Oxidation of screws in external structures

30/05/2024

Introduction

Oxidation of screws used in outdoor structures, such as bridges and other constructions exposed to adverse environmental conditions, represents a significant issue for structural integrity and safety. This article explores oxidation processes in such environments, the specific causes, available treatments and best prevention strategies.

Oxidation Process

Oxidation is a chemical reaction in which a metal reacts with oxygen to form oxides. This process is accelerated in the presence of moisture, salt and other pollutants, common in outdoor structures such as bridges. For steel screws used in these applications, rust (iron oxide) is the primary oxidation product:

4Fe+ 3O2 + 6H2O = 4Fe(OH)3

Causes of Oxidation in External Environments

Humidity and Rain: Constant exposure to humidity, rain and condensation accelerates oxidation. Water acts as an electrolyte, facilitating the transfer of electrons and promoting the oxidation reaction.
Salinity: Structures near sea shores or exposed to road salt during the winter face more aggressive corrosion. Chlorides accelerate rust formation.
Temperature excursions: Freeze and thaw cycles, together with temperature variations, can create microcracks in protective coatings, exposing the bare metal to oxidation.
Air Pollutants: Oxides of sulfur and nitrogen, produced by industrial pollution, combined with humidity form acids that accelerate corrosion.
Galvanic Corrosion: Occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte, such as salt water. The less noble metal (anode) corrodes more quickly, while the more noble one (cathode) is protected. In exterior structures, the combination of dissimilar metals can cause this form of accelerated corrosion.

Treatments to prevent and counteract oxidation

Hot Dip Galvanizing: Coat the screws with a layer of zinc by immersing them in molten zinc. Zinc forms a protective barrier and, even if scratched, oxidizes before the underlying steel.
Polymeric Coatings: Application of polymer-based coatings (such as epoxy resins and specific polymers) that create a physical barrier against moisture and pollutants.
Advanced Surface Treatments: Techniques such as fluorination or anodization improve corrosion resistance by creating very hard and resistant protective layers.
Design and Planning: Design screws and structures to minimize water accumulation areas and facilitate drainage. The use of gaskets and sealants can prevent moisture from entering.
Regular Maintenance: Periodic inspections to identify and repair damage to protective coatings. Maintenance includes removal of oxides and reapplication of protective coatings.
Controlled Environments: Application of paints and protective coatings in controlled environmental conditions to ensure optimal and long-lasting adhesion.
Cathodic Protection: Using sacrificial anodes in extremely corrosive environments, which oxidize instead of screws, thus protecting the structural metal.
Use of special materials and alloys: Prefer materials such as those described below

Special Alloys Used

Stainless Steel

Type 316: Contains molybdenum, which significantly improves corrosion resistance, especially in high salinity environments, such as coastal areas. It is widely used in bridges and marine structures.

Type 2205 (Duplex Stainless Steel): This alloy combines an austenitic and ferritic structure, offering superior corrosion resistance and greater mechanical strength than standard stainless steels. It is particularly useful in highly corrosive environments.

Nickel alloys

Inconel 625: A nickel-chromium-molybdenum based alloy with excellent corrosion resistance in marine and industrial environments. It is used in applications where both high mechanical strength and corrosion resistance are required.

Other similar alloys are Inconel 600 and 617.

Titanium alloys

Titanium Grade 5 (Ti-6Al-4V): One of the most common titanium alloys, it offers high mechanical strength and excellent corrosion resistance. Although expensive, it is ideal for critical applications where corrosion resistance and durability are essential.

Titanium Grade 2: A cheaper alternative with good corrosion resistance and sufficient mechanical properties for many structural applications.

Nickel-Chromium Based Alloys (Incoloy)

Incoloy 800: This alloy is known for its excellent resistance to oxidation and carburization in high temperature environments.

Incoloy 825: Contains molybdenum and copper, which improve corrosion resistance, especially in acidic environments.

Other similar alloys are Incoloy 832, 901 and 925.

Nickel-Molybdenum-Chromium Alloys (Hastelloy)

Hastelloy C276: One of the most versatile nickel-molybdenum-chromium alloys, with excellent corrosion resistance in a wide range of aggressive chemical environments.

Hastelloy X: Known for its resistance to oxidation and carburization at high temperatures, it is used in aerospace and petrochemical applications.

Zirconium alloys

Zirconium 702: Offers exceptional resistance to corrosion, especially in environments where strong acids such as hydrochloric and sulfuric acid can develop.

Zirconium 705: A niobium-alloyed version, offering a combination of superior mechanical strength and excellent corrosion resistance.

Special protective coatings

PTFE (Polytetrafluoroethylene)

Advantages: Excellent chemical resistance, low friction, heat resistance up to 260°C.

Applications: Used in corrosive and high temperature environments, such as chemical plants and marine structures.

Rilsan (Nylon 11)

Advantages: Excellent corrosion and abrasion resistance, good mechanical strength, maintains flexibility even at low temperatures.

Applications: Used in marine, industrial and infrastructure applications where superior resistance to corrosion and abrasion is required.