Analysis of the problem of salt spray resistance test

Metals are exposed to the natural environment and are highly susceptible to environmental erosion and corrosion damage. The purpose of coating is not only to decorate, but also to pay attention to its protective effect. A coating system protection performance is good or bad, must get the data in advance, salt spray test is based on this artificial
acceleration
simulation test

.
Metallic salt spray corrosion principle
metal material corrosion in the salt spray environment, mainly conductive salt solution seeps into the metal electrochemical reaction, forming a "low potential metal
-
electrolyte solution
-
hodium potential impurities" microcrystyte system, the occurrence of electronic transfer, as an anode of the metal dissolved, forming a new compound i.e. corrosion. The main factors affecting corrosion are water, oxygen and ions.
The main role in the process of salt spray corrosion damage is chlorine ions, which have a strong penetrating skills, easy to penetrate the metal oxide layer into the metal interior, destroy the metal's blunt state;
coating is a highly polymer film, making ions difficult to penetrate, which is the key to the coating corrosion resistance.
Salt Mist Test
Salt Mist Test is an environmental test that mainly uses artificial simulated salt spray environmental conditions created by salt mist test equipment to assess the corrosion resistance of products or metal materials.
Compared with the natural environment, the salt concentration of chloride in its salt mist environment can be several times or dozens of times the salt spray content of the general natural environment, which greatly improves the corrosion rate, and the salt spray test on the product, and the time to get the results is greatly shortened. Of course, the natural environment is extremely complex, and the artificial acceleration simulation test is still different from the natural environment, so it cannot be completely replaced or conversion.
mainly the following types:
neutral salt spray test (
NSS
), acetate fog test(ASS)
, copper acetate accelerated salt spray test
(CASS)
, cross-variant salt spray test;
test criteria for salt mist test conditions, such as temperature, humidity, concentration of sodium chloride solution,
PH
value, salt spray time, storage time, test cycle, fog collection, etc. have clear provisions, in addition to the performance of salt spray test box technical requirements.
the difference between the various salt mist test parameters:
transmal salt spray test
is a comprehensive salt spray test, it is actually a neutral salt spray test plus constant wet heat test
test, mainly used in cavity-type whole machine products. The test parameters are the same as between the neutral
test
NSS test.
In addition to the factors affected by the salt spray test
in addition to the standard test provisions such as test temperature and humidity, solution concentration, sample placement angle,
pH
value, salt spray subsidion and spray method, there are still many aspects of the salt spray test results will have a significant impact:
1
, film-forming resins:
different film-forming resins have their own performance, different types of film system performance differences are greater. Mainly manifested in the performance difference:
1
) water resistance: salt spray test environment itself humidity is very high, test board close to the immersion state, if water resistance is not good, then the salt solution penetrates rapidly, the protective effect of coating film is greatly reduced.
2
) alkaline: the corrosion products produced by steel are alkaline.
3
) cross-link density: this has a certain relationship with paint type, ratio ratio (two or more parts), drying type and drying time. The higher the cross-link density, the stronger the shielding effect of the coating on water, oxygen and ions, the better the protective performance.
4
) adhesion: the better the adhesion, the closer the coating to the substrate, but also more difficult to bubble, shedding and so on.
2
, Formula Structure:
1
) Rust-proof pigments: one is a flaky structure that increases the permeation path of the corrosive medium to play a delay effect, and the other is to rely on chemical reactions to change the properties of the surface or the properties of the reaction to achieve anti-rust purposes.
2
) Pigment ratio: Although adding certain pigments to salt mist resistance has some help, but also to a certain limit, in order to take into account other properties and other powders added. When the paint's face ratio is too high, the film tightness decreases, water resistance, flexibility, adhesion, etc. will change accordingly, thus affecting the salt spray resistance of the coating film.
3
) Adhesion Promoter: The addition of the appropriate amount of adhesion promoter to the formula can effectively enhance the grip of the paint film on the substrate, preventing foaming and shedding. In addition, some cross-linked adhesion promoters can provide a certain cross-linking point, improve cross-link density, thereby improving salt spray resistance.
4
) deblisters: When the lacquer deboy effect is not good, the micro-bubbles in the paint film greatly shorten the penetration path of the corrosive medium, thereby aggravating corrosion.
3
, construction process
1
) paint film thickness: the process of salt spray corrosion is through the coating film penetration process, the thicker the coating film, the more difficult penetration, there is better salt spray resistance.
2
) substrate type: different substrates and coating matching properties have certain differences, their own corrosion resistance is also different.
3
) substrate treatment: substrate treatment includes polishing or not, cleaning or not. Obviously, polished clean substrates are more beneficial for the protection of substrates.
4
) thin multi-channel: under the same paint film thickness conditions, thin multi-channel is more advantageous than a single coating.
5
) coating maintenance: sufficient drying time (including coating interval), good maintenance environment, but also more conducive to salt spray resistance performance.
addition, the sealing, dashing and evaluation methods of the model will also affect the evaluation of salt spray resistance level to varying degrees.
the evaluation of the test results
From the factors affected by the salt spray test, it is unreasonable to emphasize the salt spray resistance time of the resin or lacquer liquid without fully limiting the influence of other factors. Because salt spray resistance time can be affected by many performance, such as the above mentioned formula, substrate, supporting processes.
, in the case of fixed experimental conditions, due to experimental habits, subjective factors, etc. will also produce different results.
the results of the salt-resistant spray experiment can be evaluated by reference to
GB/T 1766-2008 rating method for aging paint and varnish coatings or
ISO 4628-1 to 5 paint and varnish coatings The identification of the number and size of aging evaluation defects and the degree of uniform change in appearance is mainly judged from the following aspects:
1
, rusty
rust evaluation can be divided into two situations: dashing and unlined.
is not cross-lined, the general eligibility criteria are "no rust (
" Ri0
), in the case of dashing, one-way corrosion is generally required to spread no more than
1mm
or
2mm
.
Rust degree evaluation
2
, foaming
foaming is a number of water-resistant coatings in salt spray tests often occur in the phenomenon, usually poor water resistance or poor adhesion of the paint is more likely to occur.
general, the "no bubble" as the eligibility criteria.
foaming degree evaluation
3
, peeling off
paint film peeling is a more serious paint film damage, the test time in the case of paint film peeling, salt spray test is certainly not passed.
addition, the paint film in the test may also appear including loss of light, color change, adhesion reduction and other different types and degrees of paint film changes, in different application requirements, but also to pay attention to.