keep moisture from getting trapped.
- Low-cost protection: in many places, shop primers or a single coat of zinc-rich primer can make things last two or three times as long.
- Strong protection: hot-dip galvanizing or duplex systems (galvanizing and painting) work very well for coastal and industrial sites.
- Maintenance: cleaning and painting over damaged areas on a regular basis greatly extends the life of the service.
Conclusion
Untreated MS rods can last a long time in dry, protected areas, but they may not last as long in coastal or heavily polluted areas. To get the most out of your service life, use realistic estimates of how long things will last in their environment, check them often, and use simple protection and design methods. This will often cost a lot less than replacing them.